Random Uncertainties

Dew Point Devices —^

Humidity Ratio

Absolute Humidity

Psych rometers

Ideal Gas Law

NATL INST OF STANDARDS & TECH R.I.C

A1 11 01 98751

/NBS building science series TA°°.; llfiB ^157:1983 C.I NBS-PUB-C 1974

Systematic Error

Wet Bulb Temperature

Lithium Chloride Sensor

Sensor Calibration

Dew Point Temperature

Relative Humidity

Chilled Mirror Hygrometer

Dry Bulb Temperature

NBS BUILDING SCIENCE SERIES 157

General Guidelines for

the On-Site Calibration of

Humidity and Moisture

Control Systems in Buildings

435

.U58

ITMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS

1983

C. 2
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering^

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. ^Some divisions within the center are located at Boulder, CO 80303.
Of 8TW<DAiU:<8 NBS BUILDING SCIENCE SERIES 157 C^'^c.

General Guidelines for the On-Site /

Calibration of Humidity and Moisture f

Control Systems in Buildings ^

Richard W. Hyland

Temperature and Pressure Measurements

and Standards Division

Center for Absolute Physical Quantities

C. Warren Hurley

Building Equipment Division

Center for Building Technology

National Bureau of Standards

Washington, DC 20234

Prepared for: U.S. Navy

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued September 1983
Library of Congress Catalog Card Number: 83-600574

National Bureau of Standards Building Science Series 157

Natl. Bur. Stand. (U.S.), Bldg. Sci. Ser. 157, 56 pages (Sept. 1983)

CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1983

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $4.00

(Add 25 percent for other than U.S. mailing)
ABSTRACT

The control of the moisture content of the air in buildings directly affects

the comfort of the building occupants and is one of the more important tasks

of an energy management and control system (EMCS). Unfortunately, a lack of

understanding of the principles of operation and calibration of the various

instrumentation used in monitoring the moisture content of air often makes

this one of the most difficult and most neglected tasks for those assigned the

responsibility of maintaining and operating building heating, ventilating and

air-conditioning (HVAC) control systems.

This report was written for the purpose of assisting the managers and operators

of EMCS in buildings in the understanding and use of various techniques for

on-site measurement of the moisture content in the air. In addition, various

methods of on-site calibration of typical types of moisture monitoring instruments are also discussed.

The ideal gas law is reviewed because of its direct application in converting

the various humidity units. Relative humidity and dew point temperature are defined and their relationship described. Typical types of moisture monitoring

instruments are described together with their method of operation. Several

calibration techniques are discussed together with their typical advantages and

disadvantages. The last section of this report is devoted to assessing and re- porting the errors or uncertainties associated with the calibration and

functioning of various moisture monitoring instruments. This very important

area is often neglected and can result in excessive operating and maintenance

costs

.

Key words: chilled mirror hygrometer; dew point temperature; humidity

generator; humidity sensors; lithium chloride humidity sensors;

moisture content of building air; relative humidity; saturated

salt solutions; sensor calibration; sensor precision; specific

humidity; wet-bulb temperature

Cover: The cover shows an outline of a psychrometric chart and the key words

covering the contents of this report on the on-site calibration of

humidity and moisture control systems in buildings.

iii
SI CONVERSIONS

The contents of this report are directed toward assisting field personnel in

the calibration of instrumentation monitoring the moisture content of air

supplied by mechanical equipment in buildings. In view of the presently

accepted practice of the building industry in the United States and the

reference material readily accessible to field personnel managing and operating

the mechanical equipment in buildings, common U.S. units of measurement have

been used in this report. In recognition of the fact that the United States

is a signatory to the General Conference of Weights and Measures, which gave

'official status to the SI system of units in 1960, appropriate conversion

factors have been provided in the table below. The reader interested in

making further use of the coherent system of SI units is referred to NBS SP 330,

1972 Edition, "The International System of Units,"; E380-72, ASTM Metric Practic

Guide (American National Standard 2210.1); or ASHRAE "SI Metric Guide for Heatin

Refrigerating, Ventilating, and Air-Conditioning , " 1976.

Metric Conversion Factors

To convert from To Multiply by*

Area

ft^

in.

2 / 2n metre (m ) 2 2 metre (m^)

9.290304E-02

6.451600E-04

Energy

Btu (Int'l Steam Table) joule (J)

calorie (Int'l Steam Table) joule (J)

erg joule (J)

1.055056E+03

4.186800E+00

l.OOOOOOE-07

Force

pound-force (Ib^)

kilogram-force

newt on (N)

newton (N)

4.448222E+00

9.806650E+00

Length

ft

in.

metre (m)

metre (m)

3.048000E-01

2.540000E-02

Mass

gram

lb

kilogram (kg)

kilogram (kg)

l.OOOOOOE-03

4.5359237E-01

"The notation "xE+y," where x and y are numbers, is a standard form for

indicating multiplication of the number x by the number 10 raised to the

power + y.

iv
Mass per unit vol.

g/cm

lb/ft3

lb/in.

3

kg per m-^ kg per m-^ kg per

Pressure (force per unit area)

atmosphere

In. of mercury (60°F)

mm of mercury (32°F)

in. of water (60°F)

in. of water (39.2°F)

dyne/ cm^

Temperature

degree Fahrenheit

degree Celsius

degree Fahrenheit

degree Celsius

degree Rankine

Velocity

ft/min

ft/s

in. /s

Volume

ft3

in. 3

Volume per unit time

f t^/min

ft3/s

pascal (Pa)

pascal (Pa)

pascal (Pa)

pascal (Pa)

pascal (Pa)

pascal (Pa)

degree Celsius

degree Fahrenheit

kelvin

kelvin

kelvin

metre per second (m/s)

metre per second (m/s)

metre per second (m/s)

metre-^ (m ) 3 3 metre (m )

3 3 metre per second (m /s)

metre3 per second (m3/s)

1 . OOOOOOE+03

1.601846E+01

2.767991E+04

1.013250E+05

3.376850E+03

1.33322AE+02

2.488400E+02

2.490820E+02

l.OOOOOOE-01

subtract 32 and

divide by 1.8

multiply by 1.8

and add 32

add 459.67 and

divide by 1.8

add 273.15

divide by 1.8

5.080000E-03

3.048000E-01

2.540000E-02

2.831685E-02

1.638706E-05

4.719474E-04

2.831685E-02

V
TABLE OF CONTENTS

Page

ABSTRACT iii

SI CONVERSIONS iv

LIST OF FIGURES vil

LIST OF TABLES viii

NOMENCLATURE ix

1. INTRODUCTION 1

2. DEFINITIONS OF CALIBRATION AND STANDARD 2

3. MOISTURE MEASUREMENT UNITS AND CONVERSION FACTORS 3

3.1 THE IDEAL GAS LAW 3

3.2 RELATIVE HUMIDITY 6

3.3 DEW POINT TEMPERATURE 6

4. GENERAL TYPES AND PROPERTIES OF HUMIDITY SENSORS 7

5. CALIBRATION METHODS 10

5.1 CALIBRATION USING SATURATED SALT SOLUTIONS AS STANDARDS .... 10

5.2 CALIBRATION BY COMPARING WITH A STANDARD DEVICE 10

6. SUGGESTED METHODS AND PRECAUTIONS IN CALIBRATING AND USING

SEVERAL APPLICABLE DEVICES 14

6.1 FORCED AIR PSYCHROMETERS AND THE PSYCHROMETRIC CHART 14

6.2 DEW POINT DEVICES 17

6.3 THE LITHIUM CHLORIDE DEW POINT INDICATOR (DEWCELL) 20

6.4 USE OF SATURATED SALT SOLUTIONS 22

6.5 GENERAL 25

7. ASSIGNING ERRORS TO THE CALIBRATED DEVICES 26

7.1 RANDOM UNCERTAINTIES AND PRECISION 26

7.2 SYSTEMATIC UNCERTAINTIES - FIRST TYPE 27

7.3 PRESENTATION OF RESULTS 27

7.4 SYSTEMATIC ERRORS - SECOND TYPE 29

7.5 ADDITIONAL FACTORS 31

7.6 CALIBRATION RECORDS 32

REFERENCES 34

APPENDIX A A-1

APPENDIX B A-2

vi
LIST OF FIGURES

FIGURE ?a§e_

1 Schematic diagram of a generalized humidity generator for

providing a flow of moist air 12

2 A simple humidity gienerator for calibrations which do not

require large flow rates 13

3 Psychrometric chart No . 1 15

4 Psychrometric chart No , 2 16

5 The estimated accuracy of a chilled mirror dew point

hygrometer as a function of the measured dew point temperature 19

6 Schematic diagram of a saturated salt solution calibration

chamber 23

7 An example of poor placement of sampling port with respect

to sensor position in a duct 25

8 Plot of calibration results showing instrument error as a function of its indicated reading (solid line) . 30

vii
LIST OF TABLES

Table ' Page

1 Values of the universal gas constant, R, for various units . . 4

2 Sensor properties 8

3 Results of a hypothetical calibration of a device using

saturated salt solutions at 68°F (20°C) 28

4 Example for the calculation of overall uncertainties when

repeated measurements of the same relative humidity are made with two instruments which have nearly equal random

uncertainties 32

A-1 Saturation pressures of water vapor over ice as a function

of temperature A-2

A-2 Saturation vapor pressures of water as a function of

temperature A-4

B-1 Equilibrium relative humidity of selected saturated salt B-3

solutions from 32 - 212°F (0 - 100°C)

viii
NOMENCLATURE*

Sirmbols

°C = temperature in degrees Celsius

d = the absolute humidity: the ratio of the mass of water over the

total volume of a given sample

e(t) = the saturation water vapor pressure at temperature t(°F)

e_j^(t) = the saturation vapor pressure of water over ice at temperature tCF)

°F = temperature in degrees Fahrenheit

= mass of dry air in a given sample, pounds

m^ = mass of water vapor in a given sample, pounds

n = sum of the number of moles of dry air and the number of moles of

water vapor in a given volume of the mixture

= the number of moles of dry air in a given volume of the mixture

n^ = the number of moles of water vapor in a given volume of the mixture

N = a reading from an instrument

N = the average value of a series of readings from an instrument, Nl

,

N2, N3, Nn

P = total pressure of mixture, psia

P = partial pressure of dry air, psia

aP = partial pressure of water vapor, psia

w

psia = pounds per square inch, absolute

P^ = the pressure in the mirror chamber, psia

q = the specific humidity; the ratio of the mass of water vapor over

the total mass of the mixture

R = universal gas constant (see table 1)

RH = the relative humidity: 100 times the ratio of the mole fraction of

water vapor in a sample of air to the saturation mole fraction of

water vapor in the air at the same temperature and pressure.

's^ = the estimate of the standard deviation about the mean of a series of

measurements (see section 7)

*The values noted in this nomenclature are given in U.S. units of measurement

(see page iv for conversion to SI units).

ix
NOMENCLATURE (continued) "

T = absolute temperature, degrees Rankine , °R

t = temperature of air in degrees Fahrenheit

tj = measured dew point temperature of sample in degrees Fahrenheit

d

3

V = volume of mixture, ft

W = the humidity (mixing) ratio: the mass of water vapor divided by the

mass of dry air in a given sample

X = the mole fraction of dry air: the number of moles of dry air in

a given sample divided by the sum of the number of moles of dry

air and the number of moles of water vapor in the sample

X = the mole fraction of water vapor: the number of moles of water

w

vapor in a given sample divided by the sum of the number of moles

of dry air and the number of moles of water vapor in the sample

*The values noted in this nomenclature are given in U.S. units of measurement

(see page iv for conversion to SI units).

X
1. INTRODUCTION

The measurement and control of the moisture content of air are crucial parameters

for the design and operation of building energy management and control systems

(EMCS). The efficiency of the personnel working in a building is determined by

their comfort, which in turn depends on combinations of temperature, relative

humidity, and air motion, among other things. Some laboratories and commercial

and industrial facilities must be maintained within certain ranges of temperature

and relative humidity to permit operations to be carried out, or prevent product

or equipment damage. An engineer or architect selects the climate control not

only to provide the desired conditions, but to do it in the most energy efficient

manner. Under normally encountered air-conditioning situations, the latent energy

associated with humidif ication or dehumidif ication can represent as much as 20 percent of the total energy being used. An error of 10 percent in the measurement

and control of the moisture content in the air could have a significant impact in

the total energy use. Therefore, it is necessary that the devices being used to

monitor and control the moisture content in the air be maintained within an acceptable level of accuracy to avoid dollar loss from malfunction, over design,

or under design of air-conditioning equipment and its controlling systems. The

term "acceptable" must be defined for the individual building or areas of a building being controlled.

This report provides an overview of methods which must be used when performing

on-site calibrations of sensors used to determine the moisture content of air for

control purposes. It is not to be construed either as a set of universal recom- mended practices or as a documentary standard. In the context of this report, an on-site or field calibration will mean one not performed under strict laboratory

conditions, although it is obvious that the more attention is paid to details,

the more reliable the results are likely to be.

The moisture content of gas, air in particular, may be expressed in terms of

relative humidity, dew point, parts per million of water vapor per unit volume

of the mixture, or various other units. The most common of these units used in

EMCS and the factors to convert from one to another are presented in this report

followed by a brief discussion of various types of humidity sensors and their

accuracy limitations. Next, some convenient methods and devices which may be

used as field calibration standards are presented, along with examples of the sensors which may be calibrated by each. Finally, the actual calibration techniques and presentation of the results are discussed, including the assigning of errors to the calibration devices.

There are two underlying assumptions throughout this report. The first is that

the accuracies required in the discussed calibrations will not require consideration of sophisticated interactions among the various gases; that is, air-water

vapor mixtures will be assumed to be uniform gases which may be described by

ideal gas relationships. It is further assumed that precision, high-capacity

humidity generators, such as those described in reference [1], are not readily

available to EMCS personnel. The reader should be aware that such devices are maintained at the National Bureau of Standards and are generally used to calibrate

high quality humidity measuring devices. In general, the high-precision devi ces are not required for typical EMCS monitoring. Further, the cost of such devices

is out of the EMCS range . Therefore, such devices are not included in this report.

1
2. DEFINITIONS OF CALIBRATION AND STANDARD

It is appropriate to define the terms "calibration" and "standard" as used in

this report. Calibration is the comparison of the moisture content indication

of sensors with the moisture content generated by or measured by a standard method

or device. Calibrations may be made over all or part of the range of the sensors

in an EMCS. The calibration must also account for all parameters, particularly

temperature, which affect the final indication.

A standard is understood to be a moisture indicating instrument whose indication

and overall uncertainty at particular values have been determined. It might be

any such device; for example, a hair hygometer, an aspirated (motor driven)

psychrometer , or a sophisticated dew point device. It may also be an apparatus

or technique which produces particular moisture contents, within known error

bounds, when certain operating conditions are met. Examples are saturated salt

solutions or a variety of humidity generators.

2
3. MOISTURE MEASUREMENT UNITS AND CONVERSION FACTORS

The definitions of several humidity measurement units are given in this section.

These definitions are structured to contain measurable variables such as pressure,

volume, and temperature. Use has been made of ideal gas relationsips , the

details of which are discussed in reference [2].

3.1 THE IDEAL GAS LAW

A brief review of the ideal gas law is given because its application permits

conversion among the various humidity units. Fortunately, only negligible errors

result for EMCS applications by using the ideal gas law for treating moist air

as a mixture of independent, perfect gases; namely, dry air and water vapor. The

general equation is written,

PV = nRT (1)

where P = pressure (psia)

^ V = total volume of mixture (ft ) n = total number of moles in the mixture

^ ^ R = universal gas constant [10. 73151 (Ib^/in. )ft /lb-mole °R]

T = absolute temperature (°R)

(See table 1 for values of R when other units are used.)

The pressure, P, equals the sum of the partial pressure of dry air and the

partial pressure of water vapor in the mixture,

P = P + P (2) a w

where P = partial pressure of dry air (psia), and

P = partial pressure of water vapor (psia). w

The total number of moles, n, equals the sum of the number of moles of dry air

and the number of moles of water vapor in the volume, V, of the mixture,

n = n + n (3) a w

where n = the number of moles of dry air, and

n = the number of moles of water vapor, w

Combining these equations, we have

(Paw+ P ) V = (naw+ n ) RT (4)

Considering the ideal gas equations for dry air and for water vapor, we have

P V = n RT, and (5) a a

P V = n RT (6) w w

3
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Pi Pi (Ud 4-1 CO CO o d

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MH MH 0) jd 0) OJ OJ O B ex rH T3 rd OJ 5-1 T3 •H 03 03 •H 4-1 B CU 4= d OJ rH O u CO d ni: O d d rH B CO CU II Cfi rH d o •H 4-1 03 03 o M Ph <: PH H H e > 4
Dividing eq (5) by eq (4), the mole fraction of dry air, X , is obtained.

Si n P P

X =

a ^ a _ __a a naw+ n Paw+ P P

Similarly, the mole fraction of water vapor, X , is found by dividing eq (6)

by eq (4), "

n P P w n+n aw P+P aw P"

The humidity ratio, W, is defined as the mass of water vapor to the mass

of dry air contained in the given sample,

m -

,

y = ^

lbs water vapor

m ' lbs dry air

a

Considering the molecular weights of water and air and their relative mole

fractions, we have,

X P

_ 18. 015 . W _ /-oTno W

^ - 287964 X- = °'62198 — , or, a a

0.62198 Pw lbs moisture /-.^s ^ =

P - P ' lbs dry air » ^^^^ w

Other values which may be obtained from the above equations and may be found

useful in field calibration are listed below.

1. The absolute humidity, d, is the ratio of the mass of water

vapor to the total volume of mixture:

18.015 P . J w lbs of water vapor /tin d = , 3

. (11)

ft of mixture

2. The specific humidity, q, is the ratio of the mass of water

vapor to the total mass of the moist air sample,

18.015 Pw iu . lbs water vapor . . ^ ~ 18.015 P + 28.964 P ' lbs of mixture ' ^ w a

It will be noted from the above equations that the one common variable which

must be determined is P^, if the total pressure, P, is known since P^ = P - P^^.

5
The amount of water vapor which can be mixed with air before the air becomes

saturated is strongly dependent on the air temperature. (There is also a small

pressure effect, which can be considered insignificant for normal EMCS applications). The relationship of the air temperature to the saturated partial vapor

pressure is well known in the pressure-temperature regions of interest in EI4CS

.

(See tables A-1 and A-2 in appendix A, and references [4,5,11, and 14]).

The saturation may occur over water or ice, and the saturation vapor pressures

are designated respectively as e(t) and e^(t), the (t) emphasizing that the

values are different at each temperature, t. We will simply use e(t) unless it

is necessary to differentiate between ice and water. If the air mixture is

saturated, then e(t) replaces in eq (10), (11), and (12), and those equations

then yield the saturation mixing ratio, the saturation absolute humidity, and

the saturation specific humidity, respectively.

3 . 2 RELATIVE HUMIDITY

Normally, the air is not saturated, and it is convenient to express the water

vapor content as a percentage of the saturation at the same temperature . This is defined as the relative humidity, RH, and may be written as follows:

P

RH = X 100 percent (13) e(t)

Thus, if the relative humidity is measured at a known temperature, eq 13 can be

used to find P for use in the other humidity expressions, w

3.3 DEW POINT TEI-IPERATURE

The final quantity which must be identified is the dew point temperature. Since

the maximum amount of moisture which the air can hold is a function of temperature,

we could start with non-saturated air at temperature t, and begin to cool it. As

it is cooled, the maximum possible saturation vapor pressure, e(t), becomes less

and less, while the actual vapor pressure, P^^^, will remain unchanged . The relative

humidity in the air, defined by eq 13, becomes closer and closer to 100 percent

until a temperature, t,, is reached at which e(t) is the same as P^,, and at which

temperature the gas is saturated. That temperature is called the dew point temperature (or frost point temperature, if the saturation occurs over ice). At this

temperature, the moisture in the air will just begin to condense.

In some types of dew point measurement devices, a surface whose temperature is

being measured is cooled until the water vapor in the ambient air begins to condense. At the condensation temperature, t^, the saturation vapor pressure, e(t),

may be read from tables. Since no water was added to or taken from the air being

monitored before condensation on the cooled surface, this partial (saturated)

pressure, e(t^), must be the same partial vapor pressure, P^, (not saturated)

which was in the air before it was cooled. Thus, from the dew point temperature

one obtains the partial pressure of moisture, and may be used to obtain the

other humidity units described above. The relationship among dew point and

relative humidity are quickly seen on the psychrometric charts (figures 3 and 4)

which are described in section 6 of this report.

6
4. GENERAL TYPES AND PROPERTIES OF HUMIDITY SENSORS

Table 2 lists a variety of sensor types, their general principles of operation,

the moisture ranges covered, and their inherent accuracies. The table is not

all-inclusive; see reference [11] for additional types. Also, the range and

accuracy statements are general in nature, and particular devices may behave

better or worse than indicated here. The manufacturer's literature should be

used as a guide for specific, devices. For further discussions of their properties and operational principles, consult the manufacturer's literature, and see references [6,7,8, and 11].

7
CD

w CU

rrtM 1—

Qj (J

4-J 4-J ri r!

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5. CALIBRATION METHODS

As described above, even though the mass of water vapor in a given volume of

air remains constant, the relative humidity will vary as the temperature of

the air changes. Therefore, both the relative humidity at which an instrument

is being read or calibrated and the temperature must be specified. In fact,

many devices will have to be calibrated at enough different temperatures to

permit interpolation between calibrated temperatures to obtain the resulting

relative humidity vs. instrument indication curves. Check the manufacturer's

literature for guidance.

Calibration methods can be divided into two broad categories: 1) calibrations

in which the reference (standard) relative humidity is known because of the

manner used to obtain it, and 2) calibrations in which the moisture level is

initially unknown and must be determined through use of a standard; that is,

through use of a previously calibrated (by whatever means) device. Note that

by this definition, the standard may be the same type of device as that being

calibrated. In this latter category, the readings of the devices will be

compared to the readings of the standard.

5.1 CALIBRATION USING SATURATED SALT SOLUTIONS AS STANDARDS

Salt solutions are widely used to generate known relative humidities. If

certain salts are mixed with enough water to form a slush, the relative humidity

in an undisturbed gas volume at uniform temperature above the surface of the

solution will be maintained at a constant level. This means that if the salt

solution is placed within a chamber which is kept at a uniform temperature,

the entire volume within that chamber will reach an equilibrium relative

humidity, determined by the type of salt and the temperature. Humidity sensors which do not require a forced flow of gas across their sensing elements can be

placed in the chamber and calibrated. Table B in appendix B lists some common

salts, the relative humidities their saturated solutions will maintain, and the

applicable temperatures. The uncertainties shown in this table are for ideal

conditions, and cannot be realized in the field. For further information see reference [9]. A wide range of humidities may be generated in a reasonable time

for calibration purposes through this simple method. A more detailed discussion

of the method and precautions is given in section 6.4.

5.2 CALIBRATION BY COMPARING WITH A STANDARD DEVICE

The methods which may be used for intercomparing with a standard device are

as numerous as ingenuity permits. For clarity, the types of standards will be

divided into two types. The first type includes devices which require or use

a gas flow, such as some dew point (cooled mirror) and psychrometric (wet-dry bulb)

devices. Dew point devices and psychrometers may be most easily used to calibrate

instruments which have been allowed to equilibrate with either room or outdoor

temperature and humidity. These conditions usually, but not always, vary slowly

enough to permit equilibrium to be achieved, yet rapidly enough to provide

several closely-spaced points in a reasonable period of time. Buildings often

have rooms controlled at different temperatures and/or humidities, thus providing

stable calibration points. Note: Only your standard will tell what the room RH

is at its current temperature. Do not assume the room specifications are being

met I

10
It may be convenient to build a simple humidity generator for which the output

may not be predictable, but for which the moisture output in a gas can be made

constant within the required accuracy. The moist gas then can be led through

a calibration chamber which contains both a standard to determine the moisture

content and instruments to be calibrated by comparison with the standard. A

generalized schematic diagram of such a system is shown in figure 1. The rate

of flow of the moist gas across the standard and the instruments must fall v/ithin

the specified limits given by the manufacturer of each device.

The generator may not contain all of the elements listed, but any generator of

this type will have a gas supply, a throttling valve, moisture source, and

calibration chamber. The degree of sophistication is a matter of need and choice.

The moisture source, for example, can be a flat surface of water over which the

gas flows, or a water supply with a wick exposed to the gas flow.

The moisture sources may be provided with a heater to raise the moisture level

in the gas. Two flow paths with separate valves as shown in figure 1 might be

used to proportion the moist and dry gas to vary the relative humidity output.

If there is much turbulence over the moisture source, a droplet trap (such as a bottle stuffed loosely with glass wool or metal mesh) would be desirable as shown in figure 1. The heat exchanger might be needed if heaters are used in

the moisture source to bring the temperature of the moist gas from the generator

back to the temperature of the calibration chamber which contains the devices

under test. A calibrated hygrometer can be used to monitor the sample gas flow

through the calibration chamber. A flow-meter is included because the flow-rate

must be measured during the use of many commercially available dew point devices

and psychrometers.

If the instrumentation is located in a return air duct, the temperature and

relative humidity of the air passing through it should be very close to those

of the air in the room or hall near the intake port. One can therefore measure

the intake conditions with a standard, see whether or not the duct instrumentation

reads the same value (within uncertainty limits, see section 7), and adjust it

accordingly. Disagreement in results may arise from sources other than a miscalibrated instrument, and that possibility must be checked carefully before

adjusting the device in the duct. Ambient temperature and moisture changes

caused by the persons adjacent to the standard and performing the calibration

are typical examples.

The second type of standards mentioned in section 5 do not require an air flow

past their elements, although such flows may be used. These standards can be any

convenient device whose humidity indication as a function of RH and temperature

is known. Thus, in addition to the methods discussed above for generating moist

atmospheres, closed calibration chambers may be used. A pan of water can be

placed in the chamber, left long enough for the humidity to achieve some nominal

level as determined by the standard, and then removed or covered (see figure 2)

to allow general equilibration to occur. However, it is often just as easy, and

indeed perhaps desirable, to use saturated salt solutions (see section 6.4).

This section has pointed out some general calibration techniques xvrhich may serve

as guides for individual calibration schemes. The following section will discuss

some of the details and precautions associated with the above calibration methods.

11
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13
6. SUGGESTED METHODS AND PRECAUTIONS IN CALIBRATING AND USING SEVERAL

APPLICABLE DEVICES

This section will consider the use of forced air psychrometers , true dew point

(cooled surface) devices, and the lithium chloride dew point Indicator. The

final paragraphs will cover use of saturated salt solutions as humidity generators.

One general comment can be made which applies to any device one may use. Read

and understand the manufacturer's Instructions . If questions remain, ask the

manufacturer for assistance, and consult any literature which he might suggest.

6 . 1 FORCED-AIR PSYCHROMETERS AND THE PSYCHROMETRIC CHART

There are two kinds of forced-air psychrometers ; motor driven and hand-held

(sling) psychrometers. The Instruments contain two temperature sensors In close

proximity, one of which must be kept wet . This Is accomplished by surrounding

one sensor by a moistened wick. As the air whose moisture content Is being

measured Is drawn across the sensors, the dry one simply records the moist air

temperature. If the air happens to be saturated, no water will evaporate from

the moist element once the wlcklng Is at the same temperature as the moist air.

If the air Is not saturated, water will evaporate from the wlcklng, cooling it

in the process. The cooling continues until equilibrium is established with

the moist air. The dryer the air, the higher the rate of evaporation must be,

and therefore the cooler the wet element will become before equilibrium is

reached. The wet-bulb temperature depression relative to the dry bulb temperature

is therefore a measure of the relative humidity, as will be discussed.

It is surprising how often the user xjIII fail to keep the wick wet and that is

therefore the most important rule. Rule number two is that the water which is

used should be distilled . The solid particles, usually salts, found in non- distilled water will lower the saturation vapor pressure of the water and the

rate of evaporation and the cooling of the wick will be less than it should be.

Also, the wick Itself will absorb these salts and, if not cleaned or changed

before the next usage, the salts will go back into solution even if distilled

water is used next. The third rule, therefore, is to rinse or change the wicks

often, even if distilled water is used.

The instruments are used either by turning on the motor, or swinging by hand to cause air to pass over the wicks. Every 10-20 seconds, read the wet-bulb

temperature (the dry bulb should not change significantly) . This should reach

a minimum value after several readings, at which point both temperature sensors

are read. Be sure that the wet-bulb temperature has reached a minimum, and

that the wick is still wet . If the air is very dry, the initial water temperature

warm, and the water mass in the wlcklng small, the wick may dry out before the

equilibrium temperature is reached. If so, note the minimum temperature achieved

then re-wet the wick with water which has nearly that temperature (or at least

is cooler than on the initial attempt), and continue taking readings, Psychroraetric charts, such as figures 3 and 4, allow the user to determine the relative

humidity by knowing the wet and dry sensor temperatures (usually called wet and

dry bulb temperatures).

14
9

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(M) OlUil AiiaiHnH 15
16
The ASHRAE (American Society of Heating, Ref rigerationg , and Air-Condltioning

Engineers) psychrometrlc charts (figures 3 and 4) display, at standard barometric

pressure, relationships among the following parameters which relate to moist air:

relative humidity, wet and dry bulb temperatures, humidity (mixing) ratio,

specific enthalpy, and the volume of moist air per pound of dry air. Knowledge

of any two of these parameters permits determination of the others. For example,

in figure 3 if the dry bulb temperature is 77°F (25°C) and the wet-bulb temperature

is 58°F (14.4°C) then the relative humidity is 31 percent, the specific enthalpy

is 25 Btu/lb dry air, the humidity ratio is .0064 pounds of moisture per pound

of dry air, and there are 13.65 cubic feet of moist air per pound of dry air.

The wet-bulb temperature is not the same as dew point temperature unless the air

is saturated and therefore the RH is 100%. The reader should realize that the

two terms are not synonymous except under these very special conditions. For

example, if the relative humidity is 100% at 70°F (23.7°C), the wet-bulb and dew

point temperatures are also 70°F (23.7°C). If the relative humidity is 50% at

70°F (23.7°C), the wet-bulb temperature is about 58.7°F (14.4°C) and the dew point

temperature is about 50°F (10°C) . If one uses a dew point device and measures a dew point of 45°F (7.2°C), then the humidity ratio is .0064 lbs/lb = .0064g/g.

If the moist air temperature before cooling to the dew point was 68°F (20°C) , then

the relative humidity at 68°F (20°C) was 44%. This result can also be obtained

from equations (10) and (13) and table A-2. Using the pressure in psla,

.622 P

w

which yields = .1497 psla. Since the air before cooling was at 68°F (20°C)

,

table A-2 shows that the saturation vapor pressure, e, is 0.339 psla. The

relative humidity, from eq (13), is .1493/. 339 = 44%. A final precaution; be

sure that the temperature sensors read correctly, either by calibration or by

comparing them with temperature sensors which are known to be reliable.

More details regarding the theory and use of psychrometers can be found in ref- erences [10] and [11]. More details in the use of the psychrometrlc charts can

be found in reference [14]. The reader should read ASHRAE Standard ANSI/ASHRAE

41.6-1982, section 8, before using a psychrometer [15].

6 . 2 DEW POINT DEVICES

A true dew point (cooled surface) device cools a surface whose temperature can

be measured until the relative humidity immediately above the surface is 100%

and condensation just begins. As with psychrometers, the temperature sensors must be known to be reliable. The saturation temperature is called the dew point

temperature (or frost point temperature, if the saturation temperature and con- ditions are such that ice is the condensate). Equilibrium is reached when

cooling of the surface, which is usually a mirror, causes the observed area of

condensation to grow, while heating the mirror causes the condensation to shrink.

In other words, the desired temperature is such that the area of condensation

observed on the mirror is approximately constant. Precision dew point instruments

electronically sense a light being reflected from the mirror and scattered by

the condensate. The temperature of the mirror is maintained constant to keep

the Intensity of the scattered light constant, meaning that the temperature of

17
the scattering surface Is not changing. Manually operated devices require

practice to obtain stable areas of condensation. A method of visually

observing the mirror surface must be provided, since It Is the only way to

distinguish whether the condensate Is Ice or water. Either may exist for

dew point temperatures between about -22°F (-30°C) and 32°F (0°C) . Initial

undercooling to assure frost formation Is sometimes used since automatic

systems do not distinguish between supercooled water and frost. See reference [11] for further details.

All materials used to connect the dew point temperature device to the moisture

source must neither absorb nor give off water vapor . Of course, there will

be a layer of surface water molecules which will vary as conditions change and

time must be allowed for this layer to come to equilibrium. However, water

must not enter or exit the bulk material. Clean metal, glass, and Teflon

tubing meet this criterion. Polyvinyl chloride (Tygon) does not .

If a pump is used to pull the moisture through the tubing, it must be located

downstream of the mirror. Further, it should not cause a flow fast enough to

Introduce a pressure drop greater than .5% of the source pressure between the source and mirror chamber.

The mirror must be cleaned from time to time to avoid a buildup of surface

contaminants which can cause a lowering of the equilibrium vapor pressure.

The cleaning schedule can be established by noting changes in readings before

and after cleaning (assuming the same conditions can be sampled). Otherwise,

the mirror should be cleaned before each use.

The length of time required to establish equilibrium in a dew point device

depends on the moisture content, since at very low dew or frost points it will

take a long time for enough condensate to accumulate to permit visual operation.

Approximate times for equilibration to occur as a function of dew point are listed below:

Dew Point Equilibration Time

32°F (0°C) 5 min

-4 to + 32°F (-20 to 0°C) 5-20 min

-40 to -4°F (-40 to -20°C) 20 min to 1 hr

-76 to -40°F (-60 to -40°C) 1-2 hr

-94 to -76°F (-70 to -60°C) 2-6 hr

The estimated accuracy of the dew point hygrometer, as shown in figure 5, is valid

only for "constant" ambient humidity. The estimated accuracy varies from

+0.7°F (+0.4°C) for condensation temperatures above freezing to + 3.6°F (+2°C)

at a condensation temperature of -94°F (-70°C). All uncertainties are at the

95% confidence level (see section 7). The following criteria should be used to

determine whether the ambient humidity is "constant," If the standard deviation

(section 7) of the dew point temperature is equal to or less than the values

listed below, the ambient humidity is assumed to be sufficiently "constant" so that the accuracy curve given in figure 5 is valid.

18
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DEW/FROST POINT,

25

Figure 5. The estimated accuracy of a chilled mirror dew point

hygrometer as a function of the measured dew point

temperature. The above curve is valid only when the

hygrometer is properly operated and the ambient humidity

is constant

19
Condensation Duration of Number of Readings

Temperature oampxxng ±±iuc idKen uver jtiquaxxy Calculated Standa:

(min ) Spaced Time Intervals Deviation

X z u XU to LD -1-0 0°^ (A-Ci '^°r\ y u X J to Z J

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14°F (-10°C) 15 10 to 25 +0.18°F (+0.1°C)

140°F (60°C) 15 10 to 25 +0.18°F (+0.1°C)

The following is an example of the use of the above table: If the indicated

condensation temperature is -58°F (-50°C) , take 10 to 25 readings at equally

spaced time intervals for a period of approximately 60 minutes. Compute the

average value, in this case -58°F (-50°C) , and the standard deviation. If this

calculated standard deviation is equal to or less than +0.54°F (+0.3°C), the

ambient humidity is assumed to be "constant" and the expected accuracy of the

reading at -58°F (-50°C) is +2.16°F (+1.2°C) (figure 5).

Once the dew point or frost point temperature has been determined, refer to

table A-1 or A-2 in appendix A to determine the saturation vapor pressure at

that temperature. The relative humidity in the sampled air is determined from

the equation:

P„ e(t,)

RH(%) = rr-. X 100 (14) s(t ) c a

where and are respectively the pressure and temperatures of the air being

sampled, t^ is the measured dew point temperature, is the pressure in the

mirror chamber (which should not differ by more than .5% from P^) , and e(t^) and

e(t^) are the saturation vapor pressures at temperatures and t^. Between

32°F (0°C) and roughly -22°F (-30°C) , the condensate may be either supercooled

water or ice. Visual inspection is required to avoid errors in using table A-1

to find the vapor pressure since the vapor pressure curves for saturated water

over ice and over supercooled water are significantly different. The presence

of supercooled water is usually temporary and an initial undercooling of the

surface to ensure frost formation can be used (see references [11] and [14] for

further details. For further information on the use of dew point hygrometers,

consult ASTM D4230 [16].

6 . 3 THE LITHIUM CHLORIDE DEW POINT INDICATOR (DEWCELL )

One of the most popular devices in the EMCS field is the lithium chloride dew

point indicator. This device is attractive for use either as a sensor or standard.

This section will describe its operation, some precautions for its use, and a simple calibration technique.

The equilibrium vapor pressure over a saturated lithium chloride salt solution

is a known function of temperature. If the solution temperature is monitored and

adjusted so that the water vapor pressure over its surface is in equilibrium with

the vapor pressure in the ambient air, then the vapor pressure in the ambient air

is known. If, in addition, the temperature of the air is knowTi, its dew point can

be determined.

20
The electrical resistance of the salt solution at a given temperature decreases

if there is not enough salt to keep the solution saturated, and increases if

the solution begins to dry out. This characteristic, along with the dependence

of the equilibrium vapor pressure over the salt as a function of temperature,

is used in the construction of the LiCl dew point device.

A bobbin is wrapped with a wick that is impregnated with LiCl and bifilar

electrical current leads are wrapped around the wick. The bobbin, along with

a thermometer, is inserted into an open structured cavity which permits ambient

air to bathe the bobbin. Current from a constant voltage source passes through

the leads and then through the salt solution, and the heat generated in the wick

causes the cavity and salt solution temperature to change until equilibrium is

reached. The cavity temperature is displayed and the operator reads tables or

a graph (or uses a computer program) to determine the dew point of the surrounding

air. If this dew point is to be used to convert to the relative humidity in the

ambient air, the temperature of the ambient air before it enters the cavity must

also be known. To obtain the relative humidity, the vapor pressure at the dew

point is divided by the saturation vapor pressure at the temperature of the ambient

(unheated) air and multiplied by 100, as shown in eq (13),

There are many contaminants which must be avoided with this device; the most

obvious being substances such as other salts or alcohols which will readily

dissolve in water to change the equilibrium vapor pressure or those which v/ill

create explosive atmospheres. Since EMCS are used in commercial and industrial

settings, there may be contaminants of these types in the air. If so, consult

the manufacturer of the instrument before use. Other considerations concerning

the use of these devices include the response time, which is a function of the

air flow past the sensor and the control circuit. Under high flows and/or high

moisture conditions, the heating and cooling may not be rapid enough to permit

either a proper response to changing RH or to prevent excessive wetting of the

wick, while for very low flows it may take an excessively long time for the probe

to react to changes. A guard may be placed around the part of the sensor facing

the moving gas to prevent direct exposure to high flows. Flow must be within

manufacturer's specs. It should be clear that exposure to moisture conditions

which cause excessive wetting and dripping can damage the bobbin - the salt

solution can be washed away or become non-uniform on the wick. At least one manufacturer places on the bobbin a heater which is automatically activated if

exposure to excess moisture occurs, but this clearly cannot help if the drenching

occurs too quickly for the heater to respond. Even with care, the bobbin will

eventually become contaminated and have to be replaced or reactivated. The

manufacturer will supply instructions on how the bobbin can be reactivated, if

that step is practical.

The ultimate accuracy which can be expected from field devices is approximately

3°F (1.5°C) at dew points between 10°F and 93°F (-12°C and +34°C) . At dew points

from -29°F and 10°F (-34°C and -12°C) and again from 93 and 106°F (34°C and 41°C)

the expected uncertainty increases to about +6.3°F (3.5°C) because of possible

changes in the hydration phase of the LiCl solution.

21
Many types of LlCl dew point sensors are currently available. Each has its own optimum operating and maintenance specifications which should be consulted

before installation, reactivation or calibration. A more detailed discussion

of this device can be found in reference [12].

6 . 4 USE OF SATURATED SALT SOLUTIONS

Saturated salt solutions may be used as generators of known relative humidities.

These may be used as calibration standards in two ways. One method is to calibrate

one device at a time over each of several salt solutions. In this case, a different

saturated salt solution is put in a tray on the bottom of each of several small

volume test chambers. Provision must be made for reading the temperatures in the

chambers. The sensor is sealed into each chamber and time is allowed for temperature and relative humidity equilibration. The sensor is read and compared to the

value (table B-1) kno\m to be generated by the salt solution at the indicated

temperature

.

The second method is to use a test chamber large enough to accommodate a group

of sensors. As above, one salt solution at a time is used. Temperature measuring

devices are placed at several points in the chamber to monitor any gradients and

to determine v;hen equilibrium has been reached. Circulation systems may be added;

but one must be careful that a fan doesn't induce unwanted heating, either by the

motor itself or by the blade churning the air. Moving the motor away from the

wall of the container solves the first problem, but the second is not so easy.

Often the components are arranged as sho\\m in figure 6 where the circulating fan

is outside of the insulated calibration chamber. DoxTOstream of the fan, the air

passes through a heat exchanger to return it to the temperature of the calibration

chamber. In general, some air circulation V7ill be required . However, too much

circulation will cause spray or droplets to deposit salt throughout the system;

a situation which can damage some sensors. The need to insulate the test chamber

from the room will depend on how well the room temperature is controlled, and the

temperature homogeneity of the chamber interior. The chamber temperature can be

raised by putting a heater in the circulation system as shown in figure 6 or by

automatically controlling the entire volume with thermostats. Similarly, chilled

water can be circulated around the chamber walls for cooling if desired.

It will require some experimentation to set up a calibration chamber which is

best for a particular purpose and which bypasses problems arising from temperature

gradients and salt spray. Commercial units are also available.

Tables in appendix B, taken from reference [9], list numerous salt solutions and

the relative humidities they generate as functions of temperature. Also given

are estimated uncertainties in those relative humidities as actually generated by

the solutions under i deal conditions. The uncertainties shown are not the

accuracies with which one ordinarily can calibrate field instruments. In general,

one can expect to prepare atmospheres whose relative humidities are known within

approximately +4%. The actual accuracy of a particular calibrated sensor will,

or course, depend upon the sensor itself and the calibration accuracy.

22
w o 1 •Ha

t.M tu

r\ U-J •H1 r*i-t to tj rw1 to 1 t tu o f , i-i

4-*

O 'H AC/) CO

O CJ

C/3

tu

r~H [ ty Ut

03 o o a •H T3 e 4-1 (U o >-l 4-1 u CO COU D 0)

4-1 o

to •H 4-1

Cfi !-i CO c

cfl > o

U-i CO o c

4-1 •H

B TJ

03 14-1

C

)-i o cu tJO a

CO OJ •H -o

T3 < O >. •H J-l 4-1 CO CO cn e cn

(U 6 (U CO a u j:: OJ u c

(U3

•H

23
6 . 5 GENERAL

When instruments are being compared with standards for calibration, the primary

concern must be that they are all sampling air at the same temperature and pressure,

and that the moisture content of the air is the same at the location of each

device being calibrated. While this may seem obvious, it bears repeating because

it is easy to ignore simple precautions which may lead to significant errors.

Consider the use of devices such as motor driven psychrometers or dew point

devices, which must draw an air sample past a sensor. It has already been

indicated that their use as standards will be limited to situations where there

is sufficient gas flow or the sample volume is large enough to permit the

psychrometer to sample the volume of moist air without upsetting the pressure/

temperature conditions at the device being calibrated. Suppose a humidity sensor were located on the interior wall of a duct as sketched in figure 7. To illustrate

the effect of improper location, a valve has been added to permit a motor-driven

psychrometer to draw air out of the duct but right next to the sensor being checked.

VJhen the valve is opened and the psychrometer turned on, the pressure immediately

drops in the vicinity of the sensor. By the ideal gas law, each partial pressure

will be lowered, and the relative humidity will decrease in proportion to the

total pressure change. If the valve offers a restriction to the flow out, a second

effect occurs; namely, the valve will cool the air as the air expands across it. The

magnitudes of these effects on the sensor being calibrated will be highly dependent

on the particular situation. However, this type of problem is easily avoided by

placing the sampling port far away from the sensor to avoid the problem. Choosing

the best location may require some experiments with placing pressure/ temperature

sensors along a laboratory duct.

Finally, when comparing or calibrating dial-type devices, it is wise to tap the

instruments lightly before reading to reduce the errors caused by the vast

differences in the static and kinematic friction coefficients in the mechanical

parts.

24
DC

CO

LU

CD

00 Oo

4-1 -P c

g a -u

0) 0) CJ

ni (/) i-l 0) ex (-1 CO

O 4-1 O -H

U-l 4-1 -H O S-< 4-1 O -H rH O

&, W)

X rH O

0) CO e C

CT3 0)

3

•H

25
7. ASSIGNING ERRORS TO THE CALIBRATED DEVICES

A calibration task has not been completed until the associated errors and

uncertainties have been assessed and reported. This section will indicate

how the error and uncertainty determinations might proceed. It is understood

that the instruments or devices being calibrated are relative humidity indicators, although the principles discussed in this section are general.

7 . 1 RANDOM UNCERTAINTIES AND PRECISION

Random uncertainties arise because instruments and people possess limited

resolving pov/er and are subject to influences which either change the operation

being used to make the measurement, or cause the relative humidity being measured

to vary, or both. Random variations are always present to some degree. Rowever, if the instrument making the measurement is capable of detecting changes

of only + 5% RH and the actual variations v;hich are taking place are only

+. 1% RK, the actual variations probably will not be noticed. The smallest

m.eaningful change that an instrument can detect is often referred to as its

resolution. The precision which is reported after a calibration reflects all

random errors associated with repeated measurements and will depend on the

stability of the relative humidity being measured (and therefore on the stability

of moisture content, temperature, and pressure) and on the skill of the operator,

as well as the fundamental instrument resolution. The reported precision is

simply an estimate, and will not likely be the same from one calibration to the

next

.

A statistical quantity called the estimated standard deviation of the measurement,

designated as "s", can be computed to give information on the random variations.

For the moment, let us assume that any actual variations in the generated

humidity (or in the standard against which a device is being calibrated) are

small compared to the variations observed in the device itself. The calculation

of s then proceeds as follows:

At each RH, a series of n separate readings N, N2 , N^ is taken using the

instrument being calibrated, and the average indicated value N is calculated.

The differences (Nj - N) , (N2 - N),etc. between each individual reading N and N are computed, and those differences are squared. These squared differences are then added to obtain a sum E, that is, Z = (N-^ - N) 2 + (N2 - N) ^ + . . + (N^ - N)

?

This quantity is then divided by one less than the number of readings, giving a number E/(n-l). The estimated standard deviation of the series of measurements,

s", is the square root of this number. The value of s is such that if a known RH

is measured again and again, approximately 68 percent of the measurements would

lie within the range N + "s. The more readings used to compute 's', the closer it

becomes to representing the true random processes in the calibration.

The expected random uncertainty, or instrument precision, at each N is given by

s' or some multiple of "s, and this number should be stated in any documentation

of the calibration. It is not good practice to use N + "s for the random un- certainty since one expects about 32 percent of the readings to lie outside of

this range. The use of either 2's or 3's is common; the latter generally being

considered conservative. The value 2s is often used as an approximation to the

95 percent confidence interval. The multiple of s to be used is the calibration

choice but it must be clearly stated in the calibration document.

26
When the calibration has been completed, and if s is almost the same at each

of the N values, then one may use the largest of these as representative of

the Instrument's precision over its range of calibration, remembering that the

statement of precision should preferably be made in terms of 2s or 3|. If 's varies, the instrument precision may be reported at each individual N.

7 . 2 SYSTEMATIC ERRORS - FIRST TYPE

Systematic errors are the difference between N and the "true" value of the

relative humidity being measured. There are two types of systematic errors. The first type is the difference between the N values of the device being

calibrated and the corresponding value as indicated (or generated) by the

standard. Evaluating and reducing these differences is the main object of

performing the calibration. The second type of systematic error is discussed

in section 7.4.

7.3 PRESENTATION OF RESULTS

Let us review an example of how the results of a calibration might be presented to indicate the magnitude of systematic errors of the first type as well as the instrument precision at different values of RH of the calibrated

instrument. An instrument was calibrated over salt solutions at 68°F (20°C)

and the standard deviations of the calibration readings were determined at

each value. The calibration data are presented in table 3. Columns A and B,

respectively, show the relative humidities known to be generated by the salts

and the average values , N, determined by the instrument being calibrated.

Column C gives the value of 3'§^ at each point, N, and column D gives the

difference between the average instrument reading and the salt solution value.

Column E results from adding and subtracting Column C from Column D. The

table by itself will not tell the entire story, but a plot such as the one shown in figure 8 shows all details at a glance. The differences (column D

of the table) have been plotted against the values of N (column B) , and a curve (solid line) has been drawn to indicate the systematic differences

between the instrument being calibrated and the standard (salt solutions in

this case) as a function of the indicated RH, The question should immediately

arise, "How do we know that these differences aren't within the expected random

variation of the instrument readings at the respective points, and therefore

not really systematic?" This question may be answered by including the vertical

random error bars which range between N + 3s at each point. All readings which

lie within the limits of the random error bars cannot be considered as being

actually different from each other. We see that for the first four points, these

random errors overlap the zero difference line, indicating that systematic errors

of the first type are smaller than the precision of the device below about 20% RH.

After that, however (with one exception), a systematic deviation is obvious.

Note that the sample data have a point at about 27% RH which simply doesn't fit

the curve. It is often hard to recognize such a point from a table. In this

particular example, a single point like this is usually considered as a human

error and is ignored in the evaluation of the instrument. However, if the data were not spaced as closely as this example shows, or if the instrument were to

be used often near 35% RH (that is, near an indicated 20% RH) the user might

want to check the calibration of the instrument at this questionable calibration

point, or add another one close to it.

27
Table 3

Results of a hypothetical calibration of a device, using

saturated salt solutions at 68°F (20°C)

Rel. Hum. over salt %

6.61

11.31

18.56

23.11

33.07

43.16

54.38

59.14

69.90

79.23

85.11

97.59

B

Measured

Rel. Hum. %

8.5

10.0

18.2

24.0

27.5

47.0

60.0

65.2

73.3

80.0

82.0

89.8

3^

%RH

3.0

2.5

2.2

2.1

1.2

2.5

1.9

2.1

3.0

3.0

2.2

3.1

D

Difference

%, B-A

1.89

-1.31

- .36

.89

-5.57

3.84

5.62

6.06

3.4

.77

-3.11

-7.79

E

Expected range

of difference, %

-1.11 to 4.89

-3.81 to 1.19

-2.56 to 1.84

01.21 to 2.99

-6.77 to -4.37

1.34 to 6.34

3.72 to 7.52

3.96 to 8.16

.4 to 6.4

-2.23 to 3.77

-5.31 to -91

-10.89 to -4.69

28
If, upon recalibration of the device, a curve such as the one shown by the

dashed line in figure 8 were to be obtained (lying within the precision

[random error bars] of the individual points from the first calibration),

then one would say that the instrument has not significantly changed between

the calibrations.

Please note : This example was stated to be for a calibration temperature of

20°C. The calibration is not complete unless similar sets of data are obtained

at enough different temperatures to permit correlation to be made over the

entire temperature range in which the device is to be used. Some instruments,

such as dew point devices which do not read relative humidity but moisture

content, can be used with only a single set of curves.

7.4 SYSTEMATIC ERRORS - SECOND TYPE

Also shown in figure 8 are reasonable systematic error limits to be expected

when using salt solutions (see section 6.4). These limits of uncertainty

remain even after the corrections obtained from the curve have been applied,

and represent a second type of systematic error. In the case of saturated

salt solutions, table B-1 , appendix B, shows that the bes t obtainable values

under ideal conditions are uncertain by magnitudes on the order of tenths of

a percent RH, depending upon the salt and its temperature. The "true" value

over the salt solutions cannot be any better than this value. To this value,

one must add the possible effects of the uncertainties in the temperat ure and temperature gradients as well as to account for the non-uni formity of

moisture distribution throughout the calibration chamber and any other effects

that may exist . This must be done realistically, and requires no small amount

of thought. Obviously, if the error source (s) and effect (s) are known, a correction can be applied. However, here the "what if" types of errors are being considered which were not revealed in the calibration experiments. These

sources of error will be present in any calibration procedure, not just when

using salt solutions.

When all imaginable sources of systematic error have been listed and estimates

made of their possible effects on the relative humidity, these must now be

totaled in some way. If most of the effects listed would cause the RH which is

believed to have been generated and measured by the standard to read high (or low)

,

then one would add them algebraically to obtain the maximum possible systematic

error in the "true" RH. It is more often the case that most of these possible

sources could cause errors in either direction. In this case it is perfectly

reasonable to compute the square root of the sum of the squares of the error magnitudes, and assign this number as the possible systematic error in the

"true" RH. This error will remain even after the calibration is completed.

Referring again to figure 8, it is therefore possible for the entire curve to

be shifted up or down by 4% RH.

The 4% RH number is considered to be a reasonable estimate of residual unknown

systematic errors when reasonably careful calibrations are performed using

saturated salt solutions, and has been obtained by the process just described.

Systematic errors should not be taken for granted; each source must be considered . In fact, the possible systematic error sources for any process being used to

generate or determine a "true" or "known" relative humidity for calibration

purposes must be considered. The calibration curve and random error bars of

figure 8 are not sufficient descriptions of final errors. A statement concerning

29
o

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5-i CO CO

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j-J u CJ CU C

CO 01 S-< CO

5-1 4-' D •H M ^ H dJ CO •H <Cfl X. C

t-H M-( 4-1 -H

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4-1 O ^

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Indicated RH- RH Measured by Standard, % RH

30
possible residual systematic errors should also be included with the calibration

report. Of course, it is possible that they may be completely negligible, but even this must be documented.

7.5 ADDITIONAL FACTORS

When the instrument being calibrated and the standard both have significant random variations , one other situation must be considered. Suppose the standard

happens to be a device with a precision which is similar to the instrument being

calibrated. That is, suppose the use of two hair hygrometers, one of which has

been calibrated elsewhere and is now the standard. Now the random variations of

the standard are not negligible with respect to those of the device being calibrat

as was the case in section 7.1. Measurements made using the device being calibrated will be designated with the subscript x while the standard is designated by the subscript y.

Suppose one is measuring a relative humidity which is near 25% RH at some temperature. The residual systematic errors were discussed in section 7.4

above and won't be considered here. Suppose for then that five repeated pairs

of measurements RH^ and RHy were made and that the respective results are listed

in table 4. Compute the difference x-y for each of the five readings, as well

as a column of values (x-y)2. Find the totals of each column. In this example,

the mean values RH^ and RHy and their respective estimated standard deviations

have been computed for comparisons, but their values would not be used in the

calibration report. Compute the quantity C, which is the square root of the

average value of Z(x-y)2, and the quantity d, which is the average value of x-y.

The quantity d is the_systematic difference between the two instruments at this

particular value of RH as determined by the standard (25.4% RH) . Finally

compute s - I

-—/ . The value of the estimated standard deviation, x, for the

calibration^test readings of the two instruments lies between the estimated value

for and Sy. Therefore, the value of s for this calibration should be reported

since the residual random errors of both instruments are reflected in this value.

7.6 CALIBRATION RECORDS

The maintenance of calibration records is often neglected in the field. However,

the value of a complete file on the calibration of each instrument and each

standard used in EMCS cannot be over emphasized. The date and source of the

calibration of a standard being used to calibrate other instruments in the system

is a good place to start. The time interval for returning the standard to a qualified laboratory for recalibration can be determined by reviewing the dates

and the status of the standard when it was examined by the calibrating laboratory.

Calibration records of standards should be made available to the laboratory

performing the subsequent calibrations on the standard. Complete records on the use and calibrations of a standard can often reveal factors which require particular attention in a calibration process.

Similar record keeping is applicable to the calibration of each instrument in

the EMCS. The calibration records of the instruments monitoring the moisture in an EMCS can reveal the source of many differences in the computation of a heat

31
Table 4

Example for calculation of overall uncertainties when repeated

measurements of the same relative humidity are made with

two instruments which have nearly equal random uncertainties

Reading No. RH X RH y x-y (x-y)^

1 ZZ . J

OK. A —J . J in Q

Q

2 22.6 25.4 -2.8 7.84

3 22.2 25.8 -3.6 12.96

4 22.3 25.1 -2.8 7.84

5 22.6 25.3 02.7 7.29

Totals 112.0 127.2 -15.2 46.82

RH = 22.4, X

s = .19

X

RH = y

25. 4, s = y

c ^J'^f

1- =3.06 d = -15. 2/5 = -3

's =\ 1 = .25 (single system standard deviation)

32
balance being performed on a heat exchange system or a particular portion

of the system. One of the most important items to be included in calibration

records are any possible effects of the uncertainties in the temperatures

being recorded during calibration. Ambient conditions and any other factors

which may affect the calibration should be noted. Systematic errors cannot

be determined without this information.

Records on the calibration of LiCl dew point temperature sensors are often

used to indicate the need for reactivating the bobbin. The contamination of

the bobbin is often slow. However, a review of the calibration records can often point out when the need for replacement will occur and, as well, indicate

the need for replacing the LiCl sensor with a sensor of another type which is

less susceptible to contamination.

Calibration files should contain the individual readings of the points measured

at each temperature during the calibration and include identification of the

instruments used as standards in the calibration process. Calibration records

on all devices should include detailed information such as that shown in tables

3 and 4 in addition to any plots made to improve the recognition of points that are off scale such as that shown in figure 8.

33
REFERENCES

[I] Hasegawa, S. and Little, J.W. , The NBS Two-Pressure Humidity Generator,

Mark 2, J. Res. Nat. Bur. Stand. (U.S.), 81A, No. 1, 81, 1977.

[2] Harrison, L.P., Fundamental Concepts and Definitions Relating to Humidity,

Humidity and Moisture , Vol. 3, A. Wexler and W. Wildhack (Eds.), Reinhold

Publishing Corp., N.Y., 1964.

[3] Harrison, L.P., Imperfect Gas Relationships, Humidity and Moisture , Vol. 3,

A. Wexler and W. Wildhack (Eds.), Reinhold Publishing Corp., N.Y., 1964.

[4] Wexler, A., Vapor Pressure Formulation for Water in the Range 0 to 100°C,

A Revision, J. Res. Nat. Bur. Stand. (U.S.), BOA, 775, 1976.

[5] Wexler, A., Vapor Pressure Formulation for Ice, J. Res. Nat. Bur. Stand.

(U.S.), 81A, 5, 1977.

[6] Wexler, A., Measurement of Humidity in the Free Atmosphere Near the

Surface of the Earth, Meteorological Monographs , Vol. II, No. 33, 262,

1970 (American Meteorological Society).

[7] Kao , J.Y. , and Snyder, W.J., Application Information on Typical Humidity

Sensors Used in Heating, Ventilating, and Air Conditioning (HVAC) Systems,

NBSIR 81-2460, January 1982.

[8] Kao, J.Y. , and Baker, D.W. , A Commentary on the Instrumentation for Building

Energy Monitoring and Control Systems (EMCS), NBSIR 80-2104, September 1980.

[9] Greenspan, L. , Humidity Fixed Points of Binary Saturated Aqueous Solutions,

J. Res. Nat. Bur. Stand. (U.S.) 81A, 1977.

[10] Harrison, L.P., Some Fundamental Considerations Regarding Psychrometry

,

Humidity and Moisture, Vol. 3., A. Wexler and W. Wildhack (Eds), Reinhold

Publishing Corp., N.Y. , 1964.

[II] ASHRAE, Brochure on Psychrometry, ASHRAE, 1791 Tullie Circle, N.E.,

Atlanta, GA, 1977.

[12] Nelson, D.E., and Amdur, E. J. , The Mode of Operation of Saturation

Temperature Hydrometers Based on Electrical Detection of a Salt-Solution

Phase Transition, Humidity and Moisture , Vol. I, R.E. Ruskin, (Ed.)

Reinhold Publishing Corp., N.Y. , 1964.

[13] Wexler, A., Humidity and Moisture, ISA Transducer Compendium, 2nd Edition,

Part 3, G.F. Harvey, (Ed.), Instrument Society of America, 1972.

[14] ASHRAE Fundamentals Handbook, ASHRAE, 1791 Tullie Circle, N.E. , Atlanta, GA,

1981.

[15] ASHRAE Standard Measurement Guide: Section on Standard Methods of Measuring

Humidity with Cooled Surface Condensation (Dew Point) Hygrometer, ASHRAE

Standard 41.6-1982, ASHRAE, 1791 Tullie Circle, NE, Atlanta, GA, 1983

[16] ASTM D4230 - Standard Method for Measurement of Moist Air Properties, ASTM,

1916 Race Street, Philadelphia, PA, 1983

34
Appendix A

Tables of Saturated Vapor Pressures

(Tables in this appendix were selected from reference 11)

A-1
Table A-1

Saturation Pressures of Water Vapor Over Ice

as a Function of the Temperatures

Absolute Pressure Absolute Pressure

Fahr.

Temp

.

t(F)

5 P X 10

Fahr.

Temp.

t(F)

P X 10

Lb/Sq In. In. Hg Lb/Sq In. In. Hg

-160 0 004949 0 01008 -30 0. 3440 0. 7003

-155 0 009040 0 01840 -28 0. 3876 0 7891

-150 0.01620 0.03298 -26 0.4363 0.8882

-145 0.02850 0.05803 -24 0.4905 0.9987

-140 0 04928 0. 1003 -22 0. 5509 1.122

-135 0 08380 0. 1706 -20 0 . 6181 1.259

-130- 0.1403 0.2856 -19 0.6545 1.333

-125 0.2312 0.4708 -18 0.6928 1.410

-120 n 37 'S7 0 7ft49 -17 0 7339 X1 • 493 H Z/ -J -115 0 fiOl 9 1 996 -16 0 77 57 1 579

-110 0.9517 1.938 -15 0.8204 1.670

-105 1.486 3.025 -14 0.8676 1.766

-100 9 9 Q1 H . U -13 n Q1 79 X1

. 8A7 o u / -95 7 1

/ . xuo -12 nu . QAQA y

o

X1 . Q77 / A4 -90 5.260 10.71 -11 1.024 2.086

-85 7.841 15.96 -10 1.082 2.203

-80 11 S7 -9 1 1 A"^ 9 9 7 -75 16 89 34 39 -8 1 907 9 457

-70 24.43 49.74 -7 1.274 2.594

-65 35.01 71.28 -6 1.344 2.737

-60 49 79 1 01 9 -5 1 419 2.888

-55 70 01 142 .

6

-4 1 .496 3. 047

-50 97.76 199.0 -3 1.578 3.213

-48 111.5 227.0 -2 1.664 3.388

-46 127.0 258.5 -1 1.754 3.572

-44 144.4 294.0 0 1.849 3.764

-42 164.1 334.0 1 1.948 3.966

-40 186.1 379.0 2 2.052 4.178

-38 211.0 429.5 3 2.161 4.400

-36 238.8 486.2 4 2.276 4.633

-34 270.0 549.7 5 2.396 4.878

-32 304.9 620.8 6 2.521 5.134

A-2
Table A-1 , Continued

Saturation Pressures of Water Vapor Over Ice

as a Function of the Temperatures

Fahr

.

Temp.

t(F)

Absolute Pressure

P

Lb/Sq In. In. Hg

7 0.02653 0.05402

8 0. 02791 0 05683

9 0.02936 0.05977

10 0.03087 0.06286

11 0.03246 0.06608

12 0. 03412 U • U U 7 H U 13 0.03585 0.07300

14 0.03767 0.07669

15 0.03957 0.08056

16 0. 04156 0 08461

17 0.04363 0.08884

18 0.04581 0.09326

19 0.04808 0.09789

20 0. 05045 0.1027

21 0.05293 0.1078

22 0.05552 0.1130

23 0.05823 0.1186

24 0.06105 0.1243

25 0.06400 0.1303

lb 0.06708 0.1366

21 0.07030 0.1431

28 0.07365 0.1500

29 0.07715 0.1571

30 0.08080 0.1645

31 0.08461 0.1723

32 0.08858 0.1803

32 0.088586 0.18036

A3
Table A-2

Saturation Vapor Pressures of Water

as a Function of the Temperature

Fahr.

Temp

.

t(F)

Absolute Pressure

p3

Lb/Sq In. In. Hg

33 u . uyz/z

/

n T Q 7 7 Q U . lo / / O 34 0.095999 0.19546

35 0.099908 0.20342

0.10396 0.21166

-J / 0.10815 0.22020

38 0.11249 0.22904

39 0.11699 0.23819

4(1 0. 12164 0.24767

41 0.12646 0.25748

42 0.13145 0.26763

43 0.13660 0.27813

44 0. 14194 0. 28899

45 0.14746 0.30023

46 0.15317 0.31185

47 0.15907 0.32387

48 0, 16517 0. 33629

49 0.17148 0.34913

50 0.17799 0.36240

51 0.18473 0.37611

52 0. 19169 0. 39028

53 0.19888 0.40492

54 0.20630 0.42003

55 0.21397 0.43564

56 0. 22188 0.45176

-J / 0.23006 0.46840

58 0. 23849 0.48558

59 0.24720 0.50330

fin 0.25618 0.52160

61 0. 26545 0. 54047

62 0. 27502 0. 55994

63 0. 28488 0 58002

D M- 0.29505 0.60073

D 3 0.30554 0.62209

6ft 0.31636 0.64411

67 0.32750 0.66681

68 0.33900 0.69021

69 0.35084 0.71432

70 0.36304 0.73916

71 0.37561 0.76476

Fahr

.

Temp

.

t(F)

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

Absolute Pressure

P

Lb/Sq In.

0.38856

0.40190

0.41564

0.42979

0.44435

0.45935

0.47478

0.49066

0.50701

0.52382

0.54112

0.55892

0.57722

0.59604

0.61540

0.63530

0.65575

0.67678

0.69838

0.72059

0.74340

0.76684

0.79091

0.81564

0.84103

0.86711

0.89388

0.92137

0.94959

0.97854

1.0083

1.0388

1.0700

1.1021

1.1351

1.1688

In. Hg

0.79113

0.81829

0.84626

0.87506

0.90472

0.93524

0.96666

0.99900

1.0323

1.0665

1.1017

1.1380

1.1752

1.2136

1.2530

1.2935

1.3351

1.3779

1.4219

1.4671

1.5136

1.5613

1.6103

1.6607

1.7124

1.7655

1.8200

1.8759

1.9334

1.9923

2.0529

2.1149

1786

2440

3110

3798

A-4
Table A-2 (continued)

Saturation Vapor Pressure of Water

as a Function of the Temperature

Fahr.

Temp.

t(F)

Absolute Pressure

Lb/Sq In. In. Hg

108 1.2035 2 .4503

109 1.2390 2.5226

110 1.2754 2.5968

111 1.3128 2.6728

112 1.3510 2.7507

113 1.3902 2.8306

114 1.4305 2.9125

115 1.4717 2 . 9963

116 1.5139 3.0823

117 1.5571 3.1703

118 1. 6014 3. 2606

119 1. 6468 3. 3530

120 1.6933 3.4477

121 1.7409 3.5446

122 1. 7897 3. 6439

123 1.8396 3. 7455

124 1.8907 3.8496

125 1.9430 3.9561

126 1. 9966 4.0651

127 2.0514 4 . 1768

128 2.1075 4.2910

129 2.1649 4.4078

130 2.2237 4.5274

131 2.2838 4.6498

132 2.3452 4.7750

133 2.4081 4.9030

134 2.4725 5.0340

135 2 . 5382 5. 1679

136 2 . 6055 5 . J049

137 2.6743 5.4450

138 2.7446 5.5881

139 2.8165 4.7345

140 2.8900 5.8842

141 2.9651 6.0371

142 3.0419 6.1934

143 3.1204 6.3532

Fahr.

Temp

.

Absolute

P

Pressure

Lh/Sa In In He

_L144'-t "-r 3 .2006 6 . 5164

145 3.2825 6.6832

146 3.3662 6.8536

147 3.4517 1 .0211

1 48 3 5390 1 .2056

149 3.6282 7.3872

150 3.7194 1 .5121

151 3.8124 7.7622

152 3 . 9074 7 . 9556

153 4.0044 8.1532

154 4.1035 8.3548

155 4.2046 8.5607

156 4.3078 8 .7708

157 4.4132 8.9853

158 4.5207 9.2042

159 4.6304 9.4276

160 4.7424 9. 6556

161 4.8566 9.8882

162 4.9732 10.126

163 5.0921 10.368

X1 64U H- 5 ?1 34 10.615

165 4.3372 10.867

166 5.4634 11.124

167 5.5921 11.386

X vj O 5 7? 33 11.653

169 5.8572 11.925

170 5.9936 12.203

171 6.1328 Iz . 48

/

172 6.2746 12.775

173 6.4192 13.070

174 6.5666 13.370

175 6.7168 13.676

176 6.8699 13.987

177 1 .0259 14.305

A" 5
Table A-2 (continued)

Saturation Vapor Pressure of Water

as a Function of the Temperature

Fahr.

Absolute Pressure

p3

Temp.

t(F) Th/^G Tn Tn He

178 7.1849 14.629

179 7.3469 14.959

180 7 . 5119 1 5 295

181 7.6801 15.637

182 7.8514 15.986

183 8.0258 16.341

184 8. 2035 16. 703

185 8.3845 17.071

186 8.5688 17.446

187 8.7565 17.829

188 8 . 9476 18.218

189 9.1422 18.614

190 9.3403 19.017

191 9.5420 19.428

192 9 7473 1 9 846

193 9.9563 20.271

194 10.169 20.704

195 10.386 21.145

196 X U . DUO 71 SQA

197 10.830 22.050

198 11.058 22.515

199 11.290 22.987

200 11 S? 6 23 468

201 11.767 23. 957

202 12.011 24.455

203 12.260 24.961

204 12.513 15.476

205 12.770 26.000

206 13.031 26.532

207 13.297 27.074

208 13.568 27.625

209 13.843 28.185

210 14.123 28,754

211 14.407 29.333

212 14.696 29.921
Appendix B

Tables of Equilibrium Relative Humidity of Selected

Saturated Salt Solutions from 32°F to 212°F (0°C to 100°C)

(Tables in this appendix were selected from reference 9)

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B-5
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

I. PUBLICATION OR

REPORT NO.

NBS BSS 157

2. Performing Organ. Report No 3. Publication Date September 1983

4. TITLE AND SUBTITLE

GENERAL GUIDELINES FOR THE ON-SITE CALIBRATION OF HUMIDITY AND MOISTURE

CONTROL SYSTEMS IN BUILDINGS

5. AUTHOR(S)

Richard W. Hyland and C. Warren Hurley

6. PERFORMING ORGANIZATION (if joint or ott^er than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Naval Civil Engineering Conmiand

Code L62

Port Hueneme, CA 93043

10. SUPPLEMENTARY NOTES

Library/ of Congress Catalog Card Number: 83-600574

[]J Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificqnt information. If document includes a si gnificant

bibliography or literature survey, mention it here)

The control of the moisture content in the air in buildings directly affects the

comfort of the building occupants and is one of the more important tasks of an energy

management and control system (EMCS). Unfortunately, a lack of understanding of the

principles of operation and calibration of the various instrumentation used in monitoring the moisture content of air often makes this one of the most difficult and most

neglected tasks for those assigned the responsibility of maintaining and operating

building heating, ventilating and air-conditioning (HVAC) control systems.

This report was written for the purpose of assisting the managers and operators of

EMCS in buildings in the understanding and use of various techniques for on-site

measurement of the moisture content in the air. In addition, various methods of onsite calibration of typical types of moisture monitoring instruments are also discussed.

The ideal gas law is reviewed because of its direct application in converting the

various humidity units. Relative humidity and dew point temperature are defined and

their relationship described. Typical types of moisture monitoring instruments are described together with their method of operation. Several calibration techniques are discussed together with their typical advantages and disadvantages. The last section of

this report is devoted to the assessing and reporting of the errors or uncertainties

associated with the calibration and functioning of various moisture monitoring instruments. This very important area is often neglected and can result in excessive

operating and maintenance costs.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) chilled mirror hygrometer; dew point temperature; humidity generator; humidity sensors;

lithium chloride humidity sensors; moisture content of building air; relative hunidity; saturated salt solutions; sensor calibration; sensor precision; specific

13. AVAILABILITY humidity; wet-bulb temperature

[x~| Unl imited

I I For Official Distribution. Do Not Release to NTIS

[x~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

56

15. Price

$4.00

•i; U.S. GOVERNMENT PRINTING OFFICE : 1983 O—380-997 (5191

)

USCOMM-DC 6043-P80
NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from lime to time are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year Annual subscription; domestic

$18; foreign $22.50. Single copy, $5.50 domestic; $6.90 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies. Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical r 'ormation developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safely characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on N BS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's tech- nological marketplace. Order the above NBS puhlicalions from: Superintendent of Documents. Government Printing Office. IVashington. DC 20402. Order the following NBS publications— Fl PS and NBSIR s—from

the National Technical Information Service. Springfield. VA 22161

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Service , Springfield, VA 22161,

in paper copy or microfiche form.
U.S. Department of Commerce

National Bureau of Standards

Washington, D.C. 20234

Official Business

Penalty for Private Use $300

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