U.S. Department of the Interior

U.S. Geological Survey

Techniques of Water-Resources Investigations 8-A3

Use Of Submersible Pressure Transducers 

In Water-Resources Investigations

Chapter A of

Book 8, Instrumentation

Section A, Instruments for Measurement of Water Level
U.S. Department of the Interior

U.S. Geological Survey

Use of Submersible Pressure Transducers 

in Water-Resources Investigations

By Lawrence A. Freeman, Michael C. Carpenter, Donald O. Rosenberry, 

 Joseph P. Rousseau, Randy Unger, and John S. McLean

Chapter A of

Book 8, Instrumentation

Section A, Instruments for Measurement of Water Level

Techniques of Water-Resources Investigations 8-A3
U.S. Geological Survey, Reston, Virginia: 2004

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center, Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government
iii

Contents

ABSTRACT ......................................................................................................................................................1

INTRODUCTION .............................................................................................................................................1

Background ...........................................................................................................................................1

Purpose and Scope .............................................................................................................................2

BASIC PRINCIPLES OF PRESSURE AND PRESSURE MEASUREMENT ..............................................2

Physics of Pressure .............................................................................................................................2

Types of Pressure Measurements ....................................................................................................3

Overview of Direct Current Circuits ..................................................................................................4

Overview of Direct Current Circuit Analysis ...................................................................................5

The Wheatstone Bridge, Thevenin Equivalent Circuit ..................................................................7

PRESSURE TRANSDUCER CHARACTERIZATON .....................................................................................9

Basic Types of Transducers for Measuring Pressure ...................................................................9

Piezoelectric Transducer ..................................................................................................................10

Capacitive Transducer ......................................................................................................................10

Inductive and Reluctive Transducers .............................................................................................10

Potentiometric Transducer ...............................................................................................................11

Vibrating-Wire and Vibrating-Cylinder Transducers ...................................................................11

Strain-Gage Transducer ....................................................................................................................12

Understanding Pressure-Transducer Specifications ..................................................................13

PLANNING CONSIDERATIONS FOR SENSOR SYSTEMS ....................................................................14

Study Duration and System Reliability ...........................................................................................15

Required Accuracy ............................................................................................................................16

Installation Location and Site Accessibility ..................................................................................16

System Components and Compatibility ..........................................................................................16

Water Quality ......................................................................................................................................17

Number of Wells .................................................................................................................................17

Well Location, Diameter, and Depth ...............................................................................................17

Data-Collection Frequency ...............................................................................................................18

Data Transfer .......................................................................................................................................18

Cost...... .................................................................................................................................................18
iv

ASSEMBLY, CALIBRATION, AND TESTING ...........................................................................................18

Familiarization With Transducer Performance .............................................................................18

Linear Transducer Calibration .........................................................................................................20

Sources of Error in Linear Calibrations ..........................................................................................21

Sources of Error When Making Direct-Current Measurements ...............................................21

Voltage Measurements ............................................................................................................21

Current Measurements ............................................................................................................23

Resistance Measurements .....................................................................................................23

Temperature-Corrected Transducer Calibration ..........................................................................24

INSTALLATION .............................................................................................................................................25

Care and Handling ..............................................................................................................................25

Shelter .................................................................................................................................................26

Power Requirements .........................................................................................................................26

Hanging Transducers in Wells .........................................................................................................27

Measuring System Drift ....................................................................................................................28

Desiccation Systems .........................................................................................................................28

Transducer Field Calibration ............................................................................................................29

Optimizing Measurement-System Performance ..........................................................................31

DATA COLLECTION ......................................................................................................................................31

Frequency of Visits .............................................................................................................................32

Field Checks ........................................................................................................................................32

Data Recording and Retrieval ..........................................................................................................32

Verification ..........................................................................................................................................34

Use With Data Loggers .....................................................................................................................34

Field Documentation ..........................................................................................................................34

DATA PROCESSING .....................................................................................................................................34

Adjustments ........................................................................................................................................34

Documentation ...................................................................................................................................34
v

APPLICATIONS TO WATER-RESOURCES INVESTIGATIONS ..............................................................36

Ground-Water Monitoring ................................................................................................................36

Long-Term Monitoring .............................................................................................................36

Short-Term Monitoring ............................................................................................................39

Reducing Well-Bore Storage ...........................................................................................................39

Surface-Water Monitoring ...............................................................................................................39

Installation ..................................................................................................................................40

Limitations ..................................................................................................................................41

Ancillary Data ............................................................................................................................42

Unsaturated-Zone Monitoring .........................................................................................................42

Tensiometers .............................................................................................................................42

Soil-Gas Piezometers ...............................................................................................................43

COMMON PROBLEMS AND SOLUTIONS ...............................................................................................43

Leakage ................................................................................................................................................43

Open and Short Circuits ...........................................................................................................43

Grounding Problems ..........................................................................................................................44

Diaphragm Failure ..............................................................................................................................44

Power-Supply Failure ........................................................................................................................44

Data logger Channel Failure .............................................................................................................44

Voltage Surges ...................................................................................................................................44

Faulty Shielding ..................................................................................................................................44

Over–Range Problem ........................................................................................................................44

SELECTED REFERENCES ............................................................................................................................45

TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

OF THE U.S. GEOLOGICAL SURVEY ............................................................................................48
vi

Boxes

Earlier Systems ................................................................................................................................................... 1

Resistors in Parallell .......................................................................................................................................... 5

Resistors in Series .............................................................................................................................................. 5

Example Mixed Circuit Analysis ...................................................................................................................... 6

Reduction of the Wheatstone Bridge to a Thevenin Equivalent Circuit ................................................... 7

Waterproof Housing ......................................................................................................................................... 19

Inexpensive Silicon Strain-Gage Pressure Transducers .......................................................................... 20

Tables

 1. Pressure-unit conversion factors ............................................................................................................. 4

 2. Conversion of measurement precision in percent to decades 

(Keithly Instruments, Inc., 1992) ..................................................................................................... 8

Figures

1–9. Diagrams showing:

 1. Relation between force and pressure for an unconfined unit liquid at rest. ........................... 3

 2. Relation between force and pressure for a confined fluid at rest. ............................................ 3

 3. Differences between absolute pressure and gage pressure. .................................................... 4

 4. Resistors in a series. ........................................................................................................................... 5

 5. Resistors in parallel. ............................................................................................................................ 5

 6. Mixed-circuit analysis. ....................................................................................................................... 6

 7. Wheatstone bridge. ............................................................................................................................ 7

 8. Wheatstone bridge with input shorted. ........................................................................................... 7

 9. Thevenin equivalent circuit. .............................................................................................................. 7

10. Graph used for computing minimum parallel and maximum 

series resistance (Keithly Instruments, Inc., 1992). .................................................................... 8

11. Graph showing envelope of minimum parallel and maximum 

series resistance (Keithly Instruments, Inc., 1992). .................................................................... 8

12. Examples of different types of force-summing devices. ...................................................................... 9

13–15. Cutaway view of:

13. A piezoelectric pressure transducer using a diaphram 

as a force-summing device. ........................................................................................................... 9

 14. A captive pressure transducer using a bellows as a 

force-summing device (modified from CEC Instruments, no date). ....................................... 10

15. An inductive (active) pressure transducer using a 

diaphram as a force-summing device. ....................................................................................... 10

16. A reluctive (passive) pressure transducer using a 

Bourdon tube as a force-summing device. ................................................................................ 10
vii

17. A potentiometric pressure transducer and 

resistance-measuring circuit. ...................................................................................................... 11

18–21. Cutaway views of:

18. A vibrating wire pressure transducer 

(modified from CEC Insruments, no date). .................................................................................. 11

19. A vibrating cylinder pressure transducer 

(modified from CEC Insruments, no date). .................................................................................. 11

20. An unbonded strain gage. ................................................................................................................ 12

21. A bonded strain gage (modified from CEC Insruments, no date). ............................................ 12

22. Electrical schematic of a compensated Wheatstone bridge. ........................................................... 13

23–31. Graphs showing:

23. Illustration of zero drift and sensitivity. ......................................................................................... 13

24. Illustration of zero error. ................................................................................................................... 14

25. Illustration of span error. .................................................................................................................. 14

26. Illustration of nonlinear error. ......................................................................................................... 14

27. Illustration of error band. ................................................................................................................. 15

28. Illustration of hysteresis. .................................................................................................................. 15

 29. Illustration of resolution. .................................................................................................................. 15

 30. Illustration of sensitivity. .................................................................................................................. 15

31. Illustration of time constant. ............................................................................................................ 15

32–38. Diagrams showing:

32. Housing for low-cost pressure transducer with a thermistor. .................................................. 19

33. Housing for low-cost pressure transducer. .................................................................................. 19

 34. Input-current loading error. ............................................................................................................. 22

35. Shunt resistance loading error. ...................................................................................................... 22

 36. Voltage-burden error. ....................................................................................................................... 22

37. Two-wire configuration. ................................................................................................................... 23

38. Four-wire configuration. ................................................................................................................... 23

39–40. Photographs showing:

39. Equipment used for temperture-corrected transducer calibration. ........................................ 24

40. Shelter design for a pressure-transducer installation. .............................................................. 26

41. Submersible transducer in an observation well. ................................................................................. 27

42. Example calibration worksheet for submersible transducers. ......................................................... 30

43. Ground-water inspection sheet. ............................................................................................................. 33

44. Ground-water record station analysis. ................................................................................................. 35

45–51. Diagrams showing:

45. Drain-cleaner packer. ....................................................................................................................... 37

46. Drop-pipe protection of a submerged transducer. ..................................................................... 38

47. Surface-water monitoring installation. ......................................................................................... 38

48. Soil-water tensiometer. .................................................................................................................... 40

49. Field tensiometer installation. ......................................................................................................... 40

50. Pressurized housing for pressure transducer. ............................................................................. 41

51. Housing for cable splice. ................................................................................................................. 41
viii

GLOSSARY

The terms in this glossary were compiled from numerous sources. Some definitions have 

been modified in accordance with usage of submersible pressure transducers for groundwater-level monitoring and may not be the only valid definitions for these terms.

Absolute Pressure Transducer: A pressure transducer that has an internal reference chamber sealed at or close to 0 psi absolute (full vacumn).

Accuracy (absolute): A measure of the closeness of agreement of an instrument reading 

compared to that of a primary standard having absolute traceability to a standard sanctioned by a recognized standards organization. This definition of accuracy is rarely used 

in manufacturer’s specifications.

Accuracy (relative): The ratio of the error of an instrument reading to the full-scale output 

of the instrument or the ratio of the error to a specified output expressed as a percent. 

Normally this statement of accuracy will include the effects of non-linearity, hysteresis, 

and repeatability. This definition of accuracy is most commonly used by manufacturers.

Analog output: Transducer output that is a continuous function of the measurand (value 

of the physical property) as distinguished from digital (discrete) output.

Best-fit straight line: A line chosen to represent the sensitivity of a transducer; from it 

non-linearity errors may be calculated. The best-fit straight line may be determined from 

a least-squares linear regression fit of the measurand input and transducer output. 

Bridge circuit: An instrument for measuring electrical values by comparing the unknown 

value with known values. When all values are equal the bridge is said to be ‘balanced’ 

or ‘nulled’.

Burst pressure: The specified maximum pressure that may be applied to the sensing 

element of the transducer without rupture of either the sensing element or the transducer 

case.

Compensation: Provision of a supplemental device, circuit, or special material to counteract known sources of error such as temperature, shock, and vibration.

Creep: A change in output occurring over a specific time period while the measurand 

and all environmental conditions are held constant.

Data logger: An electronic digital data recorder used by engineers, scientists, and technicians.

Dead volume: The total volume of the pressure port cavity of a transducer under room 

temperature and barometric pressure.

Diaphragm: The force-sensing membrane that is deformed when pressure is applied.

Digital output: Transducer output that represents the magnitude of the measurand in the 

form of a series of discrete quantities, coded in a system of notation, as distinguished 

from analog output.
ix

Drift: A change in output over a period of time that is not a function of the measurand 

(measured property). Drift is normally specified as a change in zero (zero drift) over time 

and a change in sensitivity (sensitivity drift) over time. 

Error: The algebraic difference between the indicated value and the true value of the measurand. 

Error band: The band of maximum deviation of output values from a specified calibration 

curve or reference line.

Excitation: The external electrical voltage or current applied to a transducer for its operation, usually expressed as a range of voltage or current values.

Force-sensing device: The diaphragm and associated mechanical components that move and 

cause a change in position that can be sensed by the electrical components in a transducer.

Full-scale output: The algebraic difference between the outputs at the specified upper and 

lower limits of the measurand inputs.

Gage transducer: A transducer that measures pressure relative to the ambient atmospheric 

pressure.

Hysteresis: The maximum difference in output, at any measurand value within the specified range, when the value is approached first with an increasing and then with a decreasing measurand. Hysteresis is expressed in percent of full-scale output.

Inductive transducer: A transducer in which the mechanical movement is converted to an 

electrical signal through electrical induction.

Linearity: The closeness of a calibration curve to a specified straight line. Linearity is 

expressed as a percent of the full-scale output using the maximum deviation of any calibration point from a specified straight line. 

Mean time between failures (MTBF): The total time that a number of sensors operate 

divided by the number of sensors that fail during the operational period. A value of MTBF 

usually characterizes a single type of component or a production batch from one manufacturer.

Measurand: The value of the physical property being measured.

Precision: The ability to reproduce output readings when the same measurand is applied, 

consecutively, under the same conditions, and in the same direction. Precision is a measure 

of the repeatability of a measurement. Precision does not imply accuracy. If a measurement system has good repeatability, then consecutive readings will be densely grouped 

together when the readings are plotted.

Pressure-sensing system: As used in this report, a pressure-sensing system is composed of a 

data recorder, pressure transducer, electrical connections, power supply, atmospheric pressure sensor or vent tube, and the transducer suspension apparatus.
x

Pressure transducer: A type of measurement device that converts pressure-induced 

mechanical changes into an electrical signal.

Proof pressure: The maximum pressure that may be applied to the sensing element of a 

transducer without changing its performance beyond the specified tolerances.

Range: The upper and lower limit of the measurand within which a transducer is 

intended to measure.

Reference pressure: The pressure to which a differential pressure transducer measurement is compared. 

Reliability: The probability that an item will perform its intended function for a specified 

interval under stated conditions.

Resolution: The smallest change in the measurand that can be measured or detected in 

the output reading.

Response time: The length of time required for the output of a transducer to rise to a 

specified percentage of its final value as a result of a step change in the measurand.

Shunt resistor: A precision resistor with a known value that is used to calibrate a pressure 

transducer.

Sensitivity: The ratio of the change in transducer output to a change in the measurand. 

Sensitivity may vary over the operational range of the transducer. 

Specific Gravity: The ratio of the mass of a body to the mass of an equal volume of water 

at 4 °C or other specified temperature.

Stability: The ability of a transducer to retain its performance characteristics for a period 

of time. Stability is normally expressed in percent of full-scale output over a set period 

of time.

System effectiveness (reliability): The probability that a system can successfully meet an 

overall operational demand within a given time when operated under specified conditions.

Test envelope: A line or lines bounding the measured values on a graph of true values 

versus measured values of a single property (for example, pressure).

Time constant: The length of time required for the output of a transducer to rise to 63 

percent of its final value as a result of a step change in the measurand.
xi

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

Multiply By To obtain

 inch (in.) 25.4 millimeter

 square inch (in.2) 6.452 square centimeter

 foot (ft) 0.3048 meter

 pounds (lbs) 0.4536 kilogram 

Temperature: Water and air temperature are given in degrees Celsius (°C), which can 

be converted to degrees Fahrenheit (°F) by use of the following equation:

°F = 1.8 (°C) + 32

Absolute temperature in degrees Kelvin (°K) can be converted to degrees Celsius (°C) by 

use of the following equation:

(°C) = (°K) – 273.15

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 

1929 (NGVD of 1929)—a geodetic datum derived from a general adjustment of the firstorder level nets of both the United States and Canada, formerly called Sea Level Datum 

of 1929.

Abbreviations

AC alternating current

DBLS depth below land surface

DC direct current

EDL electronic data logger

EMF electromotive force

FPS foot-pound-second measurement system

Hg mercury

H2O water

lbs/in.2 pounds per square inch

MKS meter-kilogram-second measurement system

MTBF mean time between failures

mV millivolt

μV microvolt

N Newtons

Pa Pascals

psi pounds per square inch

PVC polyvinyl chloride

USGS U.S. Geological Survey

V volt
ABSTRACT

Submersible pressure transducers, developed in the 

early 1960s, have made the collection of water-level and 

pressure data much more convenient than former methods. 

Submersible pressure transducers, when combined with electronic data recorders have made it possible to collect continuous or nearly continuous water-level or pressure data from 

wells, piezometers, soil-moisture tensiometers, and surface 

water gages. These more frequent measurements have led 

to an improved understanding of the hydraulic processes in 

streams, soils, and aquifers. 

This manual describes the operational theory behind 

submersible pressure transducers and provides information 

about their use in hydrologic investigations conducted by the 

U.S. Geological Survey.

INTRODUCTION

With the improvement of instruments used to collect 

and record ground-water level data, provision of consistent 

guidelines for the use of this equipment is important to those 

who collect the data. Submersible pressure transducers (pressure-sensing devices) and electronic data loggers (digital data 

recorders) have become an integral part of U.S. Geological 

Survey (USGS) hydrologic investigations that collect timeseries data on water-level fluctuations in wells. Because of 

the perceived ease of installation and operation of submersible pressure transducers and data loggers, data provided by 

these systems commonly are not adequately supported by 

quality-assurance procedures and documentation. Submersible 

pressure transducers occasionally are misused, either because of 

a lack of understanding of the capability of a particular transducer, or because of a lack of attention to its calibration and 

maintenance. This manual describes the operational theory fundamental to all transducers and provides detailed information 

about the use of submersible pressure transducers in hydrologic 

investigations conducted by the USGS. 

Background

Electrical pressure transducers designed to be immersed 

in water (submersible pressure transducers) have been used 

by ground-water scientists since the early 1960s (Shuter and 

Johnson, 1961; Garber and Koopman, 1968). These pressuresensing devices, typically installed at a fixed depth in a well, 

sense the change in pressure against a membrane. Pressure 

changes occur in response to changes in the height, and thus in 

the weight of the water column in the well above the transducer. 

Substantial improvements in design, operation, and accuracy of 

electronic submersible pressure transducers and data recording 

systems have led to a significant increase in their use, replacing 

older systems (described in the box) in some applications. 

Submersible pressure transducers also are used to monitor 

surface water elevations (Meyers, 1994) and variations in soil 

moisture tension in the unsaturated (vadose) zone (Healy and 

others, 1986). Several types of water-level sensors can provide time-series data to an electronic data logger, but for many 

applications only submersible pressure transducers are suitable 

(Freeman, 1996). Because many types of submersible pressure 

transducers are available, it can be difficult to determine which 

USE OF SUBMERSIBLE PRESSURE TRANSDUCERS 

IN WATER-RESOURCES INVESTIGATIONS

By Lawrence A. Freeman, Michael C. Carpenter, Donald O. Rosenberry, Joseph P. Rousseau, 

Randy Unger, and John S. McLean

Earlier Systems

Early systems that recorded continuous water-level 

changes in wells consisted of a float connected to a steel 

tape or beaded cable and a counterweight connected to 

a wheel. The rotating wheel turned a drum, upon which 

a pen, connected to a clock, recorded water levels on a 

paper chart.

Another system, in use since the 1960s, uses an electrical device to sense the water surface in a well, either with 

a series of electrodes or with a membrane deflected by 

the water surface. These probes control an electrically 

activated drum located at the top of the well. The drum 

rotates either clockwise or counterclockwise until the sensor 

is situated at the water surface. The data are recorded on a 

continuous paper chart or by a digital recorder.

A third type of sensor that has been in use since the early 

1980’s is an acoustic velocity device such as the Polaroid acoustic distance-measuring device. It measures the 

changes in depth to the water surface by measuring the 

variations in elapsed time between sending and receiving 

sound waves.
2 

type is most appropriate for a particular hydrologic investigation or application.

Purpose and Scope

This manual is intended to be a guide to the proper 

selection, installation, and operation of submersible pressure 

transducers and data loggers for the collection of hydrologic 

data, primarily for the collection of water-level data from 

wells. The manual describes the basic principles, measurement needs, and considerations for operating submersible 

pressure transducers, including a discussion of the systematic 

errors inherent in their use. Standard operational procedures 

for data collection and data processing, as well as applications 

of transducers for specific types of hydrologic investigations 

are included. Basic concepts regarding the physics of pressure 

and the mechanics of measuring pressure are presented, along 

with information on the electronics used to make and record 

these measurements. Guidelines for transducer calibration, 

proper use, and quality assurance of data also are presented. 

Field applications of pressure transducer systems are discussed 

for ground water, surface water, and the unsaturated zone; as 

are common problems that may corrupt data, and suggestions 

for field repairs.

BASIC PRINCIPLES OF PRESSURE AND 

PRESSURE MEASUREMENT

The physics of pressure and types of pressure measurements described in this section provide the background 

understanding for the application of pressure transducers to 

hydrologic investigations. An overview of the basic principles 

of direct current (DC) circuits is provided, since an understanding of the operational theory behind sensing and recording systems requires a working knowledge of DC circuitry. 

The application of these principles and operations to measure 

pressure with pressure transducers is addressed in later sections of this manual.

Physics of Pressure

Pressure (P) is defined as the force (F) exerted per unit of 

surface area (A) over which the force is applied. In classical 

mechanics, when two solid objects are in contact with each 

other, the pressure exerted by one object on another object is 

defined as:

P = dF/dA, (1)

where:

dF = force or weight (mass x acceleration)

 acting on the contact surface and

dA = contact area over which the force is applied.

In fluid mechanics, pressure is similarly defined. For 

purposes of this discussion, fluid refers to both liquids and 

gases. Unlike solids, however, fluids possess negligible shear 

strength and will completely or partially occupy the volume of 

the container in which they are placed. A liquid, if it does not 

completely fill the container, will present a free liquid surface. 

A gas, on the other hand, will always fill the volume of its 

container.

When a gas is confined in a container, molecules of the 

gas strike the walls of the container. The collision of the 

molecules against the container walls results in a force exerted 

against the surface area of the container. Pressure in the container is this force divided by the surface area of the container. 

Pressure within the container is the same everywhere. For 

a given mass of gas, pressure inside a container will vary in 

response to changes in the volume of the container and in 

the temperature of the gas in accordance with the relations 

expressed in the combined (Charles’s and Boyle’s) gas law:

PV = constant x T (2)

(for a given gas, the volume is inversely 

proportional to the pressure and directly 

proportional to the absolute temperature),

and in the ideal gas law:

PV = nRT, (3)

where:

P = pressure,

V = volume,

n = moles of gas (mass),

R = universal or molar gas constant, and

T = temperature 

The pressure at a point in a gas that is not confined to the 

volume of a container, and is at rest, such as atmospheric air, 

is dependent upon the density and height of the column of gas 

above the point of measurement. Because gas is compressible, its density will vary in response to position or elevation. 

Temperature differences, in an otherwise static column of 

air, also will affect gas densities. For example, atmospheric 

pressure decreases with increasing altitude; the relationship is 

not linear, however, because air density and temperature also 

decrease with increasing altitude. Gas pressure exerted at a 

point is identical in all directions around the point of measurement.

For a liquid at rest that is not confined, the pressure 

exerted by the liquid at any point depends upon the density 

and the height of the column of liquid above the point of 

measurement. Because water is only slightly compressible, 

its density can be assumed to be constant, provided temperature and salinity do not vary significantly above the point of 

measurement. Pressure at any point in a liquid acts perpendicularly against the surfaces it contacts. Within the liquid, 

the pressure at a point is identical in all directions from the 
 3

point of measurement. This principle is illustrated in the use 

of open-ended manometers to measure pressure differentials 

as shown in figure 1.

In figure 1, pressure P1 is greater than P2 by an amount 

equal to the height (h) of fluid in the manometer times the 

specific gravity of the fluid. In the absence of capillary forces, 

the diameters of the reservoir (D) and manometer (d) have no 

influence on the differential height of fluid between the two 

liquid surfaces. 

Whenever an external pressure is applied to a confined

fluid at rest, either liquid or gas, the pressure increases at 

every point in the fluid by the amount of the external pressure 

applied. This principle, known as Pascal’s law, is the basis for 

the design of the hydraulic lift, closed-end manometers, and 

many other types of pressure-sensing or transduction devices. 

This principle is illustrated in figure 2. 

In figure 2, a downward force (F) of 100 pounds (lbs) 

acting across an area of 1 square inch (in2) produces a pressure 

(P) of 100 pounds per square inch (lbs/in2 or psi). Because 

the fluid is confined, pressure is the same throughout the fluid, 

regardless of position. The downward applied force of 100 lbs 

produces an equivalent force of 5,000 lbs when the pressure is 

transmitted across an area of 50 in2. Unlike the example of the 

unconfined fluid system shown in figure 1, fluid displacement 

is dependent on the size of the reservoirs and tubes. Mechanical advantage is gained by difference in the area of the pistons. 

Pressure may be expressed in any one of the three major 

systems of units. The standard unit of pressure in the English, 

or foot-pounds-second (FPS), system is pounds per square 

inch, abbreviated as psi. In the meter-kilogram-second (MKS) 

system of measurement, pressure is expressed as kilograms 

per square meter. In standard international (SI) units, pressure 

is expressed in Newtons (N) per square meter, or Pascals (Pa). 

In ground-water hydrology, the length unit of measurement commonly is used to represent pressure because it is easily referenced to the total energy at a point in the ground-water 

flow system. The total energy at a point in a body of water 

equals the sum of its potential energy and kinetic components. 

For any incompressible fluid, either at rest or in motion, this 

relation is expressed as:

h = P/γ + z + v2

 / 2g (4)

 potential kinetic

 energy energy

where:

h = total energy potential, or head,

P = pressure,

γ = density of fluid,

z = position potential or elevation with respect to a 

 specified datum,

v = velocity, and

g = acceleration of gravity

Total energy potential or head is simply the summation of position (z) above or below some reference or eleva-

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tion datum, height of the column of water above the point of 

measurement, and a kinetic energy term expressed in units of 

length. In most ground-water hydrology applications, velocity 

(v) is very nearly zero and the kinetic energy term is therefore 

assumed to be zero. Because pressure in a static water body 

is determined by the height of the column of water above the 

point of measurement, it is convenient to express pressure in 

units of length. At the water surface, pressure head is zero 

and total head (or potential) is simply the height of the water 

surface above or below the reference datum. In ground-water 

hydrology, potential commonly is referred to as “head.” In the 

FPS system, it is expressed in terms of feet or inches of water; 

in the MKS system, as meters or centimeters of water.

Pressure-unit conversion factors commonly used in 

hydrology are listed in table 1. Take care when applying these 

conversion factors. Pressure expressed in terms of mercury 

(Hg) or water (H2O), for example, depends on the specific 

gravity or density of the fluid, which is temperature dependent. 

In the case of water, the effects of total dissolved solids and 

suspended solids (turbidity) on density also must be considered when quoting pressure head in terms of length. 

Table 1. Pressure-unit conversion factors. 

Multiply FPS

lb/in2

MKS

kg/m2 (1)

SI

N/m2

(pascal)

Temperature

Specification

lb/in.2 (psi) 1 1.422 x 101 6.895 x 10 3

ft of H2O 4.336 x 10-1 3.048 x 10 2 2.989 x 10 3 4oC

in of Hg 4.912 x 10 -1 3.453 x 10 2 3.377 x 10 3 0oC

mm of Hg 1.934 x 10-2 1.359 x 10 -3 1.333 x 10 2 0oC

atmosphere 

(atm) 1.470 x 101 1.033 x 10 4 1.013 x 10 5

N/m2

(Pascal) 1.450 x 10 -4 1.020 x 101 1 

bar 1.450 x 101 1.020 x 10 4 1.000 x 10 5

kg/m2(1) 1.422 x 101 1 9.807

(1) One kilogram mass under standard gravitational acceleration.

Types of Pressure Measurements

Absolute pressure is measured in reference to a vacuum 

or zero pressure. Pressure at sea level is 1 atmosphere, 101.3 

kPa, or 14.70 psi. Pressures measured by an absolute pressure 

transducer are always positive because these devices are referenced to a perfect vacuum in which absolute pressure is zero. 

Zero referencing is accomplished by completely evacuating 

and sealing the interior or reference side of the transducer.

Gage pressure is measured in reference to atmospheric 

pressure at mean sea level. Pressures measured by a gagepressure transducer are positive for pressures greater than 

sea-level pressure and negative for pressure less than sea-level 

pressure. Thus, atmospheric pressure measurements above 

sea-level datum are negative because atmospheric pressure 

decreases with altitude. Sea-level referencing is accomplished 

by sealing the interior or reference side of the transducer to 

atmospheric pressure at sea level. The term “gage pressure” 

is sometimes used to describe pressure measurements referenced to ambient atmospheric pressure other than to sea level. 

The relation between absolute pressure and gage pressure is 

illustrated in figure 3.

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Figure 3. Differences between absolute pressure and 

gage pressure.

Differential pressure is measured with respect to a varying pressure reference such as ambient atmospheric pressure 

or some other pressure source that is allowed to vary independently of the primary measurement. Pressure transducers constructed in this manner actually sense the difference between 

two independent pressure sources simultaneously. The output 

of the differential pressure transducer is proportional to the 

pressure difference between the two independent sources. 

Pressure measurements made with open-ended manometers or 

vented submersible pressure transducers are examples of differential measurements.

Sealed-reference pressure is measured with respect to 

a sealed reference pressure other than atmospheric pressure at 

sea level. A sealed-reference pressure is created by evacuating (pressuring down) or pressuring up the interior or reference side of the transducer to a prescribed absolute pressure. 

Because the sealed-reference side contains a constant volume 

of gas, the transducer must be maintained at a constant temperature to avoid changes in the reference pressure.

Overview of Direct Current Circuits

The most common type of pressure transducer is the 

strain-gage pressure transducer. Understanding the operational theory behind these transducers requires a working 
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knowledge of basic DC circuitry. This knowledge applies as 

well to the operation of the electronic components used to 

excite and read the device. Many problems encountered in the 

field can be analyzed and diagnosed by applying some simple 

techniques that require a working knowledge of DC circuitry. 

In the following illustrations, V is voltage, I is current, and R 

is resistance.

Overview of Direct Current Circuit Analysis

The overview of DC circuit analysis illustrates how a DC 

circuit can be reduced to an equivalent single voltage source 

and a single resistor. For example, in the top part of figure 4, 

a circuit consists of a battery and two resistors, R1 and R2, in 

series. The battery supplies a voltage (V) causing a current (I) 

to flow in the circuit. Each resistor produces a voltage drop 

(V1 and V2). Current flow is continuous through the circuit, 

Figure 4. Resistors in a series.

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Figure 5. Resistors in parallel.

Resistors in Parallel:

V = IR

(Ohm’s law) 

1/R = 1/R1 + 1/R2...+ 1/Rn

(the reciprocal of resistance is the sum of 

the reciprocals of each resistance)

R = R1

R2

 R1+ R2

(special case for two resistors in parallel)

I = I

1

 + I2 ... + In

(current is proportional to resistance)

V = V1 = V2= Vn

(voltage is constant across each resistor)

Resistors in Series:

V = IR

(Ohm’s law) 

R = R1+ R2...+ Rn

(resistance is additive)

I = I1 = I2 = In

(current is constant across each resistor)

V = V1 + V2 ... + Vn

(voltage drops are additive)

V = I (R1+ R2...+ Rn)

(voltage divides in proportion to resistance)

so I = I1 = I2. The total resistance of the circuit is the sum of 

the individual resistances (R = R1 + R2); therefore the circuit 

in the lower part of figure 4 is functionally the same as the 

circuit in the top part. 

For resistors in parallel, the voltage drop across each 

resistor is the same (V = V1 = V2). From Ohm’s Law, 

V1 = I1R1 and V2 = I2R2, so I1R1 = I2R2 and the total current 

flow in the circuit is the sum of the current in each branch 

(I = I1 + I2). Because I1 = V/R1 and I2 = V/R2, I=V/R1+V/R2

and I=V/R as shown in the lower part of figure 5.
6 

For multiple parallel and series components the procedure 

is virtually the same. First, convert independent or isolated 

parallel components to an equivalent resistor, then convert any 

resulting series components to an equivalent resistor. Repeat 

this process until all resistors are reduced to one equivalent 

resistor as shown in the box and in figure 6.

Complex resistance circuits cannot be broken down into a 

simple equivalent resistor. More complex DC circuits require 

the application of Kirchhoff’s laws to analyze the separate 

components of the circuit. Kirchhoff’s laws state:

1. the algebraic sum of all the currents flowing toward a 

junction is zero, and 

2. the algebraic sum of all the potential-source voltages is 

equal to the algebraic sum of all the voltage drops in any 

loop or closed conducting path in the circuit. 

Figure 6. Mixed-circuit analysis.

An example of a circuit requiring the application of 

Kirchhoff’s laws is the Wheatstone bridge, variations of 

which are used in many strain-gage pressure transducers (for 

example, in the common bonded-foil strain-gage transducers). The Wheatstone bridge circuit (fig. 7) consists of four 

resistors (R1 through R4) supplied with a fixed voltage (Vin). 

When all resistances are equal, the current through each 

resistor is equal (I1= I2= I3= I4) and no voltage difference is 

measured at the voltmeter (V). The bridge is then said to be 

‘balanced.’

Understanding DC circuits can be an invaluable aid in 

error analysis, selecting measurement instruments, optimizing pressure transducer performance, and troubleshooting 

system malfunctions. Circuit analysis can be applied to test 

for electrical short circuits, to locate damaged bridge elements, or to find insulation leakage in the circuit. 

Example

Mixed CircuiAnalysis 

Convert the mixed circuit in figure 6 to a single 

voltage source and resistor and find I, I1, I2, Ia, Ib,

V1,V2,Va, Vb, Rc, and Reqv

Given: 

 V = 10 volts

 R1 = 91 ohms R2 = 100 ohms

 Ra = 90 ohms Rb = 10 ohms 

Convert parallel-branch component to an 

equivalent resistor, Rc

 1/Rc = 1/Rb + 1/Ra

 Rc = (RbRa)/(Rb + Ra) = 900/100 = 9 ohms

Convert series resistors to an equivalent resistor 

 Reqv = R1 + R2 + Rc = 91 + 100 + 9 = 200 ohms

1. Compute I, I1, I2, Ic

 I = V/Reqv = 10 volts/200 ohms = 0.05 amps

 I = I1 = I2 = Ic = 0.05 amps

2. Compute V1, V2, Vc

 V1 = IR1 = (0.05 amps x 91 ohms) = 4.55 volts

 V2 = IR2 = (0.05 amps x 100 ohms) = 5.00 volts

 Vc = IRc = (0.05 amps x 9 ohms) = 0.45 volts

 V = V1 + V2 + Vc = 10.0 (volts check)

3. Compute Va, Vb, Ia, Ib

 I = I1 = I2 = Ia + Ib

 Vc = Va = Vb = 0.45 volts

 Ia = Va /Ra = 0.45/90 = 0.005 amps

 Ib = Vb /Rb = 0.45/10 = 0.045 amps

 I = Ia + Ib = 0.05 amps (check)

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 7

The Wheatstone Bridge, Thevenin 

Equivalent Circuit

A Thevenin equivalent circuit is a hypothetical circuit 

designed to simplify the analysis of complex, two-terminal linear 

networks, such as Wheatstone bridges. Thevenin’s theorem states 

that any potential source that has only two output terminals and 

is composed of resistors and a voltage or battery source can be 

represented by a series combination of a resistor (RS) and voltage 

source (VS) as shown in figure 9. VS is the open-circuit potential 

and Rs is the resistance between the output terminals when the 

Figure 9. Thevenin 

equivalent circuit.

Figure 8. Wheatstone bridge 

with input shorted.

Step 1: With the input voltage source shorted, 

reduce the circuit to an equivalent resistance (RS) 

as seen across the open circuit (or output voltage 

leads) shown in figure 8. 

Given: Vin = 10 volts 

 R1 = 2000 ohms 

 R2 = 2000 ohms 

 R3 = 1900 ohms 

 R4 = 2000 ohms 

With the input voltage shorted, the circuit reduces 

to two parallel branches that are in series, as seen 

across the output voltage leads (VS). Circuit simplification reduces each parallel component to an 

equivalent resistor followed by adding in series:

 RS = R1R2 + R3R4

 R1+ R2 R3+ R4

 RS = 1974.36 ohms 

Step 2: Determine the open-circuit voltage (VS) 

appearing across the output voltage leads using the 

input voltage (Vin). Determine voltage potential (VS) 

across the output leads, where:

 VS = I2R2 - I3R3 = I1R1 - I4R4

 Iin = I2 + I3 

 (Kirchhoff’s first law)

 I2R2 + I1R1 = I3R3 + I4R4 

 Vin = I3R3+ I4R4 

 (Kirchhoff’s second law)

 I1 = I2, I3 = I4

(current is constant across resistors in series)

Compute I3 and I4

 10V = 1500 I3 + 2000 I4

 10V = 3900 I4

 I4 = 2.5641 milliamps = I3

Compute I1 and I2

 10V = 2000 I1 + 2000 I2

 10V = 4000 I2

 I2 = 2.5 milliamps = I1

 VS = I2R2 - I3R3 = 128.2 millivolts

Compute short-circuit current

 IS = VS/RS = 0.065 milliamps

The Thevenin equivalent circuit for the example 

Wheatstone bridge is shown in figure 9.

Figure 7. Wheatstone bridge.

Reduction of the Wheatstone Bridge 

to a Thevenin Equivalent Circuit

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8 

battery or voltage source is shorted. The equivalent circuit is 

constructed by applying Ohm’s law to parallel and series components in the network, reducing the circuit to a voltage source 

(VS) connected in series with a Thevenin equivalent resistance 

(RS). The procedure for constructing a Thevenin equivalent 

circuit for the common Wheatstone bridge is described on the 

preceding page.

Constructing the Thevenin equivalent circuit is the first 

step in evaluating the adequacy of the measurement system 

to provide the necessary resolution and sensitivity to resolve 

the measurement of interest. Resolution and sensitivity are 

discussed in greater detail later in this manual. Once the 

expected signal levels (open circuit voltage and short circuit 

current) are defined, probable error sources that may affect 

the measurement can be identified. The application and 

value of this error analysis is illustrated by constructing what 

is referred to as a test envelope for the type of measurement 

(voltage, current, or resistance) being made. For purposes of 

illustration, the Wheatstone bridge in the example above will 

be used to construct the test envelope.

Construction of the test envelope is accomplished with 

reference to figure 10 and table 2. Determine the required 

measurement precision, which is 0.1% (1,000 parts per million) in this example; and determine the associated number 

of decades (table 2). The number of decades in this example 

is three. A decade is equivalent to one order of magnitude 

difference in the open-circuit voltage or short-circuit current. 

In figure 10, plot the computed open-circuit voltage (VS = 128 

millivolts) and short-circuit current (IS = 0.065 milliamps). 

Extend a line the length of the number of decades – scaled to 

either the horizontal or vertical axis – vertically downward and 

extend a line of the same length horizontally to the left. Construct an arc the length of the total number of decades connecting the horizontal and vertical axes. The resulting envelope, 

shown in figure 11, defines the minimum values of resistance 

that can be placed in parallel with the Thevenin equivalent 

resistance (about 5 ΜΩ) and the maximum resistance that can 

be placed in series (about 5 Ω) without affecting the desired 

measurement precision. All resistance within the envelope, 

whether in parallel or in series, would seriously degrade the 

measurement. A second envelope is shown in figure 11 to 

illustrate the effect of increasing the measurement precision 

requirement from 0.1% to 0.01% (100 parts per million). In 

the second case, the minimum resistance in parallel is 50 ΜΩ, 

and the maximum resistance in series is 500 mΩ (0.5 Ω). 

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Figure 10. Graph used for computing minimum parallel and 

maximum series resistance (Keithly Instruments, Inc., 1992).

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Table 2. Conversion of measurement precision 

in percent to decades (Keithly Instruments, Inc., 

1992).

Figure 11. Graph showing envelope of minimum parallel and 

maximum series resistance (Keithly Instruments, Inc., 1992).

Percent Parts per 

million Decades

10 100,000 1

1 10,000 2

0.1 1,000 3

0.01 100 4

0.001 10 5
 9

Potential sources of errors due to parallel and series 

resistance in the measurement circuit are described 

in the section “Sources of Error When Making 

Direct Current Measurements.”

PRESSURE TRANSDUCER 

CHARACTERIZATON

Pressure transducers are characterized by their 

mechanical and electrical transduction elements, 

the performance specifications of the transducer, 

and the interaction of the transducer with the other 

components of the measuring system (such as the 

power supply and data logger).

Basic Types of Transducers for

Measuring Pressure

A transducer is a device that converts energy 

from one form to another. Electrical pressure transducers, which measure changes in pressure, consist 

of a mechanical-transduction element or forcesumming device coupled to an electrical-transduction element, which is connected to a display or 

recording device, or both. There are two types of 

electrical transduction elements — active and passive. Electrical-transduction elements that convert 

pressure-induced mechanical changes directly to 

an electrical signal are referred to as active transducers. Passive transducers require an external 

excitation that causes the transducers to respond to 

pressure-induced mechanical changes. The electrical-transduction element converts mechanical energy into electrical energy 

and the force-summing device or mechanical-transduction element converts gas or liquid energy into mechanical energy.

Many types of pressure transducers consist only of mechanical-transduction elements. Open-ended and closed-ended 

manometers, barometers that record changes in the height of a 

column of liquid in response to some external pressure change, 

and spring-loaded pressure-sensing devices are examples of 

mechanical transducers. 

Electrical pressure transducers are classified primarily 

on the electrical principle or method of electrical transduction 

involved in their operation. Different electrical transduction 

elements can be coupled to a variety of force-summing devices. 

Some combinations work better than others, depending on the 

application and measurement needs. Commonly used types of 

force-summing devices are illustrated in figure 12. A piezoelectric pressure transducer incorporating the diaphragm in the 

housing is illustrated in figure 13.

Electrical pressure transducers using force-summing devices 

are described below. The most common of the many types of 

pressure transducer is the strain-gage pressure transducer.

Figure 12. Examples of different types of force-summing devices.

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Figure 13. A piezoelectric pressure transducer using 

a diaphram as a force-summing device.

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10 

Piezoelectric Transducer

The piezoelectric transducer is an example of a selfgenerating or active pressure transducer. The design of this 

type of transducer is based on the ability of certain crystals 

(quartz, tourmaline, Rochelle salt, or ammonium dihydrogen 

phosphate) and ceramic materials (barium-titanate, or leadzirconate-titanite) to generate an electrical charge or voltage when mechanically stressed. The crystal geometry of 

these materials is oriented to provide maximum piezoelectric 

response in one direction and minimal response in other directions. The transducer develops a voltage proportional to the 

change in pressure. These transducers cannot be calibrated 

using normal static-pressure calibration techniques. This type 

of transducer is used to measure rapidly fluctuating pressures.

Capacitive Transducer

A capacitive transducer employs a diaphragm positioned 

between two fixed metal plates (fig. 14). In some designs, 

the metal plates are fixed to either side of the diaphragm; 

deflection of the diaphragm changes the capacitive coupling 

between the diaphragm and the metal plates. In other designs, 

the metal plates are isolated from the moving diaphragm; 

deflection of the diaphragm causes a change in the capacitive 

coupling between the two metal plates. An alternating current 

(AC) signal across the plates can be used to sense the change 

in capacitance.

Inductive and Reluctive Transducers

In an inductive transducer, pressure-induced displacements of a diaphragm cause a change in the self-inductance of 

a single coil. In a reluctive transducer, displacements occur 

in the magnetic coupling between a pair of coils. An inductive transducer is active and operates on the principle that 

the relative motion between a conductor and a magnetic field 

induces a voltage in the conductor (fig. 15). Because the pressure-induced electrical output signal requires relative motion, 

the inductive design is limited to dynamic measurements. In 

a reluctive transducer, displacements occur in the magnetic 

coupling between a pair of coils. A reluctive transducer is 

passive and requires external AC excitation of a pair of coils. 

It operates on the principle that the magnetic coupling between 

the two coils is affected by the displacement of a pressuredriven conductor located in the magnetic field between the two 

coils. The conductor is either connected to a force-summing 

device or is itself a force-summing device. Two basic designs 

have evolved (figs. 15 and 16). 

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Figure 15. An inductive (active) pressure transducer 

using a diaphram as a force-summing device.

Figure 14. A capacitive pressure transducer 

using a bellows as a force-summing device 

(modified from CEC Instruments, no date).

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Figure 16. A reluctive (passive) pressure transducer 

using a Bourdon tube as a force-summing device.
 11

Potentiometric Transducer

The potentiometric pressure transducer consists of a 

movable contact driven by an active force-summing device 

(fig. 17). The movable contact, or wiper, travels across a 

resistive element that may be a wire-wound coil, a carbon 

ribbon, or a deposited conductive film. The motion of the 

wiper across the resistive element causes a change in the 

resistance selected by the wiper. The change in resistance 

produces an electric signal (either a change in voltage or 

current) that is proportional to the mechanical displacement 

of the wiper. This type of transducer may be excited using 

either AC or DC. 

Vibrating-Wire and Vibrating-Cylinder 

Transducers

Vibrating-wire (fig. 18) and vibrating-cylinder 

(fig. 19) transducers use a vibrating element, either a fine 

wire or a cylinder that forms a portion of one leg of a 

Wheatstone bridge circuit. The vibrating element is located 

in a magnetic field with one end of the element attached to a 

diaphragm or other type of force-summing device. Current 

flowing through the vibrating element causes the element to 

move in the magnetic field, which in turn induces a current 

in the element. The resulting voltage, amplified and fed 

back to the vibrating element, sustains the oscillations at the 

element’s resonating frequency. The resonating frequency 

of the vibrating element is controlled by the tension exerted 

on the wire or cylinder by a diaphragm or other force-summing device. Vibrating-wire transducers can be installed 

in small-diameter (0.5-in.) wells, and because they produce 

AC signals, they can be used on long wires with little signal 

degradation.

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Figure 17. A potentiometric pressure transducer and resistance-measuring circuit.

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Figure 18. A vibrating wire pressure transducer (modified from CEC Instruments, no date).

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Figure 19. A vibrating cylinder pressure transducer 

(modified from CEC Instruments, no date).
12 

Strain-Gage Transducer

The strain-gage transducer, sometimes referred to as a 

resistive transducer, is by far the most widely used type of 

pressure transducer. Its electrical transduction elements operate on the principle that the electrical resistance of a wire is 

proportional to its strain-induced length.

The strain-gage transducer uses the gage-factor property 

of the strain element to convert a mechanical displacement 

into a change in the electrical resistance of a circuit. Gage factor, defined as the unit change in resistance (R) per unit change 

in length (L), is expressed as:

 GF = ∆ R/R

 ∆ L/L (5)

Product specification sheets rarely provide gage factors. 

Instead, they commonly express pressure-transducer sensitivity as the voltage signal output ratio per unit of pressure 

change:

 ∆ V/V per unit of pressure change.

There are basically two classes of strain-gage transducers, unbonded and bonded. The unbonded strain gage uses 

a strain-sensitive wire (or wires) with one end fixed and the 

other end attached to a movable element. Strain, induced on 

the wire by the displacement of the movable element, produces a change in resistance proportional to the displacement 

of the movable element. The basic design of this type of transducer is illustrated in figure 20.

Bonded strain-gage transducers (fig. 21) can be grouped 

into those that require an adhesive to fix the gage to the pressure-sensing element (metal foil and strain-sensitive wires) 

and those attached to the strain-sensing element by techniques 

that effectively make the strain gage an integral part of the 

strain-sensing element (thin film and semiconductor). Thin 

film and semiconductor strain gages typically are mounted 

directly on the pressure-sensing element. Metal foil and 

strain-sensitive wires commonly are mounted on a secondary sensing element, which acts as the deforming member to 

produce the strain sensed by the strain gage.

Metal foil—Strain gages consist of wire or foil ribbon 

coated with a thin layer of insulation and cemented to the 

strain-sensing element. Distortion of the strain-sensing element 

is communicated by the bonding material directly to the wire 

or foil filaments. Increasing the length of the gage reduces the 

cross-sectional area of the conductor and increases the conductor’s resistance, causing a change in voltage, proportional to the 

pressure change, across the output leads.

Thin film—Strain gages employ a metal substrate on 

which are deposited thin films as an insulation layer and a resistor layer, using either a vacuum-deposition or sputtering process. The strain gage is either masked onto or etched into the 

thin film resistor layer, making the gage an integral component 

of the strain-sensing element. The strain gage can be deposited 

directly onto sensing elements of any configuration, such as 

diaphragms, beams, or tubes.

Semiconductor—Strain gages are similar to the thin-film 

strain gages in that the strain-gage circuit is an integral part of 

the strain member. In integrated silicon strain-gage pressure 

transducers, the strain elements are diffused directly into the 

pressure-sensing element, becoming “atomically” bonded to 

the sensing member. Because silicon is virtually 100 percent 

elastic to the breaking point, this type of transducer exhibits 

very little hysteresis. Because gage factors in these types are in 

some cases more than 50 times greater than those of wire gages, 

signal output is high, which commonly eliminates the need for 

signal amplification.

The Wheatstone bridge, introduced earlier in the discussion 

of the Thevenin equivalent circuit, is one of the most common 

bridge configurations for strain-gage pressure transducers. In 

its simplest form the Basic Wheatstone bridge consists of four 

resistors arrayed to form a closed loop, a pair of sensing leads, 

and a pair of excitation leads. The bridge is affixed to a pressure-sensitive diaphragm or substrate. Pressure changes distort 

the substrate or diaphragm and cause the resistance of the bridge 

to change in response to strain induced on its resistors. In 

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Figure 20. An unbonded strain gage.

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Figure 21. A bonded strain gage (modified from CEC 

Instruments, no date).
 13

some designs, all bridge elements may be active, while in 

other designs only one element may be active.

Variations on the basic configuration of the Wheatstone 

bridge are referred to as Compensated Wheatstone bridges. 

These variations include additional resistor circuits, diodes, 

and circuit components designed to provide various types of 

compensation functions or signal enhancement capabilities, 

such as zeroing, shunt calibration, temperature compensation, 

and sensitivity adjustments (fig. 22).

The strain-gage bridge may be excited by either a constant voltage or a constant current, depending on the application and the excitation method used for calibration. There are 

advantages to each method.

Voltage—Most manufacturers provide calibrations 

and transducer specifications using voltage as the mode of 

excitation. The length of the lead wire (and hence its resistance) needs to be considered when selecting a transducer 

for a remote application. Short leads usually do not create 

significant measurement problems because the voltage loss 

on the excitation lead is small as a percent of the total excitation signal. The resistance of the lead increases as its length 

increases. The resistance of 20-gage annealed copper wire is 

approximately 0.01 ohm per foot. A transducer operated using 

a long wire lead should be calibrated with the lead attached. 

With a long lead, the voltage drop that develops across the 

sensing circuit is reduced in proportion to the resistance of the 

lead; the output signal will be reduced accordingly. 

Current—Some transducers can be calibrated and 

operated using a constant current to supply excitation to the 

measurement bridge. The advantage of a constant-current 

excitation is that the effects of lead-wire resistance can be 

eliminated and the necessity of calibrating each unit with the 

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Figure 22. Electrical schematic of a compensated Wheatstone bridge.

lead wires attached can be avoided. Delivering a constant current to the device is achieved by allowing the voltage across the 

output leads to seek the necessary potential (within prescribed 

limits) required to make the current equal to the calibrated current setting. A current-generating source with the capability to 

regulate the voltage drop across the input leads is required to 

provide a constant current. Because current is controlled at the 

input end of the circuit, the same current will be present across 

the sensing circuit, provided there are no current leaks or shorts 

in the measuring circuit.

Understanding Pressure-Transducer 

Specifications

Selection of a pressure transducer requires careful review of 

the literature from prospective vendors. Comparing instrument 

specifications is a difficult and time-consuming process. Vendors 

commonly specify different sets of parameters and, typically, it is 

not clear which definitions are being applied to properly interpret 

a stated specification. Confidence levels are rarely specified and 

reporting specifications vary from one manufacturer to another. 

When in doubt, the vendor or manufacturer should be consulted 

for clarification and additional information. When selecting a 

pressure transducer, carefully consider the specifications of the 

instruments that will be used to excite and measure the output 

of the pressure transducer. These components of the measuring 

system may be the limiting factor in meeting overall performance 

objectives. An analysis of the circuits, using the principles in “An 

Overview of Direct Current Analysis,” may be helpful in selecting these instruments. The input and output characteristics of the 

transducer must be compatible with the excitation and recording 

system used. 

Terms frequently used to describe the performance characteristics of pressure transducers appear in the glossary. The 

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Figure 23. Illustration of zero drift and sensitivity.
14 

definitions of many of these terms are based on terminology adopted by the Instrument Society of America (1970). 

Terms such as drift, error, error band, resolution, hysteresis, 

and time constant require additional explanation as illustrated in the following figures.

Drift, an undesired change in output over a period of 

time that is not a function of the measurand, is normally 

specified as a change in zero (zero drift) over time and a 

change in sensitivity (sensitivity drift) over time. Zero drift 

is illustrated in figure 23 as the vertical shift in the intersection of the best fit straight line between the initial calibration 

and the later calibration. The sensitivity drift is shown by 

the difference in slope of the best fit straight line between 

the initial calibration and the later calibration, where the 

initial calibration curve, adjusted for zero drift, is shown as 

the dashed line.

Error, the algebraic difference between the indicated 

value and the true value of the measurand, can be zero, span, 

or nonlinear error. Zero error is constant throughout the 

range of true values of the measurand as shown in figure 24. 

Span error is an error that changes linearly with the value of 

the measurand, as illustrated in figure 25. Nonlinear errors 

change as a nonlinear function of the value of the measurand, as illustrated in figure 26.

The error band is the band of maximum deviation of 

output values from a specified calibration curve or reference 

line. The sum of all the defined errors cause the measured 

values to differ from the true values. The measured values 

will fall within the error band, as shown in figure 27.

Hysteresis is the maximum difference in output, at 

any measurand value within the specified range, when the 

value is approached first with an increasing and then with 

a decreasing measurand. Expressed in percent of full-scale 

output, it is usually described in terms of maximum hysteresis in the output, as illustrated in figure 28. 

The resolution of a measurement system is the smallest 

change in the measurand that can be measured or detected in 

the output reading. The ability to detect significant differences in output can be due to the construction of the transducer, noise in the system, or the numerical resolution of a 

digital data logger. The definition of resolution is illustrated 

in figure 29.

The sensitivity of transducer or system is the ratio of 

the change in transducer output (∆qo) to a change in the measurand (∆qi

). Sensitivity may either be constant, in which 

case ∆qo/∆qi will vary linearly over the operational range of 

the transducer (fig. 30a) or can vary nonlinearly (fig. 30b), 

in which case the sensitivity is commonly defined as a linear 

approximation for short segments of the curve.

The time constant is the length of time required for the 

output of a transducer to rise to 63 percent of its final value 

as a result of a step change in the measurand, as shown in 

figure 31. The time constant of a transducer is analogous to 

the time constant of an electrical circuit, in which the time 

constant is the time required for a capacitor charging through 

a resistor to reach 63 percent of the applied voltage. 

PLANNING CONSIDERATIONS FOR 

SENSOR SYSTEMS

The type and number of sensors and data recorders 

needed for automated collection of water-level data depend 

on the objectives of the study. Determine these objectives 

prior to selecting system components. Options are numerous, 

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Figure 24. Illustration of zero error.

Figure 25. Illustration of span error.

Figure 26. Illustration of nonlinear error.
 15

but once the study objectives and needs are clearly determined, 

the selection of appropriate system components will be simplified. Some considerations for planning the installation of a 

water-level collection system are presented below. For many 

installations, submersible pressure transducers may not be 

needed, nor may they be the most suitable water-level sensors. 

In the following sections, however, submersible pressure transducers are assumed to be the preferred water-level sensors.

Study Duration and System Reliability

Nearly all submersible pressure transducers are capable 

of providing accurate results for short-term studies (such as 

aquifer tests or slug tests) but as the study duration increases, 

the chance of sensor failure and the amount of zero drift 

increases. Purchasing more expensive sensors, engineered to 

withstand the added demands of long-term deployment, may be 

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Figure 31. Illustration of time constant.

Figure 30b. Illustration of sensitivity.

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Figure 27. Illustration of error band.

Figure 28. Illustration of hysteresis.

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Figure 29. Illustration of resolution.

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Figure 30a. Illustration of sensitivity.
16 

necessary. Sensor maintenance and recalibration also becomes 

a consideration when designing a long-term data-collection 

effort. 

For long-term investigations, the data logger and powersupply systems need more attention and protection. It may 

be necessary to recondition and recalibrate the data logger 

occasionally or to house it in a dry environment to prevent 

failure of components due to long-term exposure to moisture. 

Sensor cables may need to be protected with tubing or pipe to 

prevent long-term damage from ultraviolet radiation, physical 

weathering, exposure to ozone, or vandalism.

System reliability is among the most important considerations when designing a water-level monitoring system to 

be operated over a long duration. Redundancy, designed into 

the system so a partial failure will not result in complete loss 

of data, can range from multiple sensors in the same borehole 

connected to one data logger, to two or three completely separate systems logging water-level fluctuations in the same well. 

If a high degree of reliability is important, the study should be 

budgeted to provide early warning of system problems and fast 

access to replacement components to minimize down time. 

Many manufacturers use terms such as mean time 

between failure and reliability to present durability information on their products. Mean time between failure is most 

commonly defined as the total time that a number of sensors 

operate, divided by the number of sensors that fail during 

the operational period. Reliability is the probability that an 

item will perform its intended function for a specified interval under stated conditions. Specified interval refers to the 

length of the study or test. Stated conditions refer to the 

operational environment—weather, humidity, temperature, and 

electromagnetic interference. Most of the time, the specified 

interval or the stated conditions supplied by the manufacturer 

are not the same as those of the hydrologic investigation. 

Also, reliability specifications usually refer to a single component of what commonly is a multiple-component system. For 

example, a pressure transducer may have one stated reliability 

and a data logger may have a different stated reliability, and 

the reliability of the combination of the two components (the 

system reliability) will be different from either of the reliabilities of the individual components. Most of the time, the 

overall system reliability will approximate the reliability of the 

least-reliable component.

Required Accuracy

Systems of pressure transducers, data loggers, cables and 

other supporting equipment used for sensing and recording 

water levels in wells should be sufficiently accurate to meet the 

needs of most ground-water projects of the USGS. The following have been suggested as standards: a water-level sensing 

and recording system should be capable of performing within 

a measurement error of + or – 0.01 ft. for most water-level 

measurement applications. For the case of large changes in 

water level (for example, during aquifer tests), this measurement error may not be achievable, and an accuracy of 0.1 percent 

of the expected range in water-level fluctuation is acceptable. 

Where the depth to water is greater than 100 ft, an accuracy of 

0.01 percent of the estimated depth to water is generally acceptable. In summary, the measurement error and accuracy standard 

for most situations are 0.01 ft, 0.1 percent of range in water-level 

fluctuation, or 0.01 percent of depth to water above or below a 

measuring point, whichever is least restrictive.

Many hydrologic investigations in the USGS require the 

accuracy of the preceding suggested standard. While most sensor manufacturers produce devices that achieve this accuracy, the 

added complexities of the wiring, data logger, power source, and 

environmental variability may unacceptably degrade the overall 

system accuracy. Investigators may want to test for themselves 

the overall accuracy by conducting a pilot project before investing in a system that may not meet data objectives. In some cases, 

the desired accuracy may not be achievable with current technology or within budgetary constraints. For example, it is difficult 

to achieve a high level of accuracy with long leads, when depths 

to water are large, or when water-level fluctuations are large. 

Stringent accuracy constraints require frequent check measurements in the field.

Installation Location and Site Accessibility

If the study site is near the office, then frequent visits to the 

site to download data, perform site maintenance, replace failed 

components, or make accuracy check measurements may be reasonable. If, however, the site is remote or difficult to access, then 

the system needs to be designed to be operated remotely, and 

contain greater redundancy to better ensure uninterrupted collection of data. Remote sites may need an enhanced power supply, 

more robust shelters, extra data-storage capacity, equipment to 

allow communication with the site and transmission of data from 

the site, two or more transducers in a well, and automated checks 

for sensor drift.

System Components and Compatibility

When designing a data-collection system, determine which 

components are necessary and ensure that all of the components 

can communicate properly. Because power can be supplied 

by some data loggers, a short-term study might require only a 

pressure transducer connected to a data logger. For a long-term 

study, however, additional components including a power supply 

(batteries, solar panels, voltage regulator), additional data storage 

devices, a shelter or shelters, and a data-transmission system 

may be needed. Ensuring compatibility between components 

becomes more difficult as the number of components increases. 

For example, some data loggers can not interpret a digital signal 

from a transducer that makes an analog-to-digital conversion at 

the sensor. Similarly, the type of analog signal needs to be compatible; if the sensor sends an amperage signal, the data logger 

needs to be able to receive an amperage signal. Some pressure 

transducers require a separate measurement of temperature in 
 17

to contain redundancies, and may need to have a data-transmission capability. 

If well diameters are small (less than 2 in.), the choice 

of pressure transducers is limited. For example, many wells 

installed in peat deposits are of small diameter to reduce the 

time lag for pressure equilibrium due to the typically low 

hydraulic conductivity of peat. Although some transducers 

are as small as 0.39 in. in diameter, most strain-gage pressure 

transducers will not fit down a well smaller than 1.25 in. in 

diameter. The investigator may need to choose a different type 

of transducer, such as a vibrating-wire pressure transducer, 

remembering that smaller transducers are usually more fragile.

Wells with a large depth to water present special problems for instrumentation. Unusually deep wells, such as those 

at the Nevada Test Site where water levels range from tens 

of feet to several hundreds of feet deep, pose many unsolved 

anomalous data problems—spikes, drift, inexplicable rises 

and recessions, lost data—as well as correlation problems 

between manual and continuous data (O’Brien, 1993). Pressure transducers hung from long cables can be affected by 

cable stretch. The cable also can expand and contract with 

temperature changes, thus raising and lowering the transducer 

in the well and introducing errors into the data. 

Sensors are susceptible to impact-shock damage from 

hitting the sides of the well or being rapidly submerged during 

installation or calibration. Deep wells present more opportunities for this kind of mechanical damage. 

Signals can degrade in long cables. The voltage attenuation and interference in long cables can cause erroneous 

data to be stored in the data logger. In addition to the effects 

on surface equipment, great differences in temperature and 

humidity between the surface and the water level, coupled 

with dissimilar metals, may lead to galvanic effects inducing voltages and transient currents that may distort the signal. 

The signal wires and their axially wound shields can act as 

inductive and capacitive circuits, which then may lead to ferromagnetic resonant effects, inducing transient currents into 

the signal-bearing channels. One solution is to convert the 

analog signal to a digital signal at the sensor and transmit the 

digital signal up the sensor cable to the data recorder. Another 

solution is to transmit the signal using AC current from 4-20 

ma current-mode transducers. Current-mode signals are less 

susceptible to degradation than the more common DC voltagemode transmissions. 

Vent tubes on long cables have an increased chance 

of becoming clogged simply due to their greater lengths. 

Because of the great depths in some wells, environmental 

conditions may cause a variety of problems with both waterlevel measurements and data recording. The varying temperature and humidity and the atmospheric pressure gradients 

between the water level and land surface may cause vent tubes 

to become congested and ultimately allow moisture to be 

transported down into the sensitive electronic and mechanical 

portions of a pressure transducer. Preventing moisture entry is 

discussed in “Desiccation Systems.”

order to correct the transducer output for changes in temperature in the well. If a data logger is not capable of receiving 

the temperature signal, the overall system accuracy is reduced. 

The data logger must also be able to supply the excitation 

voltage or current required by the sensor. When designing the 

installation, the number of sensors the data logger can simultaneously record needs to be considered. 

Water Quality

Water quality must be considered when planning an installation. If the well will be used for water-quality sampling, the 

transducer and cable should be easy to clean before installing. 

Do not use lead or plastic-coated lead weights to apply tension 

to the cable. If the well is at a contaminated site, consider the 

possible effects of contaminants in the water that may corrode 

or otherwise degrade transducer components. Select components that are corrosion resistant, and easily decontaminated. 

Some manufacturers make chemical-resistant transducers of 

stainless steel or titanium, and polyfluroethelene-coated cables.

Number of Wells

For several wells in close proximity—for example, a 

nest of piezometers or multiple wells for a pumping test—one 

data logger that can receive signals from several pressure 

transducers usually is much less expensive than dedicating a 

data logger to each sensor. Data retrieval from one data logger 

also is much simpler. Instrumentation of many wells requires 

many pressure transducers, which can become cost-prohibitive for some studies. In some situations it may be possible to 

prioritize the need for continuous water-level data, and record 

water levels in key wells with pressure transducers and data 

loggers while manually measuring levels in other wells. If the 

study design calls for single, isolated wells to be instrumented, 

many manufacturers offer water-level sensing systems that 

allow the pressure transducer, data logger, power supply and 

cabling to be installed inside the well bore, thus protecting the 

entire system from the weather, vandalism, or theft.

Well Location, Diameter, and Depth

The setting in which the transducer is installed needs to 

be evaluated prior to installation. Wells installed in areas subject to strong electromagnetic fields, such as near generators, 

motors, pumps, power supplies, or similar devices may not 

be suitable candidates for some types of pressure transducers 

and may require additional protection and signal conditioning. Natural occurrences such as storms, precipitation, and 

lightning likewise can affect the transducer and data logger.

Wells installed in remote locations commonly require 

provision for additional data storage. If site visits are infrequent, the data-collection system should be robust, may need 
18 

Accurate check and calibration measurements are much 

more difficult when the depth to water is great. Manual 

wireline or tape measurement is more difficult because of the 

line stretch caused by weight-induced tension or temperatureinduced expansion. These problems may call into question the 

accuracy of the data record when it is compared against the 

manual measurements. 

Data-Collection Frequency

For most water-level investigations, the frequency of data 

collection is limited by the data logger and memory-storage 

system. Frequent observations require more memory in the 

data logger or storage devices. Commonly, water-level data in 

wells are collected no more frequently than hourly. For aquifertest applications, however, the pressure transducer may limit 

data-collection frequency. In some situations, where recovery 

from an aquifer test is very rapid, observations should be made 

on the order of every 0.5 second. Some pressure transducers 

may take more time than that for the output to stabilize following excitation of the sensor. Some data loggers store the average of measurements taken over a period of several seconds, so 

recorded measurements of rapidly changing water levels may 

lag behind the true water levels. 

Data Transfer

Data commonly are stored in a data logger or attached 

storage device, or both. In order to transmit the data from the 

logger to a computer in the office, a direct datalogger download can be made during a site visit or the data can be accessed 

remotely. Remote access can include automated transmission 

by satellite (Jones and others, 1991), phone line, cell phone, 

or radio signal; or the data can be stored on an onsite computer. The frequency of this transmission would depend on the 

timeliness of the data, ranging from the need for immediate 

transmissions to long transmission intervals designed to prevent 

exceeding the data storage capacity of the data logger or on-site 

storage device. Consult the manufacturers’ manuals for data 

transmission techniques.

Cost

Most study objectives are compromised to some extent by 

budgetary constraints. Developing a priority of goals, and determining the cost of these goals, will provide the greatest value for 

the funding available. For example, if accuracy is the main goal, 

but maintaining an uninterrupted data record is a lesser priority, 

a study design could include very high quality pressure transducers but no system redundancies. Conversely, if maintaining 

a continuous data record is the highest priority, the study may 

be designed to have more than one sensor in each well, multiple 

data- storage systems, and backup power supplies, but use less 

expensive pressure transducers. The availability of personnel to 

service the data-collection sites commonly is the single most 

important decision that needs to be made when designing a 

study. If the study can afford frequent site visits, the accuracy 

of data and continuity of record nearly always are increased.

ASSEMBLY, CALIBRATION, AND 

TESTING

The user must be familiar with the behavior of the transducer, its installation, calibration and sources of error in the 

calibration to ensure that the data collected are reliable and 

reproducible. Depending on the accuracy requirements of the 

study as discussed previously, the user may choose to perform 

simple field checks of the transducer characteristics supplied by the manufacturer, or perform more detailed tests on 

individual transducers in the office. Studies requiring greater 

accuracy will require more extensive testing of the transducers. User-assembled transducer systems will require the most 

extensive testing. 

Familiarization With Transducer Performance

Users should take time in the office to become familiar 

with new pressure transducers and data loggers, as well as 

with recalibrated transducers before taking them to the field. 

Failure to do so can result in much wasted time in the field. 

Before submerging the transducer, a series of performance 

tests should be done in the office to determine: 

1. the effects of temperature on transducer output, 

2. drift characteristics, 

3. hysteresis effects, 

4. the location of the transducer plane, and 

5. standpipe effects. 

To test, connect the transducer to the data logger and set 

the output interval to about 5 to 30 seconds, depending on 

transducer response time, with output to a computer so that 

numbers, a graph, or both appear on the screen. With the sensor upside down, place a few drops of silicone oil in the transducer port. Compare transducer outputs with the transducer 

in normal vertical position (as it would be in a well) upside 

down, and horizontal. A fluid mass such as silicone oil below 

the transducer plane gives a highest reading with the transducer upside down, an intermediate reading sideways, and the 

lowest reading in normal vertical position. Warm and cool 

the transducer, noting changes in output. Vary the excitation 

voltage over its specified range, noting changes in transducer 

output. Leave the transducer to record for a few days at a constant temperature, then leave it to record while the temperature 

fluctuates over a temperature range that includes the range of 

temperatures expected in the field. For gage or differential 

transducers, the output during these tests should not change. 
 19

The constant-temperature test is a test of drift, exclusive 

of temperature effect. The variable-temperature test is a test 

of drift including temperature effect. For the variable-temperature test, return the transducer temperature to the starting 

temperature and subtract the prorated drift to get residual temperature effect. For an absolute transducer, subtract the output 

of an absolute-pressure standard from the measurements to 

get drift and temperature effect. Sudden temperature changes 

applied to a transducer (such as by holding the transducer in 

your hand, or lowering a transducer into a well) may cause 

unusual time-varying changes in output. These changes commonly are due to temperature gradients across the transducer 

element or among circuit board components.

Pressure tests to measure hysteresis are relatively easy 

to perform. Apply a vacuum to the transducer and release, 

noting output; then apply positive pressure and release, noting 

output when the output stabilizes. The difference between 

the stabilized outputs is hysteresis. Temperature tests for 

hysteresis can be done in a bath of ice and fresh water by 

warming the transducer and putting it in the bath, noting the 

output; then cooling the transducer in a bath of ice and salt 

water and returning it to the bath of ice and fresh water, noting the output. Temperature tests for hysteresis are difficult 

to perform on absolute transducers because air pressure may 

have changed by the time the transducer has come to thermal 

Waterproof Housing

A rugged and inexpensive housing for these transducers can be assembled with a PVC-pipe coupling 

and two bushings (figs. 32, 33). The bottom bushing 

is center-drilled with a letter “D” bit (0.246-in diameter) for a pressed fit that allows the pressure port 

to stick out of the bottom of the housing. Fit the top 

bushing with a strain relief fitting. Insert the cable 

through the strain relief and top bushing and attach 

it to the transducer. The usual wiring convention is 

red-excitation; black-analog ground; white-signal 

high; green-signal low. The pressure port is pressed 

into the bottom bushing, while optionally sealing 

the contact between the PVC and pressure port with 

cyanoacrylic glue. Glue the bottom bushing into the 

coupling with PVC glue, fill the cavity with potting 

compound, and press the top plug into place. Purchase extra potting material and practice potting a 

few times on dummy housings without transducers to 

become familiar with the procedure. 

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Figure 33. Housing for low-cost pressure transducer.

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Figure 32. Housing for low-cost pressure transducer 

with a thermistor.
20 

equilibrium at the three different temperatures. In field installations where the water level or temperature fluctuates rapidly, 

attaining the desired accuracy may require correcting the 

transducer readings for hysteresis.

An interesting test to perform on differential pressure 

transducers is to apply the full-scale pressure to both ports. 

If the transducer were truly differential, no change in output 

would occur. In fact, the sensing element has undergone a 

volumetric strain from the pressure change, and a change 

in output will occur. In silicon strain-gage transducers, the 

change in output from application of full-scale pressure to 

both ports can be as much as one percent of the specified fullscale output. This shift is of little consequence in applications 

using high-range transducers. When using low-range differential transducers to obtain high resolution water-level fluctuations, however, the shift caused by barometric fluctuations, 

which can be as much as 0.6 ft of water, can produce errors of 

1 percent times the ratio of the barometric fluctuation to the 

full-scale output of the transducer.

The next set of performance tests is done in the office 

during and after submerging the transducer. If the transducer 

has an obvious port, fill it with water. A tiny piece of fine 

screen with openings smaller than 0.0025 inches glued over 

the port will keep the water in and sand out. Inject water 

through the screen using a disposable insulin syringe, which 

has a very fine needle. The first wet test should be the effect 

of hydration on zero shift. For this test, note the pressure in a 

fixed orientation immediately after filling the port with water. 

The next day note the pressure (after assuring that the port is 

filled to the same level) in the same orientation. Hydration 

may be noticeable in some transducers that contain silicone oil 

or gel in the sensing element or in the port. Hydration does 

not cause continuing drift, but it is necessary to ensure that the 

transducer is hydrated before it is installed to avoid having to 

subtract this small effect from the field data.

Perform the right-side-up, upside-down, and horizontal 

tests again, comparing the wet outputs with the dry outputs. 

The wet output minus the dry output, in pressure units of 

feet of water, gives the length of the port which is used to 

determine the transducer plane. If the port points down, the 

difference should be positive with the transducer upside down 

and negative with the transducer right side up. Immersing the 

transducer in water should give the result of zero output when 

the transducer plane is even with the water surface. Another 

approach to determining the negative-standpipe effect is to 

connect a small-diameter copper or stainless-steel tube to 

the port, fill the tube and port with water, and bend the tube 

around until the tip is even with the transducer plane. Determining the transducer plane allows the user to become familiar 

with the positive and negative standpipe effects of the port. 

In field operations, if the port is not filled with water, it will 

capture an air bubble. The pressure inside the bubble does 

not increase and decrease linearly with water-level changes, 

because the bubble’s air-water interface moves up and down. 

In addition, the bubble will gradually dissolve, producing drift. 

Pulling the transducer out of the water to check zero drift will 

give a negative reading equal to the column of water held in 

capillary tension below the transducer plane. Without a screen 

or small tube, some or all of the water may be shaken out, giving irreproducible results for a zero-drift check and different 

subsequent drifts for bubble dissolution. Not all transducers 

exhibit these standpipe effects, and the effects may be negative 

or positive. Experimentation and testing, which will determine the effects for a particular transducer, should be done in 

the office before going to the field.

Linear Transducer Calibration

A common calibration procedure for pressure transducers 

uses a standpipe to obtain different values of submergence and 

a linear regression or straight-line fit. The equation is:

 V = a + bP, (6)

in which V is transducer output, usually in millivolts; P is 

pressure, in feet of water; a is the y-axis intercept and b is the 

slope of the best-fit line. Equation 6 is solved for P giving:

 P = V - a (7) b 

In the calibration procedure, the variables P and V are 

commonly switched, giving:

 P = a′ + a′V, (8)

Inexpensive Silicon Strain-Gage 

Pressure Transducers

Inexpensive pressure transducers can be soldered 

to inexpensive cable, potted in waterproof housings, and submerged in water to measure water-level 

changes in wells, piezometers, and stream gages; 

pore-pressure changes in saturated sediments; and 

soil-moisture tension. Measure submergence by 

using one transducer under water and another transducer as a barometer. Submergence is the calibrated 

output of the underwater transducer minus the calibrated output of the barometer. If multiple absolute 

transducers are used for water-level measurements, 

three transducers should be used for the barometer 

to ensure redundancy. Differential-pressure transducers require a vent tube from the reference port to 

the atmosphere but do not require a barometer for 

adjustment to submergence. An ancillary transducer 

used as a barometer, however, determines the barometric effect on water levels in wells and piezometers. 

Specifications for the transducers, cable, fittings and 

potting compound are given in Carpenter, (1994).
 21

in which a′ = 1/b and b′ = -a/b from equation 7. This procedure 

is statistically incorrect because independent and dependent variables are not interchangeable. As a practical matter, however, 

when the coefficient of determination (r 2) is 0.9999 or higher, 

the coefficients programmed into the data logger are identical.

Linear calibration of submersible pressure transducers can 

be done in the office using a vertical standpipe capped on the 

bottom and kept filled to overflowing with water, or in the field 

using a well. Be sure that no air is captured in the pressure port 

because, as explained earlier, the pressure at the diaphragm does 

not increase linearly with an increase in submergence when a 

bubble is present. Ensure that water temperature is constant 

throughout the water column, because density stratification in 

the standpipe will affect the pressure readings. 

Calibration in a standpipe is done by inserting and withdrawing the transducer while maintaining the water level at 

the top of the pipe. The cable is marked and measured from 

the transducer plane at a minimum of five increments over the 

desired range of submergence. The transducer is submerged 

in the standpipe to a mark on the cable, and the distance and 

transducer output are noted. Repeat the procedure for all the 

marks. The linear regression coefficients are determined using a 

hand calculator or a spreadsheet program. Enter the coefficients 

into the data logger, and repeat the submergence procedure as a 

final calibration check. For the insertion and withdrawal phases 

transducer output may not be identical for a given submergence 

depth. This difference arises from hysteresis and lack of repeatability in positioning the transducer at precisely the same depth. 

Some experimentation is needed to determine positioning errors. 

In a standpipe calibration, it may be difficult to maintain or vary 

the temperature of the transducer in a controlled way. The transducer may also be calibrated in an observation well, as described 

in “Transducer Field Calibration.” 

Sources of Error in Linear Calibrations

The problem with linear regression for calibration of silicon 

strain-gage pressure transducers is that the procedure leaves 

significant residual errors while giving the false appearance of 

a good fit when the calibration points are plotted on the same 

graph as the straight-line fit. In fact, the r2 for linear regressions 

can exceed 0.99999, while errors of as much as ±0.3 percent 

exist between the calibration points and the individual linearregression equations (Carpenter, 1994). Major sources of error 

in linear calibration are thermal effects on the transducer and 

non-linear response of the transducer. Different manufacturers’ 

transducers exhibit different curvature and temperature effects in 

their calibrations. The critical test in the calibration procedure 

involves plotting residuals of the calibration equation minus the 

calibration points. When the range of that plot is less than the 

accuracy required by the user, the calibration is adequate.

Thermal effects in a transducer can arise when water 

temperatures change at the transducer. In wells and piezometers, geologic features such as faults, fractures, and joints can 

provide conduits of high hydraulic conductivity. If the head 

varies between these features with time, water can flow vertically in the well, producing temperature changes. Well screens 

or gravel packs spanning aquifers with intervening confining 

units can allow flow within the borehole in response to nearby 

pumping owing to differences in hydraulic conductivity and 

specific storage between the aquifers. For example, a temperature change of 3.4°C in 15 minutes was observed at a depth of 

51 m in an alluvial aquifer in response to turning on a pump in 

a nearby well (Carpenter and others, 1987).

Inadequate or nonexistent temperature compensation produces thermal effects in pressure transducers. Bridge networks 

in transducers commonly are compensated to: 

1. counteract the effects of temperature changes of the 

transducer on the output voltage, 

2. achieve an output voltage very close to zero for zerodifferential or absolute input pressure, and 

3. achieve a selected output voltage for full-scale pressure 

input. 

The compensating networks commonly consist of series 

and parallel resistors, some of which exhibit temperature 

effects opposite to those of the transducer. Because a compensated design is a solution for the average of a transducer model, 

and because individual transducers vary, a particular design 

will reduce some errors and, in the process, introduce other, 

smaller errors. A compensated design may make it difficult to 

use a transducer below a manufacturer’s error specifications 

and may make temperature correction by the user much more 

difficult than uncompensated designs. Some manufacturers 

apply a simple linear shift or offset for temperature compensation. Such compensation does not usually interfere with userdetermined temperature correction of transducers to achieve 

higher accuracy.

Sources of Error When Making Direct-Current 

Measurements

The act of measuring the response of the pressure transducer can in itself introduce errors due to the effect of the 

meter on the circuit being measured. These errors are caused 

by current loading, shunt-resistance loading, magnetic coupling, and ground loops.

Voltage Measurements

Voltage measurement errors, other than those attributable 

to instrument calibration, can arise from the physical limitations of the voltmeter and from extraneous voltages generated 

in other parts of the measurement circuit. An ideal voltmeter 

would have an infinite input resistance and would draw zero 

current from the circuit being tested. Because the voltmeter 

resistance is not infinite and is connected between two points 

of unequal potential (in parallel with the circuit), a current will 

occur in the voltage measurement. This current draw may 

change the current magnitudes in the circuit enough to cause 
22 

a considerable change in potential being measured. These 

limitations usually can be minimized with proper instrument 

selection. Some of the most important sources of error associated with voltage measurements are discussed below.

Input-current loading error, caused by the small 

amount of input current (Iin) that is drawn by the voltmeter 

during measurement (fig. 34) is proportional to the source 

resistance (Rs) of the measurement circuit. Because of this 

resistance, the source voltage (Vs) read by the voltmeter (Vm) 

is reduced by the loading current error (IinRs), so that 

Vm = Vs - IinRs. To reduce this error, a voltmeter with a small 

input current should be used.

Shunt-resistance loading error is caused by any resistance (Rshunt) that is in parallel with the test signal (fig. 35). 

Shunt-resistance sources include the resistance of the voltmeter and insulation leakage across the test leads and connectors. 

Because of shunt-resistance loading, the source voltage (Vs) 

read by the voltmeter (Vm) is reduced by the shunt resistance 

(Rshunt); so that Vm = Vs [Rshunt /(Rs + Rshunt)]. Shunt-resistance 

loading errors can be minimized by:

1. using a voltmeter with as high an input resistance 

(impedance) as possible,

2. using well insulated electrical cables and connectors, and

3. using active guarding (a grounded ring around the point 

of attachment of the signal lead) to decrease the value of 

the electrical lead cable shunt resistance.

Thermal electromotive force (EMF) errors result from 

small voltages generated at the junction of two dissimilar 

metals by the thermocouple effect. The size of these thermocouple-induced EMFs depends on the type of metals used and 

the temperature differences between junctions. These thermocouple effects will either add to or subtract from the desired 

signal, producing an unpredictable offset because the EMFs 

generated by the junctions vary in response to temperature. 

Thermal EMF errors can be minimized by:

1. using only clean, crimped-on, copper-to-copper 

connections, and

2. keeping all junctions at the same temperature.

Magnetic coupling errors occur when a magnetic field 

of varying strength passes through a loop in the measuring 

circuit. Voltage induced by the magnetic field is proportional 

to the strength of the magnetic field, the rate of change of the 

magnetic field, and the loop area of the measuring circuit. 

Magnetic field effects can be minimized by:

1. locating the measurement circuit furthest from magnetic 

field sources such as motors, transformers, and magnets;

2. keeping signal leads as short as possible to minimize 

loop area; and

3. using twisted pairs to minimize loop area.

Ground loops may produce both noise and error voltages. Ground loops arise from grounding the measurement 

instruments and output source at two or more different points 

on a common ground bus. The separation between ground 

points can result in a voltage between the output source and 

measurement instruments, causing a current to flow around 

the loop. Resistance in the loop produces an unwanted voltage 

in series with the output voltage of the transducer. To avoid 

ground loops, all test and measurement components should 

be grounded at one single point. Some pressure transducers may include a ground wire attached to the outer housing. 

When submerged, the pressure transducer itself may become 

a ground point in the circuit. To avoid creating a ground loop, 

the ground wire should not be connected to the measurement instrument if the measurement instrument itself also 

Figure 35. Shunt-resistance loading error.

Figure 36. Voltage-burden error.

Figure 34. Input-current loading error.

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 23

is grounded elsewhere. Alternatively, it may be better not to 

ground the measurement instruments but to use the pressure 

transducer ground as the common ground point for the entire 

measurement circuit.

Current Measurements

Current-measurement errors, other than those attributable to instrument calibration, can arise from the physical 

limitations of the current meter and from unknown or unwanted 

currents generated in other parts of the measurement circuit. 

An ideal current meter would have zero input resistance and no 

voltage drop across the input terminals. As in the case of the 

voltmeter, actual instruments do have some small resistance that 

can cause measurement inaccuracies. Since a current meter has 

some internal resistance, its insertion into the circuit (in series) 

may decrease the current in the measured branch. Current 

meter errors can be minimized with proper instrument selection. 

Some of the most important error sources associated with current measurements are discussed below.

Voltage burden errors, caused by the input resistance (Rin) 

of the current meter, result in a small voltage drop across the 

input terminals (fig. 36). The source current (Is) that is measured by the current meter (Im) is reduced by the input resistance (Rin) so that Iin = Vs

/(Rs + Rin) or IsRs/(Rs+ Rin). To reduce 

this effect, Rin should be as small as possible. Current meters 

measure current either by measuring the voltage across a shunt 

resistor, or by using an active circuit such as a current to voltage 

(I to V) converter. Shunt-resistor current meters tend to have 

very high voltage burdens (in some cases up to several hundred 

millivolts), whereas the voltage burden of current meters with I 

to V converters are typically less than 100 microvolts.

Generated currents or stray currents can add to the desired 

signal current and introduce errors into the measurement. The 

effects of these can be minimized with careful attention to the 

measurement environment. Stray currents may be introduced 

into the measurement circuit from a number of different sources. 

These include vibration, insulator deformation, and contamination.

Vibration generates static charge and stray currents. Stray 

currents are generated by friction between the insulation and 

the electrical cable, a phenomenon known as the triboelectric 

effect. To minimize triboelectric effects, isolate the test circuit 

from sources of vibration, such as motors, pumps, and wind, and 

securely mount or tie down wires and cables.

Insulator deformation

generates piezoelectric currents. Stray currents can be 

minimized by not overstressing terminal connections and 

using insulating materials 

that have low piezoelectricgenerating properties.

Contamination caused 

by body oil and solder flux 

combined with humidity or 

moisture may create weak 

battery effects that result in stray currents. To minimize this 

source of error, keep all critical connections clean and free of 

contamination.

Resistance Measurements

Resistance measurement requires the use of either a current meter or a voltmeter and is subject to the same instrument 

limitations, error currents, and error voltages 

as are generated in other 

parts of the measurement 

circuit. Resistance is not 

measured directly; rather 

it is calculated from a 

known input (current or 

voltage) and a measured 

output (voltage or current) 

using Ohm’s law. Resistance measurements can 

be made using either two 

wires or four wires. 

In two-wire configurations, the resultant voltage or 

current across the circuit is measured across the same pair of 

wires used to introduce the input current or voltage (fig. 37). 

Make two-wire measurements either by supplying a constant 

current and measuring voltage or by supplying a constant voltage and measuring current. A special variant of the transducer 

is the transmitter. The output current of the transmitter is measured over the same two wires used for voltage excitation.

In four-wire configurations, one pair of wires is used 

to supply a current or a voltage and the second pair of wires is 

used to measure the resultant voltage (fig. 38). Most straingage pressure transducers are operated in the four-wire configuration. The main advantage of the four-wire configuration over 

the two-wire configuration is that there is effectively no voltage 

drop in the sense leads because there is generally very little current in the sense circuit. The voltage measured in this way is 

essentially the voltage developed across the test resistance. 

A voltage source is normally employed when operating 

strain-gage pressure transducers. The voltage source generates an input current, and the resultant output voltage reflects 

changes in the bridge resistance caused by changes in pressure. 

Long lead wires can introduce errors into the voltage delivered 

to the transducer because of the resistance of the electrical 

cable. Although the resistance of the excitation leads generally is not large, the input current usually is significant and can 

result in a large voltage drop. Input voltages usually are much 

higher than the output voltage, and hence voltage losses in the 

input signal can be a source of error in the circuit. To compensate for voltage losses over long lead wires, calibrate the 

transducer with the lead wires attached. 

Using a current source to measure voltage provides a 

constant current to the measurement circuit. Voltage at the 

source will vary in order to maintain a constant current to the 

transducer. A current-supply source with a voltage-regulating 

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Figure 38. Four-wire configuration.
24 

capability is needed for this type of operation; unregulated battery supply sources will not maintain a constant input current. 

Problems associated with voltage drops using long lead wires 

can be avoided by exciting the transducer with a constant current. The transducer must be calibrated using a current source, 

and the current input needs to be adjusted to match the excitation voltage recommended by the manufacturer.

Temperature-Corrected Transducer Calibration

A procedure that is more complicated than linear calibration, but is entirely manageable, can reduce residual error to 

±0.03 percent of full-scale output or less. The pressure transducers are calibrated at five pressures at each of three temperatures in a water bath in a Dewar flask, insulated water jug, or 

wide-mouth thermos bottle (fig. 39). A calibrated field standard (precision transducer) with specified accuracy of ±0.01 

percent of the full-scale reading is used with a hand pump. 

These components are connected either by plastic tubing and 

barbed nylon fittings or by 0.25 in. diameter drip-irrigation 

tubing and fittings that also distribute pressure to several transducers. A data logger collects and stores data from several 

pressure transducers (through differential analog inputs), from 

the field standard, and from several temperature sensors in the 

water bath (through single-ended analog inputs), transmitting 

the data to a computer screen. Immerse a precision thermistor connected to a display with a specified accuracy (±0.02°F) 

in the bath for calibrating the temperature sensors. Immerse 

the transducers in the water bath, allow them to equilibrate for 

about one-half hour with an occasional stir to maintain even 

temperature distribution, and make a set of nearly simultaneous measurements of all the sensors for each of five pressures 

over the pressure range of interest. Repeat the measurements 

at two more temperatures, allowing one-half hour equilibration 

at each temperature. 

Because the field pressure standard is an absolute 

transducer, the calibration is straightforward. It has a tare 

switch for subtracting starting pressure; thus it can be used to 

calibrate differential transducers if the series of five pressure 

measurements is made reasonably quickly and the tare value 

is checked at the end of the five measurements. Reset the 

tare at each new temperature before each set of five pressure 

measurements. As a practical matter, atmospheric pressure 

does not change rapidly enough to be a problem in calibrating differential transducers, although sudden opening of 

doors (which cause changes in barometric pressure) should 

be avoided during a set of nearly simultaneous measurements. Note that the standard can be permanently damaged 

by overpressurization. Although some standards have a 

pressure-warning alarm that can be set, it is wise to choose a 

pump that cannot overpressure the standard. A bath filled with 

finely crushed ice and just enough de-ionized water to make a 

slush can be within ±0.04° of 0°F. Because of its repeatability and usefulness in calibrating transducers for temperature, 

the ice-water bath is a good choice for one of the calibration temperatures. Record the output time for each pressure 

manually, along with the readings from the pressure standard 

and precision thermometer, and use them to edit the data after 

they are transferred to a spreadsheet program. This calibration 

procedure produces values of voltage for ranges of pressure 

and temperature.

Multiple regression is used to determine the calibration 

equation for each pressure transducer. An equation that can 

reduce residual error with respect to the standard to ±0.03 percent over the calibration range of water-level fluctuation is:

 V = a + bP + cP2 + dT = eT2 + ƒTP (9)

in which V is transducer output, in millivolts; P is pressure,

in feet of water; T is temperature, in degrees Fahrenheit; 

and a, b, c, d, e, and ƒ are regression coefficients. Solving 

equation 9 for pressure gives:

 P = – (ƒT + b) + √(ƒT + b)

2 – 4c (a + dT + eT2

 – V) (10)

 2c

The value of r2 using equations 9 and 10 can exceed 

0.9999999. In the spreadsheet, columns of P2, T2, and TP are 

generated from the data columns of T and P. Multiple regression then is used to generate the coefficients a, b, c, d, e, and ƒ. 

The coefficients c, e, and ƒ of P2, T2, and TP are all small, but 

the removal of any of these terms leaves a much larger residual 

than that from equation 10. If a temperature sensor is installed 

with the pressure transducer or built into its housing, equation 10 can be programmed into a data logger or calculated in 

a spreadsheet to give temperature-corrected output. Before 

removing a group of pressure transducers from the calibration 

equipment, check the entire calibration procedure by entering 

the coefficients and checking the temperature-corrected output 

Figure 39. Equipment used for temperture-corrected transducer 

calibration.
 25

against the output from the pressure standard for representative temperatures and pressures.

The pressure standard and precision thermometer are 

expensive. However, if many transducers are to be calibrated or if accuracy of ±0.1 percent of full scale or better is 

required, the convenience and accuracy of a pressure standard 

are well worth the cost. The accuracy obtained from inexpensive, user-assembled transducers (±0.03 percent of full 

scale) can approach the specified accuracy of the standard 

(±0.01 percent of full scale). A hand pump also is a major 

convenience. Because temperature is an ancillary measurement, its accuracy is not as critical as that of pressure, and 

the precision thermometer may be excluded, although an 

accuracy of ±0.1°C or better in calibrated field temperature 

sensors can be valuable for hydrologic interpretation (Carpenter and others, 1995). A bicycle pump and Schrader valve 

can be used for the hand pump. A simple cost comparison of 

total components and materials for inexpensive transducers, 

wire, potting, and calibration equipment plus calibration time 

versus the number of commercial transducers needed gives a 

breakeven point of three to ten transducers. The benefits of 

user-assembled transducers include the ability to: 

1. maintain an inventory of ranges and types of transducers 

to quickly satisfy emerging and specialized needs, 

2. provide redundant measurements, 

3. make a larger number of measurements than can be done 

with the equivalent cost in commercial transducers, and 

4. perform measurements that would otherwise be too difficult or expensive.

INSTALLATION

For the successful collection of data, the submersible transducer and data-recording system must be correctly 

installed. This section provides general guidelines and suggestions, including the proper care and handling of transducers, considerations for designing shelters for equipment, 

discussion of suspension systems and desiccation systems, and 

field procedures for calibrating the transducer and optimizing 

the transducer’s position in the well.

Care and Handling

Proper care and handling of a transducer is essential to 

its reliable operation. Most submersible pressure transducers 

come with a factory warranty, with specific instructions on 

necessary maintenance, and with a warning about mishandling. 

Failure to follow the instructions and warnings could cause 

irreparable damage to the transducer, void any factory warranty, and cause the corruption or loss of valuable data. The 

most common causes of damage are sudden impacts, freezing 

or overheating of the transducer, and sudden, extreme pressure 

changes (water hammer). Improper electrical connections also 

are a common source of transducer damage or failure. 

Sharp impacts can break the body of the transducer and 

dislodge or break delicate electronic components. Keep the 

transducer packaged in its original shipping container until it 

is ready to be installed. After removing it from the packaging, 

take precautions to prevent the transducer from being dropped. 

Avoid sharp contact with the sides of the well or piezometer 

casing during installation and calibration. Do not over-torque 

a transducer when fitting it to an auxiliary threaded device 

such as a counterweight or packer housing. 

Exposure to temperature extremes beyond the manufacturer’s design specifications also can damage a transducer. 

Freezing or overheating can damage the sensor’s electronics 

and, in extreme cases, damage its waterproofing by causing 

warping or cracking, disintegration or melting of the waterproofing material. The severity of damage to the waterproofing material of the transducer depends on the methods and 

materials used. Exposure to temperature extremes prior to 

installation can be avoided by storing the transducer in a cool, 

temperature-controlled environment. Most transducers come 

from the manufacturer with specifications as to the extremes 

of the operating temperature. The transducers should not 

be operated under, or exposed to, conditions that exceed the 

design limitations. Submersible transducers used to monitor 

changes in water level commonly are designed to function in 

the range of 0 to 120°F, though transducers operating through 

a greater range of temperature extremes are available. 

Most submersible transducers are designed to monitor 

pressure in a variety of fluids; the specifications may state 

that the transducer can be operated at temperatures below 

32°F, but it should be remembered that fluids other than water 

have different freezing points. As common sense dictates, 

transducers submerged in water generally cannot be operated 

at or below freezing temperatures because ice crystals will 

form and damage the transducer. This prohibits their use in 

surface-water applications where temperatures can fall below 

water’s freezing point.

Sudden, extreme changes in pressure also can damage 

a submersible transducer. A common mistake is letting a 

transducer free-fall into the water during installation. Instead, 

gently lower the transducer into the water to the desired submergence depth. Be aware that transducers installed in wells 

subjected to frequently repeated and significant changes in 

water levels can show signs of wear sooner than usual (L.A. 

Freeman, U. S. Geological Survey, written commun., 1994). 

This is common with vibrating-wire transducers or transducers 

that use a diaphragm, on a long-term basis, to monitor observation wells located near pumping centers. Frequent, rapid 

rises and falls in water levels can cause the wire or diaphragm 

to fatigue at an accelerated rate, a situation which usually 

manifests itself in an accelerated rate of calibration drift. 

Another common cause of damage to an installed transducer is overpressuring. The factory specifications commonly 

list three sets of pressure limitations. The first is the calibration range of the sensor. Operating the transducer slightly 
26 

outside of this range will not damage it. The second pressure 

specification is the operating range, commonly twice the calibration range for silicon strain-gage transducers. For example, 

a transducer designed for sensing 0 to 5 psi will provide 

output up to 10 psi without damage. However, the transducer 

calibration may not hold for pressures above the specified 

calibration range. The third type of pressure limitation is the 

maximum pressure that a transducer can be subjected to before 

being physically damaged. This limit, which varies from one 

manufacturer or transducer design to another, is usually 2 to 

6 times the calibration range. This equates to 10 to 30 psi for 

a transducer designed to monitor 0 to 5 psi. If a transducer 

is subjected to pressures exceeding the operating range, but 

not exceeding maximum pressure tolerance, the unit may not 

be damaged, but data can be lost. Once the pressure falls to 

within the operating range, the transducer will recommence 

correct output, because only the output range of the electronic 

components has been exceeded. In summary, to minimize 

damage to a transducer, strictly adhere to the manufacturer’s 

specifications for care and handling. 

Shelter

Data loggers, power systems, data cables from the transducers, well casings, and other equipment must be protected 

from weather extremes, from damage by animals and insects, 

and from vandalism. It may also be necessary to shelter a 

person servicing the site from extreme weather conditions. In 

addition, equipment for some study locations in scenic areas 

must blend in with the local surroundings. All of these factors 

must be considered in designing an equipment shelter. Figure 

40 shows one type of shelter design. This design can be used 

when there are no requirements for limiting visual impact, 

there is minimal risk of vandalism, and there is very little 

other equipment needing shelter. This design mounts directly 

to the metal casing protecting the observation well or wells. 

The fiberglass environmental box provides enough space for 

one data logger, one backup storage module, one 12-volt battery, one desiccant chamber, and one solar panel charger and 

regulator. It does not provide shelter to the servicing groundwater scientist. Transducer and logger systems small enough 

to install in the well eliminate the need for above-ground 

shelters and should be considered for use in areas where 

vandalism is a problem. Some data loggers are designed to fit 

on top of the well casing, but inside the outer metal protective 

casing, whereas others are designed to be suspended in the 

well, either above or below the water level.

Power Requirements

The amount and type of power required to operate a pressure-sensing system are specific to the instruments used at a 

given site. The type and number of transducers used, the type 

of recording device, and the frequency at which the data are 

recorded all affect the power requirements.

Each type of transducer and each data logger has its 

own power consumption rate, usually expressed in milliamps. 

Transducers, data loggers, and other electronic components 

also have specified voltage ranges for their operation. The 

user should refer to the owner’s manual for an instrument’s 

power requirements. Instrument manufacturers commonly 

have staff available to advise on the best way to configure an 

instrumentation system. Most systems are operated using DC, 

but some sophisticated systems may require a combination of 

DC and AC power. The user-defined scan rate and recording 

frequency of a data logger program affect power consumption. Power consumption will be greater for installations in 

which the data logger is connected to more than one pressure 

transducer.

Proper grounding and shielding of the transducer, supplemental instruments, and data logger will minimize errors, 

as discussed in the section on “Overview of Direct-Current 

Circuits.” Grounding and shielding should include protection 

of the power supply system. A wiring diagram illustrating 

how the sensors are connected to the data logger will save time 

when changes occur, and also will allow someone who is not 

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Figure 40. Shelter design for a pressure-transducer installation.
 27

familiar with the site to troubleshoot or replace equipment. 

Because data can be lost as a result of poor wiring connections, all connections should be checked and tightened periodically. Daily temperature fluctuations in the instrument shelter 

can cause terminal screws to work loose.

Rechargeable batteries commonly are used to power the 

instruments. A good practice is to record the voltage of the 

battery or other power supply at regular intervals in order to 

monitor power consumption to help evaluate the adequacy 

and performance of the power-supply system. Using batteries minimizes problems caused by power outages or surges. 

When a battery is the primary source of power, a solar panel 

array, or a battery charger attached to AC wiring can be used. 

The power source must be compatible with the type of battery 

used and must be adequate for the power consumption requirements of the transducer and data logger. For systems that have 

large power consumption rates, AC power may be needed as 

the primary power source. In this case, batteries can provide 

emergency power for short periods of time. 

Hanging Transducers in Wells

The transducer should be suspended in place in the well or 

piezometer from a stable fixed point called the hanging point 

(fig. 41), secured either to the well casing itself, to the inside 

of an instrument shelter, or to the protective outer casing of the 

well. A pressure transducer may be suspended by its electrical 

cable in wells where the depth to water is small, but for greater 

depths the electrical cable may expand or contract, changing 

the depth at which the transducer is suspended. For deeper 

installations, or where the manufacturer requires it, a suspension system is necessary. Where local sediment compaction or 

regional land subsidence is occurring, differential movement 

of the suspension system and the well can be a problem if the 

suspension system is attached to an instrument shelf resting on 

land surface. The suspension line must be secured to a point 

attached to the well casing. This will allow the transducer to 

move along with the well casing. One method of attaching the 

cable to the wellhead is to use nylon wire ties or stainless-steel 

hose clamps secured to a hook-shaped nail designed for electrical conduit. Hose clamps can be used with mesh cable clamps 

for added security. Experiment to make sure that pulling on 

the clamp makes the clamp tighter.

Some transducers come with the suspension cable, vent 

tube, and transducer wiring incorporated into a single unit. 

Other transducers need a separate suspension line, which can 

be made of stainless steel aircraft cable. A high-test fishing 

line, preferably made from plastic-coated stainless-steel cable, 

also works well for this purpose. Cable stretch is seldom a 

problem for suspension lengths of less than 300 ft. A cable 

grip, available from an electrical supply outlet, will hold the 

vent tube and wiring cables in place for either type of suspension system. The suspension method is necessarily site 

specific and transducer specific. Secure all cables, vent tubes, 

and suspension lines so that they do not slip while still permitting the vent tube to be open. Mark all cables and suspension 

lines to help determine if there has been any slippage, and to 

provide a way to precisely measure how much slippage has 

occurred. Cable slip can be detected by marking the cable 

with indelible marker or contrasting-color silicone rubber. 

Reference this mark to a fixed point such as the measuring 

point (MP) for the well.

A MP mark can be filed or sawed from the outside of the 

casing to just touching the inside edge of the casing. Depth 

to water is measured from this point. Using the highest point 

on the well casing makes surveying the elevation of the MP 

easier. At well heads with pumps, concentric casings, and 

other complications, a different measuring point may have to 

be chosen, marked, and noted. Two considerations are 

1. ease of surveying (either by Global Positioning System 

or by leveling), and 

2. ability to precisely align the holding point of the tape or 

electrical sounder.

Some transducers should be fitted with an appropriate ballast weight to counteract the buoyancy of the transducer and 

cables when submerged. An inexpensive alternative is to slide 

stainless-steel nuts and washers over the suspension cable, 

resting them on top of the transducer. Washers should have 

an outside diameter approximately the same dimension as the 

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Figure 41. Submersible transducer in an observation 

well.
28 

transducer. If a weight is attached below the transducer, the 

weight should be attached with a fine wire securely enough to 

stay on while lowering and raising the transducer, but weakly 

enough so that it would break free and allow the transducer 

to be retrieved if the weight became stuck in the well. It 

requires some experimentation to determine the amount of 

weight necessary to keep the cable taut without breaking the 

any electrical wires. Select ballast-weight materials carefully 

to avoid introducing unwanted constituents to the well. Do 

not use lead or plastic-coated lead weights. Even if the well is 

not being currently monitored for water-quality constituents, it 

may be in the future. 

Transducers can be hung 300 ft in a well using just the 

electrical cable without unwanted stretch in the cable and with 

an analog signal in the millivolt range. Greater depths may be 

achievable with experimentation. Problems to address include 

1. signal degradation from noise in long electrical cables, 

2. resistive line loss and changes in line resistance with 

temperature changes in excitation leads, and 

3. internal breakage and separation of electrical conductors. 

Conductor breakage can be avoided by attaching the 

electrical cable to a stainless-steel cable with wire ties so that 

the electrical cable is initially slightly relaxed and will not be 

under tension when the assembly is in the well. 

The principal objective of selecting a submergence depth 

at which to hang a transducer during installation is to ensure 

that water levels do not exceed the transducer’s operational 

range or fall below it during the monitoring period. Historical 

water-level measurements can be helpful when designing a 

long-term water-level monitoring program (Taylor and Alley, 

2001). If enough measurements have been made, it is much 

easier to determine what daily or seasonal extremes can be 

expected for a given well. Thus, the appropriate range for a 

transducer can be selected, and its optimum position estimated. To minimize potential for errors and simplify subsequent data processing, reposition the transducer infrequently. 

Under ideal conditions, the transducer can be hung at a single 

depth for the entire monitoring period, although it may be 

necessary to re-position a transducer once or twice during 

a season, especially when monitoring wells in areas where 

seasonal ground-water pumping prevails. In some locations 

it is possible to have a range in water level of 100 to 200 ft 

during a year (Hollett and others, 1994). A transducer with 

a large range in output may be used; however, measurement 

resolution decreases as the transducer’s range increases. In 

this instance, the data requirements must be considered. If 

high resolution is required, a transducer with a smaller range 

will be needed, thus requiring that it be repositioned more 

frequently so that the water level remains within its range.

Measuring System Drift

As a means of checking system drift, periodically measure water levels in the well containing the pressure transducer 

using a steel tape or sounder. Drift is calculated by determining the difference between the recorded water level and the 

actual measured water level. As discussed in the previous 

section, this drift includes possible cable stretch and slip. The 

effect of cable stretch and slip can be measured by pulling the 

transducer above the water level and making a reading. The 

difference between the offset and the drift is the cable stretch 

and slip. This test requires that the transducer plane and the 

port not be separated vertically, or that some technique be 

employed to alleviate this discrepancy. If the port is below 

the transducer plane, the transducer may measure the negative 

gage pressure of the water column. If the transducer is jostled, 

some or all of the water may fall out of the port, resulting 

in a smaller magnitude negative pressure, which may not be 

repeatable from test to test. If water is lost out the port, a 

residual error will remain after resubmergence until the compressed air bubble dissolves. This problem can be alleviated 

by following the procedures described in “Assembly, Calibration and Testing”. 

Desiccation Systems

For many strain-gage transducers, it is necessary to keep 

moisture from entering the transducer’s vent tube. Connected 

to the inside of the transducer at the diaphragm, the vent tube 

transmits atmospheric pressure changes to the pressure-sensing unit in the transducer housing. Because vent tubes also 

act as direct conduits for moisture to contact the transducer’s 

components, an adequate desiccation system is necessary to 

eliminate faulty data and moisture damage. Many transducers 

are supplied with an air desiccation system, but because some 

are inadequate for very humid conditions, the person installing 

and maintaining the transducer must ensure that the air desiccation system functions properly. An adequate desiccation 

system is essential for obtaining reliable, long-term data and 

for extending the transducer’s life. Where conditions are very 

humid, for a long vent tube, or for a large-diameter vent tube, 

it may be necessary to install a supplemental desiccant system. 

Small-diameter vent tubes are highly susceptible to blockage 

by the accumulation of water droplets in the tube, which will 

cut off or adversely affect the communication of atmospheric 

pressure to the transducer, thus causing erroneous data. Eventually, these droplets make their way down into the transducer, 

wetting the components and permanently damaging the 

sensor. Because long vent tubes or large-diameter vent tubes 

hold a larger volume of air, they require a desiccating system 

of greater capacity. When the desiccation system is housed 

inside a weather-proof shelter, provisions for communication 

of changing atmospheric pressure must be made by making 

vent holes in the shelter; these can be covered with a flexible 

diaphragm that will allow for the transmission of changes in 

atmospheric pressure to the vent tube and, at the same time, 

provide an impermeable moisture barrier. If absolute- and 

sealed-reference transducers are used, a desiccation system is 

not needed because these devices have no vent tubes.
 29

The internal wire bundle can be used as a vent tube to 

depths of more than 100 ft. The cable can be vented below 

its hanging point in the well by inserting into the wire bundle 

a syringe needle (with the sharp tip ground smooth to avoid 

penetrating any insulated internal wires), and sealing the hole 

with silicone rubber cement and electrical tape. A small desiccant pack can be put in a 60-ml syringe bore (after removing 

the plunger), and the plunger end of the syringe can be sealed 

with a partially-relaxed, thin plastic glove to make a pressuretransmitting, humidity-isolating chamber. Some experimentation—squeezing on the glove and noting the change 

in transducer output—will be necessary to ensure that both 

positive and negative barometric changes are transmitted down 

the vent tube or wire bundle. If the desiccant chamber is used 

on a transducer cable below the hanging point, seal the end of 

the cable with silicone rubber cement and tape and, using a 

hand pump or syringe plunger, perform a vacuum test of the 

seal integrity. 

Transducer Field Calibration

Although most transducers are calibrated at the factory 

and come with the manufacturer’s calibration specifications, 

an individual calibration check should still be done in the field 

as each transducer is installed. A field calibration check incorporates the effects of all system components and local environmental conditions, including water density. In addition, the 

calibration for the transducer’s entire pressure range should 

be checked periodically, whenever water-level measurements 

show that the calibration is not holding, and when the transducer is removed from the well at the end of data collection. 

Perform the calibration after installing the instrumentation so 

that any signal alteration that might occur between the transducer and the data logger is incorporated. Set the data logger 

to display the transducer readings at as high a resolution as 

possible.

Calibrate the transducer in the well by submerging 

it, lowering and then raising it by known increments, and 

comparing this incremental distance change with the transducer output change. By getting multiple readings at each 

calibration point, and by moving the transducer up and down 

in the well, values are obtained that will be “averaged” during 

the regression computation, thus minimizing bias associated with hysteresis in the system. Use a minimum of five 

points, in each direction, covering the calibration range or the 

operational range of the transducer. Determine the effects of 

pressure hysteresis by taking measurements while raising and 

lowering the transducer through the entire operational range. 

Moving the transducer and cable in the water column 

displaces water in the well. Small-diameter wells in lesspermeable formations are the most prone to having water 

levels altered by displacement. Transducer readings should 

be used for calibration only after the water level in the well 

has stabilized between calibration points. The stabilization 

time will depend on the permeability of the geologic formation, and the ratio of the transducer and cable diameter to the 

well’s diameter. Verify a stable water level with a calibrated 

measuring tape. After stability is attained, take several readings (minimum of 3) of the transducer output, then move the 

transducer to the next calibration point and repeat the process, 

again waiting for the water level to stabilize. Take care to 

precisely measure the distance the transducer is raised and lowered to minimize calibration errors caused by errors in distance 

measurements. Check the measuring tape for accuracy. 

Use a standard worksheet, such as shown in figure 42, to 

record field calibration data. Some of the positions in figure 42 

have been completed as an example. Note that the calibration 

was not checked for the transducer’s full range; this is acceptable if the expected water level range is less than the range of 

the transducer, or if the calibration is being checked for a range 

of data previously recorded.

Even though the transducer may be able to withstand 

pressures greater than the operating range specified, the 

calibration slope of the instrument may be different above its 

calibration range. If it is expected that the calibration range 

will be exceeded, a separate calibration can be made for that 

range. Preferably, a transducer will be selected that is designed 

to work under the extremes of water-level fluctuation expected 

for the site, thus avoiding the problem of exceeding the calibration range. 

After obtaining the calibration points, a calibration equation for the transducer can be easily determined by computing 

a linear regression using any basic statistical software package. 

Because the slope and offset of this equation only converts the 

transducer’s output into its submergence depth, an additional 

correction must be added to the calibration equation to convert 

submergence depth to depth below land surface (DBLS). 

Obtain this by comparing the measured water level, after the 

transducer is set in its final position (set point), with the converted output from the transducer (depth below water surface) 

and applying this to the regression equation. The general form 

of a linear calibration equation is y=mx+b. The final calibration equation takes the form of equation 11, as follows:

DBLS = SP - (psi, ft or mV) (Sr) + OS , (11)

where

DBLS is Depth to water below land surface datum, in feet;

SP is Set Point Distance, in feet; the measured distance 

 below land surface datum of the pressure transducer 

 at the time of installation;

psi or mV is the transducer output, in pounds per square 

 inch, feet of water, or millivolts at a specific time; and

Sr is the slope of the Generalized Least Squares regression 

 equation, in units/foot.

OS is the offset (zero intercept) in the regression equation, 

 in feet.

The resulting equation can be applied to the output of the 

transducer either by converting directly to water level as DBLS 

in the data logger at the site, or later by applying the equation 

as the data are adjusted for the data base (Freeman, 1996). 
30 

Figure 42. Example calibration worksheet for submersible transducers.

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Document all changes to the data logger program, and 

make information available in the field to allow on-site determination of the potential for over-ranging or water levels falling below the transducer position. A preprinted form should 

include the altitude of the land surface and measuring point; 

the distances between the measuring point and hanging point, 

set point, and the hung depth; and an inventory of equipment 

installed at the well. Take the field form to the field for data 

retrieval and periodic water-level measurements.

Calibration-slope checks are possible in the field on a 

differential transducer without pulling the transducer from the 

well. After removing the desiccant, connect a hand pump and 

pressure standard to the vent tube. Apply a vacuum in increments, noting the approximate time to stabilize the pressure 

and the pressure value from the data logger. The data logger 

will exhibit increasing pressures that correspond with decreasing pressures in the pressure standard. Changes in time to 

stabilization would indicate possible leakage or wiring deterioration in the transducer cable. 

Optimizing Measurement-System Performance

In some cases, an analysis of the individual components 

of the pressure measurement system, either by laboratory calibration or by calculation from DC circuit analysis, may indicate a total error exceeding the desired accuracy of the system. 

Several procedures can be followed to enhance overall system 

performance and reduce the total error to the desired accuracy. 

These include the following:

1. Use an in-place system calibration to simultaneously 

measure the combined effects of the transducer and 

associated measurement equipment. Lowering a transducer into a wellbore to different submergence depths, 

an example of this technique, is described in “Transducer 

Field Calibration”.

2. Before recording the output of the transducer, increase 

the delay time following excitation. The output voltage 

curve of the transducer, as a function of time, must be 

established to determine when its output is stable with 

respect to the measurand.

3. Calibrate the transducer either at the operating temperature or over a controlled temperature range so that the 

data can be adjusted for the effects of temperature, which 

must be monitored simultaneously. 

4. Use a constant current instead of voltage to excite the 

transducer. Establish current settings which ensure 

that the voltage across the transducer is within specified tolerances for the unit. This technique is especially 

advantageous when operating transducers over long lead 

lines.

5. Calibrate the transducer over a narrower pressure (and 

temperature) range than its full range specification.

6. Calibrate and operate the pressure transducer in an overpressure range to enhance its sensitivity.

7. Calibrate the transducer with the full length of its lead 

wires attached so that the calibration will include the 

resistance of the leads. Calibrate the transducer at the 

excitation voltage or current to be used during field data 

collection.

8. Calibrate the transducer at a variety of temperatures and 

pressures. Fit a nonlinear, multiple-parameter (pressure 

and temperature) regression equation (second order or 

higher polynomial) to the data used to convert transducer 

output to an equivalent pressure. 

9. Use one channel of the datalogger to monitor a precision 

wire-round resistor. Use this apparent change in resistance to correct for offset and drift in the data logger.

10. Apply power to the transducer for several hours (burn-in 

time) before calibrating and using it.

11. Use proper grounding and shielding techniques.

Because the temperature error commonly is predictable, 

the pressure measurements can be adjusted if the temperature 

is recorded simultaneously. Another method of improving 

system accuracy is to artificially control the temperature of 

the transducer and associated measurement equipment by 

keeping the equipment in a heated enclosure maintained at a 

constant temperature. If the ambient temperature can be kept 

unchanged, then this source of error can be eliminated.

DATA COLLECTION

Because operating a transducer installation (or installations) in the field consists primarily of seven interrelated 

components, an overall field operation plan must be developed 

that integrates all of these components: 

1. The optimum frequency of visits to a site depends on 

the type of instrumentation being used and the accuracy 

required in the study. The use of data loggers without 

supplemental data storage modules or telemetry can limit 

the length of time between field visits.

2. The frequency and types of field checks also are based 

on the type of instrumentation being used and the needs 

of the study. 

3. The data-retrieval method and equipment may dictate, to 

some extent, the frequency of visits. 

4. The data-retrieval method depends, somewhat, on the 

instrumentation and study needs.

5. Field measurements used to verify instrument output 

may dictate the frequency of visits, as well as the method 

and frequency of data retrieval. 

6. Appropriate field documentation is important for transducer calibration and calibration checks, for recording 

equipment malfunctions, for recording changing site 

conditions, and for data computation.

After considering these components, and weighing their 

importance, a strategy for field operation can be implemented. 
32 

Frequency of Visits

Site visits should be scheduled on the basis of the study’s 

needs, stability of the transducer, and storage limitations of 

the recording device. For example, for a study involving 

long-term monitoring of ground-water levels, visits may be 

routinely scheduled at 4- to 8-week intervals. This visitation 

frequency will usually provide enough verification measurements to document transducer drift over time and over a range 

in water level. Visit the site frequently enough so that the 

primary and back-up recording devices will not run out of 

storage memory. On the other hand, a short-term study, such 

as an aquifer test, requires the operator to be on-site continuously to make frequent verification measurements and instrument checks.

Field Checks

During field site visits, check the instrumentation and 

recording system, and verify the operation of the power supply, transducers and other sensors. One simple set of checks 

includes comparing the voltage from the solar panel or AC 

trickle charger to the voltage from the regulator to the battery, 

and comparing the voltage of the power supply to the voltage present at the instrument port on the recording device. 

Inspect the condition of the suspension system, well casing, 

and instrument shelter. If the suspension system shows signs 

of slippage, measure the amount of movement of the mark that 

has occurred and correct the data accordingly. Excessive slip 

can damage the vent tube or cable, or both, which can result in 

faulty or lost data, or damage to the transducer. Damage to the 

well casing can cause a change in the measuring point, or may 

indicate possible damage to the transducer or its cable. Damage to the shelter that could expose the enclosed instrumentation to the weather needs to be repaired as soon as possible. 

Observations of land use, or changes in the general area 

of the site can be important for data analysis, record computation, and data interpretation. These observations could be 

notes on land-use changes, construction of new wells nearby, 

notes of flowmeter readings from production wells, mention of 

known floods or earthquakes that occurred since the last visit, 

or any other item that might have a bearing on data collection. 

Data Recording and Retrieval

The study’s data needs determine the recording frequency for each parameter. Record each parameter or nearly 

simultaneous group of parameters with the date and time to 

prevent many hours of file manipulation and editing as well as 

potential misinterpretation of the location of gaps in the data. 

Battery voltage and temperature are valuable ancillary data. In 

some types of data loggers, failure to maintain the appropriate 

battery voltage can cause permanent loss of the recorded data 

or the data logger program. Excessive shelter temperature, in 

particular, adversely affects the operation of the data logger 

and power supply. 

Electronic data can be retrieved in several ways, both 

remotely and on-site. Remote retrieval or transmission can be 

accomplished with satellite data-collection platforms, radio 

transmitters, and telephone modems. Data can be retrieved 

on-site by using a portable computer with data retrieval 

software, by downloading data from a data logger to a backup 

storage device, or by exchanging a fresh storage device (datastorage module or data “flash card”) for the one at the site, or 

by a combination of these techniques. The fact that the technology in this area is advancing rapidly precludes a detailed 

discussion of these systems in this manual. 

The Ground Water Inspection Sheet (fig. 43) shown in this 

report was designed for multiple well sites and data loggers that 

record multiple channels. Commonly a data logger has two 

types of data storage, temporary and permanent. Temporary 

storage is used when the measuring frequency is greater than the 

recording frequency. For example, the data logger may be reading sensor output every 5 minutes, but recording only hourly 

values. By recording both sets of values, the person servicing 

the well can see a more frequently updated transducer value for 

comparison with a measured water level. For instance, if water 

level were changing quickly, an hourly reading by the data logger would not be adequate for comparison with the actual water 

level measurement made by the field person unless it were made 

just after the hour. Remember that the purpose of making a 

physical measurement is to compare it with the sensor output to 

determine drift or calibration corrections. 

The inspection form also contains a place to record the 

raw sensor output from the data logger, as well as the value 

converted to units of measurement (feet, meters). A project 

may have a policy of recording actual sensor output (mv, psi, 

ma) rather than converting the value to a measurement unit in 

the data logger program itself. There are several advantages 

to this policy when troubleshooting system problems, looking 

at sensor performance, and determining if the sensor is within 

its operating or calibration ranges. This sheet provides a place 

to document the sensor output; then, using the calibration 

equation developed for the specific transducer, document the 

associated converted value. 

Retrieving data while in the field involves a number of 

steps that are performed in a certain order: 

1. Ensure that the sensors and recording devices are operating properly.

2. Use field measurements to verify the sensor output values.

3. Document the visit, all measurements, and any problems 

on a standard ground-water inspection sheet (fig. 43).

4 Retrieve the data and document the process using a field 

inspection form for data retrieval.

5. Review the retrieved data by viewing the file or plotting 

the original data.

6. Recheck the operation of the sensors and recording 

devices prior to departing the site.
 33

Figure 43. Ground-water inspection sheet.

Channel CH # CH # CH # CH # CH # CH # CH # CH #

Well No. Well Well Well Well Well Well Well Well

M.P. used

Hold

Cut

Water Level 

below M.P.

M.P. to Land 

Surface

Water Level 

to Land 

Surface

Mean Time 

(watch)

Logger 

Value

Converted 

Logger Value

Parameter 

Shift

U.S. GEOLOGICAL SURVEY – WATER RESOURCES DIVISION

GROUND WATER INSPECTION SHEET

Site Name: Station ID number: 

State/Local well Numbers: Party 

Date (mm/dd/yy/: / / Julian: / / Watch time: / / EST CST MST PST Daylight UTC (circle)

Data Logger Information:

Date (mm/dd/yy/: / / Julian: / / Time: / / EST CST MST PST Daylight UTC (circle)

Data Logger Type: Serial No. 

Temporary Storage Values: Time: EST CST MST PST Daylight UTC (circle) Data Block No. 

 CH 1 CH 2 CH 3 CH 4 CH 5 CH 6 CH 7

 CH 8 CH 9 CH 10 CH 11 CH 12 CH 13 CH 14

 CH 15 CH 16 CH 17 CH 18 CH 19 CH 20 CH 21

 CH 22 CH 23 CH 24 CH 25 CH 26 CH 27 CH 28

Permanent Storage Values: Time: EST CST MST PST Daylight UTC (circle) Data Block No. 

 CH 1 CH 2 CH 3 CH 4 CH 5 CH 6 CH 7

 CH 8 CH 9 CH 10 CH 11 CH 12 CH 13 CH 14

 CH 15 CH 16 CH 17 CH 18 CH 19 CH 20 CH 21

 CH 22 CH 23 CH 24 CH 25 CH 26 CH 27 CH 28
34 

Verification

Because verifying the sensor output is crucial to determining the validity of recorded data, it must be done routinely 

in the field. Measure ground water levels using a steel tape or 

calibrated electric tape. At a minimum, verify the instrument 

output each time data are retrieved at the site, but verification 

measurements also are recommended during visits when data 

are not retrieved. In either case, the verification measurements should be compared with the output from the site’s 

instrumentation at the time of the measurement. Verification 

measurements, commonly referred to as calibration checks, 

are necessary for troubleshooting, drift determination, and 

data correction during office computation. 

Field verification measurements of ancillary data also 

can be useful in determining data integrity. These data might 

include the output of voltage from the power supply, barometric pressure, shelter temperature, and outside air temperature, 

all of which should be compared with the recorded values. 

These checks serve as an alert to a variety of potential problems and will help prevent loss of data or instrument damage.

Use With Data Loggers

Data may be collected in different formats. Some 

hydrologists prefer to collect raw data from the pressure 

transducer and convert the data in the office. Others prefer 

to collect data in a format identical to the units used during field calibration checks. Another option is to program 

the data logger to provide water-level elevations. Whatever 

the preference, maintaining a record of adjustments to the 

data, whether they occur in the data logger program or in the 

office, is essential. Data loggers can be programmed in a 

multitude of ways that allow project-specific or site-specific 

data formats.

Field Documentation

Use a field note with a checklist of steps and measurements to record all field observations and the current data 

from the data logger. It provides a historical record of field 

activities.

Keep a log for the site in the instrument shelter. The log 

serves as a troubleshooting aid, as a quick historical reference 

to water-level trends, equipment problems, and as a long-term 

record of changes and occurrences at the particular site. At 

each site, keep a printed copy of the data logger program 

being used in the instrument shelter.

In the office, maintain a binder with field information 

similar to that recorded on the site log so that a general historical record is available there and can be referred to before and 

after a field trip. Other information to be included are site 

descriptions and road logs, transducer ratings, programming 

instructions, and phone numbers and addresses of important 

contacts.

DATA PROCESSING

After retrieving the recorded data in the field, the data must 

be processed and loaded into a data base. A recommended 

sequence for processing data is 

1. to archive the raw data, 

2. to convert the data into a usable format, and 

3. to load the formatted data into the National Water

Information System (NWIS) data base. 

The USGS requires that raw data be permanently archived 

prior to any alteration of the data set. Recommendations for 

archiving electronically recorded (and other) data are presented 

in Hubbard (1992) and were established as official policy by 

WRD Memorandum 92.59 with modifications contained in 

WRD Memorandum 99.33 (U.S. Geological Survey, 1992 and 

1999). After the data have been loaded into the data base, they 

can be manipulated appropriately during the records computation process, as described below.

Adjustments

The first step, if necessary, is to convert pressure data 

to water-level data. The second step is to apply corrections 

obtained from field verification measurements. If the record 

is faulty due to instrumentation or other problems, corrections 

usually cannot be applied. In general, a missing or faulty 

record of ground-water level cannot be estimated reliably. 

Four types of corrections can be applied to the record—datum 

corrections, hung-depth corrections, drift corrections, and calibration corrections. Use time proration for the first two types 

of corrections. Apply a datum correction when a change has 

ocurred to the elevation of the measuring point of the well. 

Keep a level summary sheet with the site’s permanent file to 

document reference elevation changes. A hung-depth correction can be applied if the position of the transducer changes 

relative to its original position, due either to purposeful or 

accidental raising or lowering of the transducer in the well, or 

to changes from other causes such as an earthquake. A drift 

correction—usually a linear proration with time—is applied 

to compensate for drift in the transducer’s offset calibration. 

Calibration corrections can be applied either by linear time 

proration from the time of one calibration to the next, or by 

starting and ending a constant correction for a specific period 

of time.

Documentation

The third and final step of record computation requires 

documenting and explaining in a station analysis how all conversion equations and other corrections were applied. The station analysis (fig. 44), written periodically after the final record 

is produced, should be kept with the permanent data file, along 

with the record computations, field notes, archival and data 

conversion notes, data tables, and other pertinent information.
 35

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Figure 44. Ground-water record station analysis.
36 

Data processing requires a systematic, well-planned 

process fully integrated with field procedures. USGS policy 

for archiving data should be followed. The raw data must be 

corrected systematically and consistently, then documented 

in the station analysis that will be included with the permanent record. 

APPLICATIONS TO WATERRESOURCES INVESTIGATIONS

Specific field applications of submersible pressure transducer and data logger systems are discussed in this section. 

Although submersible pressure transducers are used primarily for the collection of water-level measurements in ground 

water, they also can be used in surface-water and unsaturatedzone settings.

Ground-Water Monitoring

Submersible pressure transducers can be used for longterm and short-term applications. This subsection discusses 

both applications. A third discussion on reducing well-bore 

storage is presented so that the user can apply this technique 

to reduce the effective diameter of wells during slug tests or 

aquifer tests. 

Long-Term Monitoring

Many hydrologic investigations require continual monitoring (over periods of weeks to years) of water levels in wells. 

Examples of such studies include monitoring water levels 

for indication of earth tides (Galloway and Rojstaczer, 1988; 

Hsieh and others, 1987), indication of earthquakes (Roeloffs 

and others, 1989; Cooper and others, 1965), determination of 

temporal variation in vertical or horizontal hydraulic gradients 

(Garber and Koopman, 1968; Rosenberry, 1990), determination of timing and magnitude of recharge to ground water 

following precipitation events (Novakowski and Gillham, 

1988; Rosenberry, 1990), and monitoring of pump-and-treat 

operations at ground-water reclamation sites. For many studies, even if continual data collection is not necessary, it is cost 

effective to monitor water-level fluctuations in wells with a 

sensor rather than using human resources to collect discrete 

measurements.

Submersible pressure transducers have long been used 

for monitoring water-level fluctuations in wells (Shuter and 

Johnson, 1961; Garber and Koopman, 1968). Buried in the 

soil, these devices also have been used for decades to monitor 

pressure heads. Sensors used in this application have historically been called the “Casagrande type” pressure transducer, 

commonly used to monitor pressure heads in and around 

dams. While other automated water-level sensor systems also 

can provide continual water-level data in wells, submersible 

pressure transducers are particularly well suited for some 

applications. Typically small, and requiring little maintenance, because they are immersed in water their environmental 

conditions are relatively stable. Some examples of applications in which submersible pressure transducers are particularly well suited are listed below:

1. Submersible pressure transducers can be selected to 

monitor a small or large range of expected water-level 

conditions. Transducers designed to measure a small 

pressure range can monitor stage changes of 10 ft or 

less with a very high degree of resolution and accuracy. 

However, higher range pressure transducers can monitor 

water level changes on the order of 300 ft with little loss 

of resolution or accuracy. Pressure transducers are well 

suited when large and sometimes rapid stage changes 

are expected, such as monitoring head changes in karst 

terrain or production wells or monitoring pressure pulses 

associated with earthquakes.

2. Submersible pressure transducers can be used in nonvertical or irregular wells when other systems could not 

operate effectively. For a non-vertical well, a properly 

calibrated pressure transducer will indicate changes in 

vertical head in the well, requiring no adjustment to the 

data, whereas data from a float installed in the same well 

would require adjustment to compensate for the well’s 

non-vertical orientation. Also, severe irregularities or 

deviations in the bore of a well could render acousticvelocity devices or float mechanisms inoperative, while 

data from a pressure transducer would not be affected.

3. Submersible pressure transducers are well suited for data 

collection in severe environmental conditions, such as 

arctic or low-latitude desert climates. The relative stability of ground-water temperature provides a much more 

suitable environment for submersible pressure transducers than for sensors that are mounted above ground or 

inside a well but above the water table. During freezing 

conditions, other types of sensors mounted to the top 

of a well can be disabled by freezing of water that has 

condensed on the sensor (Rosenberry, 1990). Not only is 

the submersible pressure transducer usually not exposed 

to such extreme temperatures, if the water level in the 

well is shallow enough to freeze, the pressure transducer 

can continue to register pressure fluctuations below the 

ice lens. Relic ice lenses still frozen to the side of a well 

can hinder the operation of such sensors.

4. In flowing artesian wells (wells with potentiometric 

heads above land surface) a submersible pressure transducer can provide potentiometric-head data; it is especially well suited to provide data when the potentiometric head fluctuates both above and below land surface. If 

potentiometric head rises to the point where a standpipe 

is impractical, or if heads frequently drop below land 

surface, a submersible pressure transducer may be the 

only practical option for providing continuous potentiometric-head data.
 37

5. Wells with a depth to water greater than 300 ft present 

special problems for most submersible pressure transducers. Cable or line stretch, thermal expansion, vent-tube 

blockage, and signal loss can introduce significant errors 

in deep wells or where sensors are located far from a logging device. O’Brien (1993) noted that voltage problems 

caused by lead lengths of up to 5,000 ft, and blocked 

vent tubes, led to problems when monitoring water-level 

fluctuations in deep wells at Yucca Mountain, Nevada. 

Well-bore deviation, a problem common to deep wells, is 

magnified by the depth to water. Submersible pressure 

transducer models capable of making an analog to digital 

conversion before transmitting the signal up the well to 

the data logger can overcome many of these problems.

6. To mitigate problems associated with hydraulic lag time, 

small-diameter piezometers commonly are installed in 

wells drilled in geologic materials with low hydraulic 

conductivity. Although other types of sensors have been 

used for monitoring water-level fluctuations in smalldiameter wells (Lovell and others, 1978), most sensors 

are too large to fit inside wells with a diameter much 

smaller than about 1 in. Vibrating wire pressure transducers small enough to fit inside wells as small as 0.5 in. 

can provide reliable data when some other sensor types 

cannot. 

7. Water levels in wells installed in easily compressed 

materials, such as those in a salt marsh or a fen, can be 

altered by a person walking on the surface so that the 

water levels recorded during site visits are not representative of a site’s long-term conditions. Frequently 

these wells are of small diameter to minimize hydraulic lag time associated with low hydraulic conductivity 

materials. Submersible pressure transducers have been 

used to provide unaltered hydraulic-head data during 

intervals between site visits (Schaefer and Hemond, 

1986).

8. Submersible pressure transducers have been used to 

monitor pore pressure at earth-filled dams and in slopestability studies. Buried transducers can provide porepressure data without the aid of a well. Carpenter and 

others (1995) buried submersible pressure transducers 

in sandbars to monitor pore-pressure fluctuations in 

response to significant stage changes of the Colorado 

River in the Grand Canyon. The sensors were installed 

in areas where wells would not have been feasible 

because the river periodically inundated the sandbar. 

Figure 45. Drain-cleaner packer.

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9. Submersible pressure transducers are convenient when 

making multiple-zone pressure-head measurements in 

open boreholes containing packers that isolate intervals of the borehole. Transducers can be connected to 

threaded tubes that pass through the packers and register 

pressure head of isolated intervals without requiring the 

transducer to be located in those intervals (Hsieh, 1993; 

Robson, 1993). This type of connection can reduce 

complexity, borehole clutter, and cost.

As shown in the previous discussion, submersible pressure transducers are well suited for many hydrologic applications; however, their use for long-term monitoring of water 

levels occasionally can lead to errors if data are not corroborated. The convenience and low maintenance of submersible pressure transducers can lead to long intervals between 

calibration checks and overconfidence in the reliability of the 

sensor’s data. If checks on the calibration of sensors are not 

made, data may be erroneous to the point of leading to incorrect hydrologic interpretations. A study of vertical hydraulic head gradients at a well nest in New Hampshire showed 

that uncorrected data from submersible pressure transducers 

resulted in an interpretation of reversals in vertical hydraulic-head gradients when none actually occurred (Rosenberry, 

1990). In the New Hampshire study, linear adjustment of data 

based on monthly check measurements would have led to the 

conclusion that additional water-table fluctuations of up to 

0.17 ft occurred when weekly check measurements indicated 

that sensor drift actually was responsible for those interpreted 

water-level fluctuations. 

Gage pressure transducers usually are used to measure 

pressure in a water body open to the atmosphere, whereas 

absolute transducers are usually used as barometers and in 

sealed environments such as 

below packers. The user may 

wish to substitute absolute 

transducers for gage transducers to eliminate the need for 

vented cable, especially to 

multiple transducers in close 

proximity, connected to one 

data logger. A barometer, 

which can be an identical inexpensive absolute transducer, 

also must be operated. When 

using an absolute transducer in 

a gage transducer application, 

subtract the barometric record 

from the water-level record to 

get submergence. Three redundant barometers can be used in 

conjunction with many absolute 

transducers measuring water 

levels. Because the adjusted 

record is the difference 

between two records, noise 

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Figure 47. Surface-water monitoring installation.
 39

and drift that are not common to both transducers may 

increase by as much as a factor of two.

An absolute transducer can also be used instead of a 

gage transducer to measure changes in wells in aquifers with 

barometric efficiencies close to 100 percent. After verifying 

that the barometric efficiency is indeed close to 100 percent, 

the original record from the absolute transducer is acceptable 

as the “barometrically adjusted” record. 

Short-Term Monitoring

Submersible pressure transducers have been used extensively for monitoring water-level fluctuations during singlewell and multiple-well aquifer and slug tests. Before the use 

of automated sensors, aquifer tests were labor intensive, and 

early drawdown in the pumped well was not easily observed. 

Similarly, for single-well slug tests in sandy material, the early 

portion of the recovery commonly went unrecorded simply 

because it was not possible to get water-level measurements 

that were only seconds apart. Using submersible pressure 

transducers has reduced labor costs and has provided the 

opportunity to collect frequent data during the early portion of 

aquifer and slug tests. When combined with a programmable 

data logger, the pressure transducer can supply data frequently 

during the early portion of the test and, less frequently, as the 

test progresses and the recovery rate slows. For clean, coarse 

sand, when the recovery of a slug test can be completed in less 

than half a minute, the fast response of many types of submersible pressure transducers can allow measurement with a 

sampling interval of half a second or less.

The pressure transducer used for aquifer tests should be 

capable of reliably measuring the expected range of waterlevel fluctuations. For example, for an aquifer test, the 

pressure transducer in the pumped well should be capable of 

monitoring head changes much larger than is necessary for 

transducers installed in observation wells, where changes in 

water level are smaller and where greater accuracy may be 

desired. Similarly, for most single-well slug tests, a pressure 

transducer with a small pressure-sensitive range (such as 0-5 

psi) is adequate.

Reducing Well-Bore Storage

Due to the movement of water from the well into the formation, the water level in a well or piezometer can lag behind 

head changes in the geologic formation. Typical situations in 

which this well-bore storage effect is most significant include 

slug tests, early time in drawdown or recovery during an aquifer test, and wells in low-permeability materials. Only in slug 

or bailing tests, in which a slug or water is rapidly introduced 

into or withdrawn from the well, is the effect of reducing wellbore storage undesirable. In fact, analyzing the decay of the 

residual water level in a well to determine hydraulic conductivity is the purpose of a slug test (Lohman, 1972). 

Packers that seal parts of wells to prevent flow within the 

borehole or to isolate zones for special tests sometimes are 

used to minimize the effects of well-bore storage in aquifer 

tests. Well-bore storage is, in effect, an incremental slug test 

superimposed upon the water-level fluctuation of interest 

during the test. An inexpensive packer that can be made in 

the field from materials from a hardware store or lumber yard 

(fig. 45) encloses the transducer and seals the piezometer. 

The appropriate transducer for this application is an absolute 

device because the transducer is sealed into a zone without 

access to atmospheric reference through the well. Drain 

cleaners that expand to more than 4 in. are available; plumbing 

supply stores carry test seals of various kinds that can be made 

into packers. Straddle packers also can be assembled by using 

soldered copper tubing through brass fittings.

Small water-level differences between intervals separated 

by packers in a well can be measured with expensive, highaccuracy differential transducers that allow water in both ports 

or measured as the difference between outputs of expensive 

high-accuracy absolute transducers. Alternatively, low-cost 

differential transducers that allow water in both ports can be 

used. In one case, pressure differences of a fraction of an inch 

of water were measured between isolated intervals in a well 

at a submergence of more than 180 ft using a differential transducer with a full-scale range of about 10 ft of water. There 

was a large zero shift, but after establishing a new zero offset, 

valuable data were obtained at the beginning of the pumping 

period in spite of the fact that the pressure from submergence 

exceeded the overpressure specification by a factor of four and 

exceeded the specified range by a factor of 17. 

A drop pipe that occupies much of the cross-sectional 

area of a well can be used to reduce storage in a well bore (fig. 

46). Advantages of this design include the ability to measure 

the water level in the well without removing the pipe, and the 

ability of the well to de-gas. For wells deeper than is practical for a standpipe from the surface, the pipe can be weighted, 

sealed at the top, and suspended by the transducer cable or a 

stainless-steel cable. To prevent a change in well-bore storage 

with changes in water level, the pipe must extend above and 

below possible water-level fluctuations. The appropriate transducer for this application is a differential or vented device, 

because the transducer is in water open to the atmosphere. 

Surface-Water Monitoring

Submersible pressure transducers increasingly are being 

used for monitoring stage at surface- water gages. This is due 

to their ease of installation and maintenance, relatively low 

cost, and minimal requirements for shelter size, factors which 

provide more versatility in locating the instrumentation. For 

surface-water applications, a desirable transducer 

1. is accurate over a large range of stage and temperature, 

2. maintains its calibration,

3. is easy to install and remove for repair, and 

4. is not prone to failure induced by water leakage or voltage surges. 
40 

No submersible transducer currently is satisfactory for 

long-term, unattended use in surface-water stage monitoring 

under all environmental conditions. Because of their siting 

versatility and their low cost for short-term surface-water 

studies, submersible pressure transducers will continue to be 

used in surface-water monitoring programs. The following 

basic installation guidelines and limitations are intended to 

assist in determining the suitability of a submersible pressure 

transducer for a given surface-water monitoring program.

Installation

An important consideration in planning for the use of 

a submersible transducer at a surface water installation is to 

minimize the difficulty of servicing the transducer during 

calibration and repair. Gaging station designs should enable 

one person to remove and re-install the transducer.

Two advantages of using submersible pressure transducers for monitoring surface-water stage are the minimal space 

requirements of the data acquisition system, and lack of the 

need to provide a pressurized gas-purge system and pressure 

tank (Lamb, 1994). This allows the use of a much smaller 

weatherproof enclosure to house the data logger, battery and 

data relay equipment. If necessary, the entire sensing and 

recording system can be contained in one or two small environmental enclosures. Attach the transducer cable to the data 

logger by passing it through a small-diameter weatherproof 

strain-relief fitting in the wall of the enclosure. 

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These small enclosures can be located in places where 

larger shelters needed for pressurized gas-purge systems 

cannot. Some types of submersible transducers can be used 

with data and vent tube cables that are as long as 1000 ft, thus 

allowing shelter placement above the reach of flood waters. 

Smaller shelters can just as easily be mounted on a bridge 

deck or railing. They can be easily installed for short-term 

monitoring, then moved to another location. For example, 

portable installations were used successfully during the 1993 

mid-western floods, which allowed the USGS to continue 

monitoring the rise of floodwaters that had inundated original 

fixed gaging stations (H. Hauck, U.S. Geological Survey, oral 

comm., 1998). These systems also can be readily adapted to 

transmit data by radio or satellite. 

Two types of surface-water installations can best use submersible pressure transducers to monitor the stage of streams: 

the first type of installation uses a transducer submerged 

directly in the stream at the end of a protective steel conduit 

(fig. 47); the second type uses a submersible transducer placed 

in a stilling well. Each type of installation has benefits and 

drawbacks. A stilling well is most desirable in terms of access 

for servicing, maintenance and replacement, and for damage 

protection during high flows and freezing. However, the operation of stilling wells is subject to the safety regulations applicable to confined spaces. A steel conduit is most desirable if the 

recording instruments must be located a substantial distance 

from the stream, either horizontally or vertically. In either use, 

the transducer needs to be installed securely so that it does not 

move relative to gage datum and so that the transducer cable is 

protected. As in ground-water applications, care must be taken 

during installation and operation that moisture or other material 

does not enter the gage transducer’s vent tube. During installation, this can be accomplished by wrapping the end of the 

vent tube with waterproof tape while pulling the tube through 

Figure 48. Soil-water tensiometer.

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Figure 49. Field tensiometer installation.
 41

the steel conduit. After installation, a desiccating system that 

includes a thin, flexible moisture barrier should be used to 

keep the air in the vent tube dry, yet still allow the transducer 

to register small changes in atmospheric pressure.

When they are installed in a stilling well, submersible 

pressure transducers are mounted vertically. The most common type of surface-water installation, however, uses a steel 

conduit anchored down a sloped bank, which leaves the transducer oriented out of vertical. If the transducer is oriented out 

of the vertical, care must be used after servicing to reposition 

the transducer in precisely the same orientation. Calibration 

must then be checked against an independent measurement 

of stage. Static tubes and crest-stage-gage bottom caps can 

be used to eliminate velocity-head effects on the transducer 

(Kaehlre and Thibodeaux, 1994). A static tube, a device used 

to minimize the effects of turbulent flow, is a pipe with holes 

drilled into it at regular intervals. The transducer is secured at 

the end of the steel conduit, the static tube is placed over the 

transducer, threaded onto the end of the steel conduit, and a 

cap secured to the end of the static tube.

Other methods include installing transducers in a series 

of crest-stage gages or in a conduit with the transducer 

attached to a flexible handle that can be removed for easy 

cleaning of the transducer. Both of these methods provide 

a fixed resting point for the transducer at a known gage 

elevation. 

As with any stage sensing instrument, records must be 

maintained to help evaluate the accuracy and reliability of 

transducers used in surface-water monitoring (Kirby, 1994). 

Field observations necessary for comparison with the output 

from the transducer over the range of stage recorded should 

include water-level readings obtained from an established reference gage; and peak stages registered by crest-stage-gages 

and outside high water marks. Elevations of the reference 

and principal gages should be verified periodically by level 

surveys run from established reference or bench marks using 

approved USGS methods (Kennedy, 1990). If necessary, use 

comparisons between the field observations and the recorded 

transducer values to adjust the data. Check and re-calibrate 

the transducer when field comparisons show a substantial 

departure from the reference or principal gage readings. 

Calibration and re-calibration should be done over the range 

that reflecting the expected range of stage. Refer to sections 

in this report on calibration and field calibration for specific 

procedures.

Limitations

Instrument drift and the influence of changes in barometric pressure, temperature, and humidity on the long-term operation of transducers have been discussed earlier in this report. 

Another concern for long-term deployment is that transducers 

have, by design, a multiconductor cable connecting the transducer assembly to a data logger, which may provide a path for 

voltage surges from lightning strikes near the water.

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Figure 51. Housing for cable splice.

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Transducers and other electronic sensors placed in the 

water and having a common electrical connection to a data logger may cause a ground loop to form between the various sensors. If the data logger is attached to an earth ground and readings are erratic for one or more of the attached sensors, a ground 

loop should be suspected. A possible solution is to remove the 

earth ground from the data logger, but this may make the data 

logger more vulnerable to damage from a voltage surge. 

Variable water temperature in streams can be a problem for 

submersible transducers. Many transducers are not temperature 

compensated, and those that are may not have sufficiently accurate or responsive temperature-sensing capabilities. In any case, 

submersible pressure transducers should not be operated where 

water will freeze in the port, as that will destroy most pressure 

transducers.

Ancillary Data

As in ground-water applications, certain types of recorded 

ancillary data can be very beneficial. Monitoring the changes 

in transducer and data logger temperature can help determine 

when temperatures are outside the instrument’s operating limits. 

Because surface-water temperatures fluctuate, in contrast to 

ground-water temperatures (which usually stay nearly constant), 

it is important to monitor water temperature in surface-water 

data collection efforts because temperature compensation affects 

the transducer’s performance. Recording the battery voltage of 

the data logger also can help ascertain the integrity of the power 

supply, whether it be a solar charged battery system, a battery on 

its own, or AC power with battery backup. Review the battery 

voltage data to detect power supply problems before they cause 

the loss of data. Also, record the barometric pressure. Absolute 

(non-vented) transducers must have their output compensated for 

barometric pressure in order to record or compute the water level 

elevation. Gage (vented) transducers are self-compensating as 

long as the inside of the vent tube remains dry and unobstructed. 

In the event these conditions are not met, comparison with the 

recorded barometric pressure can aid in determining if the data 

are flawed. 

Unsaturated-Zone Monitoring

Pore space within the unsaturated zone is occupied by 

either water or air. Water is held under tension (i.e., pressure is 

less than atmospheric) by the capillary attraction of small pores. 

Tensiometers are used to measure liquid water pressure under 

unsaturated conditions. Air pressures in the unsaturated zone, 

which may be greater than or less than atmospheric pressure, 

are measured with soil-gas piezometers. Pressure transducers 

can be used in conjunction with either of these devices. 

Tensiometers

Because pressure head of liquid water above the water 

table (hp) generally is less than atmospheric, transducers used 

with tensiometers must be capable of measuring negative pressure or vacuum. Tensiometers are functional when hp ranges 

from 0 to about -400 in. of water gage pressure. Principles of 

tensiometer operation and design are described in Gardner and 

others (1922), Cassel and Klute (1986), and Stannard (1986). 

A typical design is shown in figure 48. The body of the tensiometer is a PVC tube filled with water, the bottom of which 

contains a porous ceramic cup inserted into the soil. The 

ceramic cup forms a continuum with the soil pores, allowing 

soil-water pressures (suction) to be transmitted to the tensiometer. The top end of the tensiometer is fitted with some sort 

of pressure-sensing device. Original tensiometer designs used 

mercury or water manometers or vacuum gauges, but gage or 

absolute pressure transducers, allowing automatic recording, 

are now more commonly used (for example, Yeh and others, 1986). Tensiometers usually are installed vertically from 

land surface, although constructing a trench or tunnel permits 

horizontal installation.

Measurements of hp at a number of depths in the unsaturated zone are desirable in order to measure the magnitude of 

the hydraulic gradient and infer the direction of water movement. A typical field site might be equipped with 10 or more 

tensiometers. Switchable valve systems have been developed 

that scan all tensiometers with a single transducer. Yeh and 

others (1986) used a hypodermic needle attached to a pressure 

transducer to manually record several hundred tensiometer readings in a short period of time, the needle being inserted through 

a rubber stopper on each tensiometer. When inexpensive pressure transducers became available, it became feasible to fit each 

tensiometer with a separate transducer, thus avoiding complicated switching valves and allowing automatic recording. 

When selecting the pressure range of a transducer, keep 

in mind that the pressure (P) recorded by the differential-pressure transducer will record the total head (ht) at the measurement depth of the tensiometer (Z1

, for example). That is, 

P = ht and ht accounts both for pressure head and gravity head, 

ht = hp - Z1. So the pressure head at Z1 is calculated as 

hp = ht + Z1. If the water table were at Z1, then by definition 

hp would be 0 and P would equal - Z1. The appropriate pressure range for transducers depends on the specific application: 

a range of 0 to -400 in. of water is sufficient for most field 

studies, whereas a smaller range allows for more accuracy and 

may be desirable for laboratory experiments or detailed field 

studies. In selecting the pressure range, the gravity component 

Z1 (due to the hanging water column from the transducer to 

the ceramic cup) must be considered. If the anticipated range 

of pressure heads for a study is between 0 and -5 in., but Z1 is 

20 in., then the transducer must be capable of sensing pressures in the range of -20 to -25 in. The need to account for Z1

limits the depths from land surface to which tensiometers can 

be installed. Some researchers avoid this limitation by burying both the transducer and the ceramic cup. In other cases 

trenches, tunnels, or caissons have been constructed to allow 

horizontal installation of tensiometers. This construction permits deeper installation and minimizes Z1

 so that transducers 

with smaller pressure ranges can be employed.
 43

Because the water within a tensiometer is under a vacuum, 

air tends to slowly come out of solution and accumulate within 

it. Most tensiometers are equipped with a flushing valve to 

permit periodic removal of that air. The inability to remove 

the air from buried tensiometers may severely limit their use. 

Systems of tensiometers with transducers should be designed to 

minimize accumulated air, which expands and contracts as its 

temperature rises and falls, causing pressure head oscillations 

that are correlated with temperature in the recorded data. Ideally, tensiometers and pressure transducers should be insulated 

and shielded from direct solar radiation. In field studies, this 

shielding may be accomplished by enclosing the top of the tensiometer and transducer in a container that can be buried a few 

inches below ground, as shown in Figure 49. 

Soil-Gas Piezometers

Measurements of soil-gas pressures may be made for 

determining barometric efficiency of geologic units (Weeks, 

1978) or for monitoring of pneumatic tests (Baehr and Hult, 

1991). Both operations are designed for calculating the air 

permeability of partially saturated porous media. Data from 

soil-gas piezometers constructed in the unsaturated zone are 

obtained by drilling a borehole and installing a small diameter 

tube (the diameter is usually not greater than 0.25 in.) open at 

the bottom and extending from the measurement point to land 

surface. After sealing the borehole, attach a pressure transducer 

to the top of the tube. Determination of barometric efficiency 

relies on measuring changes in barometric pressure and the 

time it takes for those changes to be propagated down to the 

piezometer, and analysis requires a barometer (an absolute-pressure transducer) in addition to the gage or absolute transducer 

attached to the tube. Gage transducers with a pressure range of 

plus and minus a few inches of water are usually preferred for 

measuring soil-gas pressures because they encompass the typical range in diurnal barometric fluctuations. Either absolute or 

gage transducers with a wider range of pressure may be used for 

monitoring pneumatic tests. 

COMMON PROBLEMS AND SOLUTIONS

Submersible pressure transducers generally require minimal maintenance, but problems can arise if they are not checked 

periodically. The following is a list of common problems 

encountered by the authors, and some suggested solutions to 

these problems. Occasionally, a problem can be solved only by 

replacing the transducer, but more commonly problems can be 

corrected in the field.

Leakage

Failure of submerged transducers commonly is caused by 

water leaking into the housing. Transducer leakage can appear 

in the data as an unexplained step or as measurements that 

cease, move out of range, or fluctuate wildly. Techniques to 

prevent leakage include the use of welded seals, O-ring seals, 

potted electronics and soldered contacts, sealed strain-relief 

fittings, and rubberized boots covering connections between 

the cable and the transducer’s housing. Any of these components, however, can fail. Multiple layers of waterproofing, 

which can be user tested, can prevent leakage, failure, and lost 

data (fig. 50). A double housing with an intervening chamber 

can be left pressurized so that if leakage occurs, dry air leaks 

into the water. Pressurize the chamber only slightly above 

the anticipated water pressure to prevent rupturing the seals. 

The chamber with strain reliefs shown in figure 50 has been 

tested to a pressure of 115 ft of water. In practice, the residual 

air pressure sometimes decays, allowing a small amount of 

leakage into the chamber. The inner housing then prevents 

that small amount of water from penetrating to the transducer. 

Chambers have been used without any leakage for more than 

five years. During one application in which two commercial 

transducers failed after only one month each in service, encasing the third commercial transducer in just the inner chamber 

produced 14 years of troublefree data.

Commercial vented transducer cable must commonly be 

spliced. A waterproof splice can be made using strain reliefs 

and PVC pipe fittings (fig. 51). Connect the vent tubes with 

tubing of about the same diameter so that the cavity does not 

act as a pressure accumulator, restricting transmission of barometric pressure changes. For added security, the cable splice 

can be doubled in the same way as the pressurized housing 

shown in figure 50. Besides their primary functions of reducing well-bore storage, the drain-cleaner packer (fig. 45) and the 

drop pipe (fig. 46) both act as additional waterproof housings 

for pressure transducers. In waterproofing transducers, user 

experimentation and testing are essential. Providing additional 

waterproofing can greatly reduce the incidence of lost data.

Open and Short Circuits

 The most frequent problems encountered when collecting water-level data with a pressure transducer and data 

logger include open and short circuits. An open circuit can 

result in data floating erratically or resting at zero or the offset 

value. A short commonly results in data values that indicate 

an over-range condition for the data logger. Solutions are to 

check the connections of the wires to the data logger wiring 

panel (where shorts most commonly occur) and to check the 

wire for cuts, kinks, and abrasions. Tinning leads reduces the 

chances of short circuits from unattached wire strands and also 

prevents or reduces noise. Find short circuits by testing the 

resistance between the drain or ground wire and each excitation and sensing lead. When resistance is not infinite, there is 

a short. Find open circuits by measuring the resistance across 

the excitation lead pair and the sensing lead pair. It should 

be equal to the resistance across the bridge to which the lead 

or excitation pair is attached, commonly about 100 ohms for 
44 

metal strain-gage transducers and 1,000 to 5,000 ohms for silicon strain-gage transducers.

Grounding Problems

Excessive levels of noise in the data sometimes can be 

attributed to one or more loose ground wires. Securely attach 

ground wires to either a ground terminal on the data logger, or to 

a grounding rod. Take care to maintain only one ground in order 

to avoid problems with ground loops, which could result in considerable noise. In some applications, the pressure transducer 

and data logger are not grounded, in which case the installation 

is termed a ‘floating system.’ In this situation, connect ground 

wires only to the ground terminal on the data logger.

Diaphragm Failure

Failure of the pressure-transducer diaphragm commonly is 

caused by transmission of pressure waves through the water column, sometimes referred to as water hammer. Strong pressure 

waves can severely over-pressure the transducer and rupture the 

diaphragm, resulting in sudden loss of data. The diaphragm 

also can fail if the transducer is dropped onto a hard surface, 

or if it is rapidly dropped to the water surface while lowering 

the device inside a well. Water freezing in the transducer can 

likewise rupture the diaphragm. Diaphragm failure requires 

replacing the transducer.

Power-Supply Failure

An interruption of the power supply to most submersible 

pressure transducers will result in the output changing to zero 

or to the offset value entered into the data logger. The solution 

is to resupply power to the transducer. Commonly, if power 

to the data logger is lost, data already collected and stored in 

random access memory (RAM) in the data logger are lost. No 

additional data will be collected until power is restored and the 

data-collection program is reentered. A partial remedy is to 

install a backup data-storage device, or use a data logger with 

a nonvolatile memory, to retain data already recorded as well 

as the data logger program. A charging system (solar or AC 

trickle) can help prevent power loss.

Data logger Channel Failure

The internal circuitry in the data logger fails infrequently. 

However, this problem is difficult to diagnose because an open 

circuit in the wiring between the data logger and the transducer 

can have the same effect on the data. If a channel in the data 

logger fails, output values drift erratically. Occasionally, the 

data logger averages this erratic drift to a value similar to the 

expected value, making the failure difficult to notice. One solution is to reconnect the sensor to a different, but still functioning, channel on the data logger; another is to replace the data 

logger. Occasionally the problem is created by contaminants 

(such as oil, water, or perspiration) that create shorts between 

data logger channels, in which case a solution is to disassemble and clean the wiring panel on the data logger.

The data logger’s wiring panel also can become corroded 

after extended operation in humid conditions. Corrosion can 

cause an intermittent open circuit between the wire and the 

wiring panel, causing intermittent periods of erroneous data. 

The solution requires cleaning the connections, and changing 

the environment in which the data logger is housed. Operation 

in a drier environment (for example, enclosing the data logger 

in a low-humidity container or frequently replacing the desiccant) prevents corrosion-related problems. Fumes released 

from a battery enclosed in the data logger’s housing also can 

cause corrosion. To avoid this problem, house the battery in a 

separate location from the data logger.

Voltage Surges 

Voltage surges (transients) can cause a multitude of 

problems, ranging from alteration of the data logger program 

or alteration of the data logger clock, to complete failure of the 

data logger and pressure transducer. A transient current from 

a nearby lightning strike can corrupt data without harming 

either the data logger or the pressure transducer. One solution 

is to turn off the data logger, resupply power, and then reenter 

the program, but proper grounding will solve most problems 

associated with transient voltage surges. A direct lightning 

strike, however, will probably destroy both data logger and the 

pressure transducer. Voltage surges coming through AC power 

lines or telephone lines can damage or destroy the transducer, 

the data logger and other connected electronic equipment. 

Surge protection devices usually are placed between these 

incoming lines and the data logger, but they are not normally 

used between the transducer and the data logger. 

Faulty Shielding

Faulty shielding can allow electromagnetic coupling to 

corrupt the signal from the sensor to the data logger. Occasionally this is caused by cuts or kinks in the transducer cable, 

but more frequently by long, exposed, unshielded wire near 

the data logger. These long wires increase the signal noise 

sent to the data logger. One solution is to minimize the length 

of unshielded wire. A second solution is to change the program in the data logger to increase the signal or the excitation 

from the data logger, effectively increasing the signal-to-noise 

ratio. A third solution is to increase signal integration time 

during the measurement cycle.

Over-Range Problem

An over-range problem develops when the signal is 

increased beyond the anticipated maximum value. Many 
 45

data loggers can be programmed to accept different ranges of 

sensor output. Sensor output rising beyond the anticipated 

maximum output programmed into the data logger creates 

an over-range situation. Over-range problems also can occur 

from submerging the pressure transducer too deeply (overpressuring), so that the transducer is subjected to pressures exceeding its maximum. Even if the pressure transducer is situated 

correctly, unexpected extreme water-level rises can exceed the 

anticipated maximum value.

Occasionally data loggers are not capable of measuring 

the entire voltage range that a transducer can output, thereby 

reducing the range of water-level measurements that can be 

sensed. Possible solutions are to reprogram the data logger 

to accept a wider range of output from the transducer, or to 

program the data logger to simultaneously log output from the 

transducer on multiple input ranges during the same measurement scan. The latter solution retains resolution while increasing the range of input signal. 

Solutions to overpressure problems are to raise the pressure transducer, or use a different model of transducer capable 

of measuring a wider range of pressure fluctuations. In some 

cases, it may be possible to continue operating the transducer 

as long as the overpressure is not too extreme. When using 

this solution, the investigator should extend the calibration 

range to include these over-pressure conditions.

SELECTED REFERENCES 

Aronson, Milton H., 1976, Low-level measurements, 

Part 1 — Noise: Pittsburgh, Pa., Measurements and Data 

Corporation, 16 p.

Aronson, Milton H., 1976, Low-level measurements, 

Part 2 — Magnetic Shielding: Pittsburgh, Pa., Measurements 

and Data Corporation, 16 p.

Aronson, Milton H., 1976, Low-level measurements, 

Part 3 — Electrostatic Shielding: Pittsburgh, Pa., Measurements and Data Corporation, 16 p. 

Aronson, Milton H., Low-level measurements, 

Part 4 — Shields and Ground Loops: Pittsburgh, Pa., Measurements and Data Corporation, 16 p. 

Baehr, A.L., and Hult, M.F., 1991, Evaluation of unsaturated 

zone air permeability through pneumatic tests: Water 

Resources Research, Vol 27, no. 10, pp 2605–2617.

Bell & Howell, 1983, Pressure transducer handbook: Pasadena, Calif., Division of CEC Instruments, 143 p.

Benedict, Robert P., 1984, Fundamentals of temperature, pressure, and flow measurements: New York, John Wiley and 

Sons, 532 p. 

Boucher, Michelle S., 1994, Precision and accuracy of manual 

water-level measurements taken in the Yucca Mountain 

area, Nye County, Nevada: U.S. Geological Survey WaterResources Investigations WRI 93-4025, 18p.

Carpenter, M.C., 1994, High-accuracy measurements with 

inexpensive pressure sensors— assembly, calibration, and 

use; in Hollett, K.J., Wilbourn, S.L., and Latkovich, V.J., 

compilers, Proceedings of a U.S. Geological Survey Workshop on the Application and Needs of Submersible Pressure 

Sensors, Denver, Colorado, June 7–10, 1994: U.S. Geological Survey Open-File Report 94-531, p.5–9. 

Carpenter, M.C., Carruth, R.L., Fink, J.B., Boling, J.K., 

and Cluer, B.L., 1995, Hydrogeology and deformation of 

sandbars in response to fluctuations in flow of the Colorado 

River in the Grand Canyon, Arizona: U.S. Geological Survey Water-Resources Investigations Report 95– 4010, 16 p. 

Carpenter, M.C., Hanson, R.T., and Riley, F.S., 1987, An 

averaging temperature sensor to correct measurements of 

aquifer compaction for interaquifer borehole flow: American Geophysical Union Transactions, v. 68, no. 44, p. 1300.

Cassel, D.K., and Klute, Arnold, 1986, Water potential: tensiometry, in Klute, Arnold, ed. Methods of Soil Analysis: 

Part 1—Physical and Mineralogical Methods, American 

Society of Agronomy, Madison, WI, 1188 p.

CEC Instruments, no date, Pressure Transducer Handbook: 

Pasadena, Calif., CEC Instruments, 150 p.

Cooper Jr., H.H., Bredehoeft, J.D., Papadopulos, I.S., and 

Bennett, R.R., 1965, The response of well-aquifer systems 

to seismic waves: Journal of Geophysical Research, v. 70, 

no. 16, p. 3915–3926.

Data Instruments, Inc., 1981, Pressure Transducer Applications: Lexington, Mass., Data Instruments, Inc., 40p.

Freeman, L.A., 1996, Time-series ground-water-level and 

aquifer-system compaction data, Edwards Air Force Base, 

Antelope Valley, California, January 1991 through September 1993: U.S. Geological Survey Open-File Report 

96–186, 32 p.

Galloway, D. and Rojstaczer, S., 1988, Analysis of the frequency response of water levels in wells to earth tides and 

atmospheric loading, in Hitchon B. and Bachu, S, eds., 

Proceedings—Fourth Canadian/American Conference on 

Hydrogeology, Fluid Flow, Heat Transfer and Mass Transport in Fractured Rocks, Banff, Alberta, Canada, June 

21–24, 1988, p. 100–113.

Garber, M.S., and Koopman, F.C., 1968, Methods of measuring water levels in deep wells: U.S. Geological Survey 

Techniques of Water-Resources Investigations, Book 8, 

chap. A1, 23 p.
46 

Gardner, W., Israelsen, O.W., Edlefsen, N.E., and Clyde, D., 

1922, The capillary potential function and its relation to 

irrigation practice: Physical Review, v. 20, p. 196.

Gould, Inc., 1966, Introduction to transducers for instrumentation: Oxnard, Calif., Measurements Systems Division, 61 p.

Gillum, D.R., 1982, Industrial pressure measurement: 

Research Triangle Park, N.C., Instrument Society of 

America, 107 p.

Healy, R. W., deVries, M. P., and Striegl, R. G., 1986, Concepts and data-collection techniques used in a study of the 

unsaturated zone at a low-level radioactive waste disposal 

site near Sheffield, Illinois: U.S. Geological Survey OpenFile Report 85-4228, 37p.

Hollett, K.J., Wilbourn, S.L., Latkovich, V.J., 1994, Proceedings of a U.S. Geological Survey Workshop on the application and needs of submersible pressure sensors, Denver, 

Colorado, June 7–10, 1994: U.S. Geological Survey OpenFile Report 94-531, 53 p.

Hsieh, P.A., 1993, Some experience gained from using packers 

and transducers in ground-water investigations of crystalline-rock aquifers, in Latkovich, V.J., ed., 1993, Proceedings of a pressure transducer-packer workshop, June 25–28, 

1991: U.S. Geological Survey Open-File Report 93-71, p. 

5–7.

Hsieh, P.A., Bredehoeft, J.D., and Farr, J.M., 1987, Determination of aquifer transmissivity from earth tide analysis: Water 

Resources Research, v. 23, no. 10, p. 1824–1832.

Hubbard, E.F., comp., 1992, Policy recommendations for 

management and retention of hydrologic data of the U.S. 

Geological Survey: U.S. Geological Survey Open-File 

Report 92-56, 32 p.

Instrument Society of America, 1970, Specifications and tests 

for strain gage pressure transducers: Pittsburgh, Pa., Instrument Society of America, S37.3. 

Jones, J.C., Tracey, D.C., Sorenson, F.W., 1991, Operating 

manual for the U.S. Geological Survey’s data-collection 

system with the Geostationary Operational Environmental 

Satellite: U.S. Geological Survey Open-File Report 91-99, 

237 p.

Kaehrle, W.R, and Thibodeaux, K.G., 1994, Testing of submersible pressure transducers installed in crest-stage gage 

pipes; in Proceedings of a U.S. Geological Survey PressureSensor Workshop, Denver, Colorado, July 28–31, 1992: 

U.S. Geological Survey Open-File Report 94-363, 53 p.

Keithley Instruments, Inc., 1992, Low level measurements: 

Cleveland, Ohio, Keithley Instruments, Inc. 172 p.

Kennedy, E.J., 1990, Levels at streamflow gaging stations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A19, 31 p.

Kirby, W.H., 1994, Analysis of errors in stage measurements 

by pressure sensors, in Proceedings of a U.S. Geological 

Survey Pressure-Sensor Workshop, Denver, Colorado, July 

28-31, 1992: U.S. Geological Survey Open-File Report 94-

363, 53 p.

Lamb, T.E., 1994, History of use of submersible and nonsubmersible transducers in Arkansas, in Proceedings of a U.S. 

Geological Survey Pressure-Sensor Workshop, Denver, 

Colorado, July 28–31, 1992: U.S. Geological Survey OpenFile Report 94-363, 53 p.

Lohman, S.W., 1972, Ground-Water Hydraulics, U.S. Geological Survey Prof. Paper 708, 70 p.

Lovell, A.D., Ellis, J.W., Brice, R.R., and Thomas, A.W., 

1978, Remote sensing of water levels in small-diameter 

wells: Agricultural Engineering, v. 59, no. 10, p. 44–45.

Measurements and Data Corporation, 1986, Pressure measurement, Part 1 — Units, Standards & Part 2 — Transducers: 

Pittsburgh, Pa., 32 p. 

Meyers, N.C., 1994, Possible designs and installation of 

submersible pressure transducers for use during floods, in 

Proceedings of a U.S. Geological Survey Workshop on the 

Application of Submersible Pressure Sensors, Denver, Colorado, June 7–10, 1994: U.S. Geological Survey Open-File 

Report 94-531, 53 p.

Novakowski, K.S., and Gillham, R.W., 1988, Field investigations of the nature of water-table response to precipitation in 

shallow water-table environments: Journal of Hydrology, v. 

97, no. 1-2, p. 23–32.

O’Brien, G.M., 1993, Monitoring static water levels in the 

Yucca Mountain area, in Latkovich, V.J., ed., 1993, Proceedings of a pressure transducer-packer workshop, June 

25–28, 1991: U.S. Geological Survey Open-File Report 

93-71, p. 31–32.

Robson, S.G., 1993, Use of packers and transducers in aquifer 

tests in Castle Pines, Colorado, in Latkovich, V.J., ed., 

1993, Proceedings of a pressure transducer-packer workshop, June 25–28, 1991: U.S. Geological Survey Open-File 

Report 93-71, p. 25.

Roeloffs, E.A., Burford, S.S., Riley, F.S. and Records, A.W., 

1989, Hydrologic effects on water level changes associated 

with episodic fault creep near Parkfield, California: Journal 

of Geophysical Research, v. 94, no. B9, pp. 12387–12402.

Rosenberry, D.O., 1990, Effect of sensor error on interpretation of long-term water-level data: Groundwater, v. 28 no. 6, 

pp. 927–936.
 47

Schaefer, M.E., and Hemond, H.F., 1986, A low-cost system 

to remotely measure piezometric head: Ground Water Moni-

toring Review, v. 6, no. 2, pp. 87–91.

Shuter, E. and Johnson, A.I, 1961, Evaluation of equipment 

for measurement of water level in wells of small diameter: 

U.S. Geological Survey Circular 453, 12 p.

Stannard, D.I., 1986, Theory, construction and operation of 

simple tensiometers: Ground Water Monitoring Review, 

Vol. 6, no. 3, pp 70–78.

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toring and the importance of long-term water-level data, 

U.S. Geological Survey Circular 1217, 68 p. 

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U.S. Geological Survey Water Resources Division Memo-

randum 92.59, accessed March 25, 1999, at URL http://

water.usgs.gov/admin/memo/WRD/wrdpolicy99.33.html

Van Doren, Tom, 1991, Grounding and shielding electronic 

systems: Rolla, Missouri, University of Missouri, 160 p.

Weeks, E.P., 1978, Field determination of vertical permeability 

to air in the unsaturated zone: U.S. Geological Survey Prof. 

Paper 1051, 41p.

Yeh, T-C., J., Gelhar, L.W., and Wierenge, P.J., 1986, Observa-

tions of spatial variability of soil-water pressure in a field 

soil: Soil Science, v. 142, no. 1, pp. 7–12.
48 

TECHNIQUES OF WATER-RESOURCES 

INVESTIGATIONS OF THE 

U.S. GEOLOGICAL SURVEY

The U.S. Geological Survey publishes a series of manuals 

describing procedures for planning and conducting specialized work in water-resources investigations. The material 

is grouped under major subject headings called books and 

is further divided into sections and chapters. For example, 

Section A of Book 3 (Applications of Hydraulics) pertains to 

surface water. The chapter, the unit of publication, is limited to 

a narrow field of subject matter. This format permits flexibility 

in revision and publication as the need arises.

The reports listed below are for sale by the U.S. Geological Survey, Branch of Information Services, Box 25286, Federal Center, Denver, Colorado 80225 (authorized agent of the 

Superintendent of Documents, Government Printing Office). 

Prepayment is required. Remittance should be sent by check 

or money order payable to the U.S. Geological Survey. Prices 

are not included because they are subject to change. Current 

prices can be obtained by contacting the Branch of Information Services (ph: (303) 202-4700; fax: (303) 202-4693; 

e-mail: infoservices@usgs.gov). When ordering or inquiring 

about prices for any of these publications, please give the title, 

book number, chapter number, and “U.S. Geological Survey 

Techniques of Water-Resources Investigations.”

A current list of Techniques of Water-Resources Investigations is available on the World Wide Web at the following 

URL: http://water.usgs.gov/lookup/get?twri.

1-D1. Water temperature – influential factors, field measurement, and data presentation, by H.H. Stevens, Jr., 

J.F. Ficke, and G.F. Smoot: USGS – TWRI Book 1, 

Chapter D1. 65 pages. 1975. 

1-D2. Guidelines for collection and field analysis of groundwater samples for selected unstable constituents, by 

W.W. Wood: USGS – TWRI Book 1, Chapter D2. 24 

pages. 1976.

 2-D1. Application of surface geophysics to ground-water 

investigations, by A.A.R. Zohdy, G.P. Eaton, and D.R. 

Mabey: USGS – TWRI Book 2, Chapter D1.116 pages. 

1974. 

2-D2. Application of seismic-refraction techniques to hydrologic studies, by F.P. Haeni: USGS – TWRI Book 2, 

Chapter D2. 86 pages. 1988. 

2-E1. Application of borehole geophysics to water-resources 

investigations, by W.S. Keys and L.M. MacCary: 

USGS – TWRI Book 2, Chapter E1. 126 pages. 1971. 

2-E2. Borehole geophysics applied to ground-water investigations, by W.S. Keys: USGS – TWRI Book 2, Chapter 

E2. 150 pages. 1990. 

2-F1. Application of drilling, coring, and sampling techniques to test holes and wells, by Eugene Shuter and 

W.E. Teasdale: USGS – TWRI Book 2, Chapter F1. 97 

pages. 1989. 

3-A1. General field and office procedures for indirect 

discharge measurements, by M.A. Benson and Tate 

Dalrymple: USGS – TWRI Book 3, Chapter A1. 30 

pages. 1967. 

3-A2. Measurement of peak discharge by the slope-area 

method, by Tate Dalrymple and M.A. Benson: USGS – 

TWRI Book 3, Chapter A2. 12 pages. 1967. 

3-A3. Measurement of peak discharge at culverts by indirect 

methods, by G.L. Bodhaine: USGS – TWRI Book 3, 

Chapter A3. 60 pages. 1968. 

3-A4. Measurement of peak discharge at width contractions 

by indirect methods, by H.F. Matthai: USGS-TWRI 

Book 3, Chapter A4. 44 pages. 1967. 

3-A5. Measurement of peak discharge at dams by indirect 

methods, by Harry Hulsing: USGS – TWRI Book 3. 

Chapter A5. 29 pages. 1967. 

3-A6. General procedure for gaging streams, by R.W. Carter 

and Jacob Davidian: USGS – TWRI Book 3, Chapter 

A6. 13 pages. 1968. 

3-A7. Stage measurement at gaging stations, by T.J. 

Buchanan and W.P. Somers: USGS – TWRI Book 3, 

Chapter A7. 28 pages. 1968. 

3-A8. Discharge measurements at gaging stations, by T.J. 

Buchanan and W.P. Somers: USGS – TWRI Book 3, 

Chapter A8. 65 pages. 1969. 

3-A9. Measurement of time of travel in streams by dye tracing, by F.A. Kilpatrick and J.F. Wilson, Jr.: USGS – 

TWRI Book 3, Chapter A9. 27 pages. 1989. 

3-A10. Discharge ratings at gaging stations, by E.J. Kennedy: 

USGS – TWRI Book 3, Chapter A10. 59 pages. 1984. 

3-A11. Measurement of discharge by the moving-boat method,

by G.F. Smoot and C.E. Novak: USGS – TWRI Book 3, 

Chapter A11.22 pages. 1969. 

3-A12. Fluorometric procedures for dye tracing, Revised, by 

J.F. Wilson, Jr., E.D. Cobb, and F.A. Kilpatrick: USGS- 

-TWRI Book 3, Chapter A12. 34 pages. 1986. 

3-A13. Computation of continuous records of streamflow, by 

E.J. Kennedy: USGS – TWRI Book 3, Chapter A13. 53 

pages. 1983. 

3-A14. Use of flumes in measuring discharge, by F.A. 

Kilpatrick and V.R. Schneider: USGS – TWRI Book 3, 

Chapter A14. 46 pages. 1983. 

3-A15. Computation of water-surface profiles in open channels, by Jacob Davidian: USGS – TWRI Book 3, 

Chapter A15. 48 pages. 1984. 

3-A16. Measurement of discharge using tracers, by F.A. 

Kilpatrick and E.D. Cobb: USGS – TWRI Book 3, 

Chapter A16. 52 pages. 1985. 
 49

3-A17. Acoustic velocity meter systems, by Antonius Laenen: 

USGS – TWRI Book 3, Chapter A17. 38 pages. 1985. 

3-A18. Determination of stream reaeration coefficients by 

use of tracers, by F.A. Kilpatrick, R.E. Rathbun, 

Nobuhiro Yotsukura, G.W. Parker, and L.L. DeLong: 

USGS – TWRI Book 3, Chapter A18. 52 pages. 1989. 

3-A19. Levels at streamflow gaging stations, by E.J. 

Kennedy: USGS – TWRI Book 3, Chapter A19. 31 

pages. 1990. 

3-A20. Simulation of soluable waste transport and buildup 

in surface waters using tracers, by F.A. Kilpatrick: 

USGS – TWRI Book 3, Chapter A20. 38 pages. 1993. 

3-A21. Stream-gaging cableways, by C. Russell Wagner: 

USGS – TWRI Book 3, Chapter A21. 56 pages. 1995.

3-B1. Aquifer-test design, observation, and data analysis,

by R.W. Stallman: USGS – TWRI Book 3, Chapter B1. 

26 pages. 1971. 

3-B2. Introduction to ground-water hydraulics, a programed text for self-instruction, by G.D. Bennett: 

USGS – TWRI Book 3, Chapter B2. 172 pages. 1976. 

3-B3. Type curves for selected problems of flow to wells in 

confined aquifers, by J.E. Reed: USGS – TWRI Book 

3, Chapter B3. 106 pages. 1980. 

3-B4. Regression modeling of ground-water flow, by 

R.L. Cooley and R.L. Naff: USGS – TWRI Book 3, 

Chapter B4. 232 pages. 1990. 

3-B4. Supplement 1. Regression modeling of ground-water 

flow – Modifications to the computer code for nonlinear regression solution of steady-state ground-water 

flow problems, by R.L. Cooley: USGS – TWRI Book 

3, Chapter B4. 8 pages. 1993. 

3-B5. Definition of boundary and initial conditions in the 

analysis of saturated ground-water flow systems – 

An introduction, by O.L. Franke, T.E. Reilly, and 

G.D. Bennett: USGS – TWRI Book 3, Chapter B5. 

15 pages. 1987. 

3-B6. The principle of superposition and its application in 

ground-water hydraulics, by T.E. Reilly, O.L. Franke, 

and G. D. Bennett: USGS – TWRI Book 3, Chapter 

B6. 28 pages. 1987. 

3-B7. Analytical solutions for one-, two-, and three-dimensional solute transport in ground-water systems with 

uniform flow, by E.J. Wexler: USGS – TWRI Book 3, 

Chapter B7. 190 pages. 1992. 

3-C1. Fluvial sediment concepts, by H.P. Guy: 

USGS – TWRI Book 3, Chapter C1. 55 pages. 1970. 

3-C2. Field methods for measurement of fluvial sediment, 

by H.P. Guy and V.W. Norman: USGS – TWRI Book 

3, Chapter C2. 59 pages. 1970. 

3-C3. Computation of fluvial-sediment discharge, by 

George Porterfield: USGS – TWRI Book 3, Chapter 

C3. 66 pages. 1972. 

4-A1. Some statistical tools in hydrology, by H.C. Riggs: 

USGS – TWRI Book 4, Chapter A1. 39 pages. 1968. 

4-A2. Frequency curves, by H.C. Riggs: USGS – TWRI 

Book 4, Chapter A2. 15 pages. 1968. 

4-B1. Low-flow investigations, by H.C. Riggs: 

USGS – TWRI Book 4, Chapter B1. 18 pages. 1972. 

4-B2. Storage analyses for water supply, by H.C. Riggs and 

C.H. Hardison: USGS – TWRI Book 4, Chapter B2. 

20 pages. 1973. 

4-B3. Regional analyses of streamflow characteristics, by 

H.C. Riggs: USGS – TWRI Book 4, Chapter B3. 15 

pages. 1973. 

4-D1. Computation of rate and volume of stream depletion 

by wells, by C.T. Jenkins: USGS – TWRI Book 4, 

Chapter D1. 17 pages. 1970. 

5-A1. Methods for determination of inorganic substances in 

water and fluvial sediments, by M. J. Fishman and 

L.C. Friedman, editors: USGS – TWRI Book 5, 

Chapter A1. 545 pages. 1989. 

5-A2. Determination of minor elements in water by emission spectroscopy, by P.R. Barnett and E.C. Mallory, 

Jr.: USGS – TWRI Book 5, Chapter A2. 31 pages. 

1971. 

5-A3. Methods for the determination of organic substances 

in water and fluvial sediments, edited by R.L. 

Wershaw, M.J. Fishman, R.R. Grabbe, and L.E. Lowe: 

USGS – TWRI Book 5, Chapter A3. 80 pages. 1987. 

5-A4. Methods for collection and analysis of aquatic biological and microbiological samples, by L.J. Britton 

and P.E. Greeson, editors: USGS – TWRI Book 5, 

Chapter A4. 363 pages. 1989. 

5-A5. Methods for determination of radioactive substances 

in water and fluvial sediments, by L.L. Thatcher, 

V.J. Janzer, and K.W. Edwards: USGS – TWRI Book 

5, Chapter A5. 95 pages. 1977.

5-A6. Quality assurance practices for the chemical and 

biological analyses of water and fluvial sediments, 

by L.C. Friedman and D.E. Erdmann: USGS – TWRI 

Book 5, Chapter A6. 181 pages. 1982. 

5-C1. Laboratory theory and methods for sediment analysis, 

by H.P. Guy: USGS – TWRI Book 5, Chapter C1. 58 

pages. 1969. 

6-A1. A modular three-dimensional finite-difference 

ground-water flow model, by M.G. McDonald and 

A.W. Harbaugh: USGS – TWRI Book 6, Chapter A1. 

586 pages. 1988. 

6-A2. Documentation of a computer program to simulate 

aquifer-system compaction using the modular finite- 
50 

difference ground-water flow model, by S.A. Leake 

and D.E. Prudic: USGS – TWRI Book 6, Chapter A2. 

68 pages. 1991.

6-A3. A modular finite-element model (MODFE) for areal 

and axisymmetric ground-water-flow problems, Part 1: 

Model Description and User’s Manual, by L.J. Torak: 

USGS – TWRI Book 6, Chapter A3. 136 pages. 1993. 

6-A4. A modular finite-element model (MODFE) for areal 

and axisymmetric ground-water-flow problems, Part 

2: Derivation of finite-element equations and comparisons with analytical solutions, by R.L. Cooley: 

USGS – TWRI Book 6, Chapter A4. 108 pages. 1992. 

6-A5. A modular finite-element model (MODFE) for areal 

and axisymmetric ground-water-flow problems, Part 3: 

Design philosophy and programming details, by L.J. 

Torak: USGS – TWRI Book 6, Chapter A5, 243 pages. 

1993. 

6-A6. A coupled surface-water and ground-water flow 

model (MODBRANCH) for simulation of streamaquifer interaction, by E.D. Swain and E.J. Wexler: 

USGS – TWRI Book 6, Chapter A6, 125 pages. 1996. 

7-C1. Finite difference model for aquifer simulation in 

two dimensions with results of numerical experiments, by P.C. Trescott, G.F. Pinder, and S.P. Larson: 

USGS – TWRI Book 7, Chapter C1. 116 pages. 1976. 

7-C2. Computer model of two-dimensional solute transport 

and dispersion in ground water, by L.F. Konikow and 

J.D. Bredehoeft: USGS – TWRI Book 7, Chapter C2. 

90 pages. 1978. 

7-C3. A model for simulation of flow in singular and 

interconnected channels, by R.W. Schaffranek, R.A. 

Baltzer, and D.E. Goldberg: USGS – TWRI Book 7, 

Chapter C3. 110 pages. 1981. 

8-A1. Methods of measuring water levels in deep wells, by 

M.S. Garber and F.C. Koopman: USGS – TWRI Book 

8, Chapter A1. 23 pages. 1968. 

8-A2. Installation and service manual for U.S. Geological 

Survey manometers, by J.D. Craig: USGS – TWRI 

Book 8, Chapter A2. 57 pages. 1983. 

8-B2. Calibration and maintenance of vertical-axis type current meters, by G.F. Smoot and C.E. Novak: USGS – 

TWRI Book 8, Chapter B2. 15 pages. 1968. 

9-A1. Preparations for water sampling, by F.D. Wilde, D.B. 

Radtke, Jacob Gibs, and R.T. Iwatsubo: USGS – TWRI 

Book 9, Chapter A1, [variously paged]. 1999.

9-A2. Selection of equipment for water sampling, edited 

by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. 

Iwatsubo – TWRI Book 9, Chapter A2, [variously 

paged]. 1999.

9-A3. Cleaning of equipment for water sampling, edited 

by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. 

Iwatsubo – TWRI Book 9, Chapter A3, [variously 

paged]. 1999.

9-A5. Processing of water samples, edited by F.D. Wilde, 

D.B. Radtke, Jacob Gibs, and R.T. Iwatsubo – TWRI 

Book 9, Chapter A5, 128 pages, 1999.

9-A6. Field measurements, edited by F.D. Wilde and D.B. 

Radtke: USGS – TWRI Book 9, Chapter A6, [variously 

paged]. 1998.

9-A7. Biological indicators, edited by D.N. Myers and F.D. 

Wilde: USGS – TWRI Book 9, Chapter A7, [variously 

paged]. 1997.

9-A8. Bottom-material samples, by D.B. Radtke: 

USGS – TWRI Book 9, Chapter A8, [variously paged]. 

1998.

9-A9. Safety in field activities, by S.L. Lane and R.G. Fay: 

USGS – TWRI Book 9, Chapter A9, [variously paged]. 

1998.
 51
52 
Lawrence A. Freeman and others—Use Of Submersible Pressure Transducers In Water-Resources Investigations—TWRI 8-A3

Printed on recycled paper