NBS

Publications

NBSIR 81-246

AlllDb DBVlflb

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Application Information on Typical

Hygrometers Used in Heating,

Ventilating and Air Conditioning

(HVAC) Systems

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Building Equipment Division

Washington, DC 20234

January 1 982

Prepared for Civil Engineering Laboratory

Ap \ Construction Battalion Center

, ^ Hueneme, CA 93010

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cil-2460

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NBSIR 81-2460

APPLICATION INFORMATION ON TYPICAL

HYGROMETERS USED IN HEATING,

VENTILATING AND AIR CONDITIONING

(HVAC) SYSTEMS

JIATIONAI, BUREA"’ PF STANDARDS

LIBRARY DEC 1 0 1982

CXC-C , '

James Y. Kao

William J. Snyder

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Building Equipment Division

Washington, DC 20234

January 1 982

Prepared for Civil Engineering Laboratory

Naval Construction Battalion Center

Port Hueneme, CA 93010

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

ABSTRACT

This report provides hygrometer selection Information for application in

heating, ventilating, and air-conditioning (HVAC) systems. A general review

of hygrometer literature has been provided and the most commonly used ones

for HVAC are discussed. Typical hygrometer parsimeters are listed to indicate

the type of performance that can be earpected. Laboratory test results of

self-regulating, salt-phase transition hygrometers are presented and discussed

in detail.

Key words; bxiilding energy monitoring; heating, ventilating, and air

conditioning controls; humidity; humidity control; humidity

measurement; humi dity sensor; hygrometer.

iii
TABLE OF CONTENTS

Page

ABSTRACT ill

LIST OF FIGURES AND TABLES v

1. Introduction 1

2. General Humidity Sensor Survey 2

2.1. Classification of Hygrometers 2

2.2. Hygrometer Characteristics 4

3o HVAC Applications 5

3.1. Self-Regulating, Salt-Phase Transition Hygrometer 5

3.2. Impedance Type Hygrometer 7

3.2.1. Dimmore Relative Humidity Sensor 10

3.2.2. Aluminimi Oxide Sensor 11

3.3. Dimensional Change Hygrometer 11

4. Sensor Performance 13

4.1. Factors Affecting Performance 13

4.2. Definition of Performance Terms 13

4.3. Measured Performance of Lithium Chloride Self-Regulating,

Salt-Phase Transition Dew Point Sensors 15.

4.3.1. Calibration Procedure 16

4.3.2. Temperature Sensing Location Test 16

4.3.3. Static Humidity Drift Test 17

4.3.4. Results and Discussion of Sensor Tests 18

4.4 Temperature Measurement 20

5. Sensor Selection and Maintenance 20

6. Conclusions 22

REFERENCES 23
LIST OF TABLES

Page

1. ''Classification of Factors Affecting Humidity Sensor Performance 24

2. An "Ideal" Humidity Sensor 25

3. First Calibration of Self-Regulating Lithium Chloride Dew Point

Sensor Number 1 26

4. Second Calibration of Self-Regulating Lithium Chloride Dew Point

Sensor Number 1 27

5. Eqiiilibrium Relative Humidity of Saturated Aqueous Salt Solutions

at 25“C 28

6. Typical Static Hximidity Test Conditions 28

7. Response Time Stmnnary for Static Humidity Test - Sensor #1

before Exposure to Sulfur Dioxide or Ammonia 29

8. Response Time Summary for Static Humidity Test - Sensor #2 30

9. Response Time Summary for Static Htmii dity Test - Sensor #1

after Exposure to Sulfur Dioxide or Ammonia 30

10. Sensor //I History in Hours of Exposure in Static Hxmiidity Test 31

LIST OF FIGURES

1. Block Diagram of a Sensor System 32

2. Sensor No^ 1 Calibration 33

3. Temperature Sensor Location Effect 34

4. Schematic of the Static Humidity Drift Test Apparatus 35

5. Static Humidity Drift Test Apparatus 36

6. Instimmentation for Static Drift Test 37

V
vi \' " I
1. Introduction

A wide variety of hygrometers are available to the engineer who is designing

or retrofitting HVAC systems. Unfortunately, there is no single hygrometer

on the market today that maintains high accuracy and fast response over the

full range of hximidity. Thus, the engineer must examine the measurement problem in terms of performance criteria such as span, accuracy, response time,

sensor lifetime, contamination effects, maintenance and recalibration

requirements

Several excellent handbooks and references [1-10]* address the general problem

of hygrometer selection but do not emphasize the HVAC application. Therefore,

the intent of this report is to describe realistic performance requirements,

to establish correct maintenance schedules, and to identify sensor development areas for humidity sensors in HVAC applications. This report will

primarily be concerned with humidity measurements, but will briefly discuss

temperature measurement, since temperature is often a hygrometer's output

signal. Because of the myriad of hygrometers available, only those hygrometers

currently tised in HVAC applications will be discussed in detail.

The energy consumption of buildings, depends to a large extent, on the

proper design and operation of the HVAC systems. Hygrometers used for

controlling outside air economizer cycles and for controlling humidifiers

may affect the building energy usage substantially.

* Numbers in brackets indicate references cited at the end of the report.
In this report, a general literature survey of hygrometers is presented to

develop a background and create an awareness of sensor types, characteristics

and problems. The detailed characteristics of hygrometers currently being

used in HVAC systems are discussed with regard to performance. Finally,

information for hygrometer selection and maintenance is presented.

2 . General Humidity Sensor Survey

2.1. Classification of Hygrometers

Most hygrometers use the principle of changing physical characteristics

caused by the absorption and desorption of water molecules on a surface. A

change in electrical impedance, ion exchange capacility, dielectric strength,

density, or polarizability can be used to indicate the sorption of water

molecules

There are many types of resistive hygrometers. Some depend on the change in

resistance or conductivity of an ionizable hygroscopic salt with humidity.

The most commonly used salt is lithium chloride Cusually less than 5% by

weight)

Capacitive hygrometers use a thin film of hygroscopic material as a dielectric

of a capacitator. The capacitance of the hygrometer changes with the humidity

of the air (actually changes with the partial pressure of water in the air).

Impedance hygrometers such as aluminum oxide are based on the change of electrical impedance (both resistive and capacitive) of the sensor with humidity.

2
Mechanical sensors rely on a change in dimension with sorption of water

molecules. Organic materials such as nylon, paper, organic skin membranes

and hair are commonly used. As an example, human hair (under a one-gram tensile load) expands in a nonlinear manner with increasing hiimidity at approximately 1.3 X 10 ^ tx) 10 X 10 ^ meter per meter for 1% of relative humidity

change JlO]

Spectroscopic hygrometers do not require a surface to sense humidity since

they depend on the absorption of electromagnetic radiation by water molecules

in the vapor state. The radiation may be infrared, ultraviolet, visible or

in the microwave region.

Piezoelectric detectors consist of a crystal coated with a hygroscopic

material. As the hygroscopic material absorbs water from the ambient, the

oscillating frequency of the crystal changes. This frequency change is

detected and is used as a measure of ambient relative humidity.

Measurement of humidity by a gravimetric method is considered as a primary

standard [5] because only mass and volume are measured. Although gravimetric

methods have the greatest accuracy (approximately 1% mass and volume) , this

method is difficult to vise as a continuous measure of moisture in air

conditioning systems.

Psychrometers give an indication of humidity from a measurement of wet-bulb

and dry-bulb temperatures. They depend upon the cooling of a wetted temperature sensor by an air stream. To obtain accurate readings the air stream

should flow across the wet wick at 3 to 5m/sec. The main advantage of

3
psychrometers is simplicity but the data must be reduced by using the

psychrometric equation or tables. The psychrometer accuracy deteriorates at

high dry-bulb temperature and low relative humidity, unless precautions are

made to reduce the wet-bulb measurement errors caused by radiation and

conduction. The accuracy also decreases if the wick becomes contaminated

but can be regained by cleaning or replacing the wick. Although the psychrometer can be used in most ambient conditions, it is difficult to obtain

measurements with good accuracy when the wet-balb is below 0°C.

The last category of hygrometer is the condensation-type dew-point detector.

This hygrometer uses thermoelectric, mechanical or other means to cool a

mirror until the water vapor in the ambient or sampling air condenses. The

temperature of the mirror surface is measured and this temperature is the

dew-point temperature of the air. The observation of condensation forming

on the mirror is done visually, by optical instrijments , or by other means.

The accuracy of this hygrometer is within + 1°C in the temperature and humidity

range of HVAC application. Automated systems are available.

2.2. Hygrometer Characteristics

There are a vast number of humidity—measuring devices. They are generally

classified as aluminum oxide, carbon, cellulose, chromatographic, Dacro thread,

dew point, Dunmore, Goldbeater skin, gravimetric, hair, ion exchange, magnetic,

nylon, piezoelectric. Pope, psychrometric, salt-phase transition, semiconductor,

single ionic crystal, and spectroscopic. The important parameters for hygrometer selection are humidity span, tenq)erature range, accuracy, drift,

hysteresis, response time, contaminant effects and maintenance.

4
Currently, no single hygroiaeter Is available which will operate over the

complete humidity range with a constant high accuracy. Also, all humidity

sensors are susceptible to contamination, which changes accuracy, response

time and life expectancy. This dxange may be irreversible depending on the

the concentration level and exposure time to the contaminant.

Psychrometers , self-regulating saturated salts, dimensional change, and

most electrical impedance- type humidity sensors are generally useful for mid-range

temperature and humidity applications. Electrolytic hygrometers are suitable

for low dew point ranges . Condensation dew-point ihygrometers cover a wide range

of temperature and humidity. The gravimetric method gives the most accurate

measurement. Condensation dew-point hygrometers generally yield more accurate

results than most other sensors, such as electrical impedance, saturated salt,

QT dimensional change type. In actual practice, accuracies are also affected

by contaminants on the sensor or in the sample lines where air sample lines

are used. In addition, the desired accuracy determines the required maintenance

and recalibration schedule. Effects of typical contaminant levels on sensor

response times are usually negligible in HVAC operations vmless sensor damage

is involved.

3. HVAC Applications

3.1. Self-Regulating Salt-Phase Transition Hygrometer

Operating Principles . A thin-walled metal tube is wrapped with a cloth sleeve

impregnated with a hygroscopic salt such as lithium chloride. A bifilar

winding of two electrodes is wound outside the cloth sleeve and is connected

to low voltage alternating current. An equilibrium temperature is reached

when the rate of moisture absorption of the salt equals the moisture

evaporation due to the heat generated in the salt by the electric current.

5
This temperature is related to the partial pressure of the water vapor

content and the dew point of the air. Therefore, this temperature is measured

to relate the dew point. Theoretically* this hygrometer does not need

calibration if the salt is not contaminated.

Performance . The operating range of this hygrometer is between -40° C and

70°C dew point and is limited to relative humidity above 11%. The typical

error is +1.5°C dew point above -5°C; it may go as high as 3.5°C below -5°C.

The response time depends on the dew point of the air. For dew points above

5°C, the response time (63%) is usually below 3 minutes.

Calibration and Maintenance . Like any other measuring equipment, the

hygrometer should be calibrated prior to purchase acceptance. Most

hygrometers need to be calibrated periodically after they are installed.

The interval of calibration of this type depends on the quality and the

stability of the thermometer, and the quality of the air the sensor is

required to measure. The calibration includes calibrating the temperature

sensor and checking the salt element temperature against the temperature-dew

point curve. When its salt element is exposed to humid air, the hygrometer

should always be energized (element heated) so that the element will not

absorb excess amounts of moisture causing the salt solution to run off.

Retreatment of the salt element is necessary if sensor current is

interrupted for a prolonged period of time, since the salt element may

absorb excess moisture or become contaminated. The retreatment intervals

depend on the air quality the element senses and may range from 100 days

to twice a year. Retreatment usually is done by washing off the contaminated

salt, applying fresh salt solution and drying off at approximately 85°C.

6
Installation Precautions. Excessive air velocity, above 15 meters per

minute, at the salt element may reduce the temperature at the thermometer

and -give lower dew point readings [3], Some manufacturers recommend using

shields. When Installed outdoors, a weather hood should be provided to

protect the sensor from contacting rain water. Condensation on the salt

element may change the calibration. Sensor locations should be avoided

where the following contaminates are present In the air: hydrogen sulfide,

sulfur dioxide, acid vapor, chlorine, ammonia, alkaline vapors, acetylene,

ethylene oxide, salt-cpntamin^ted aix, alcghpls, and glycol yapors.

Failure Modes . Total failure of this hygrometer Is not frequent but

drifting of readings due to salt element contamination Is common.

Building IP/AC Suitability . Since the dew point Is actually measured by

temperature, this type of hygrometer Is easily used In HVAC systems

to sense the outdoor dew point. The lower range of 11-15% rh of the

Instrximent should not be a limiting factor In building HVAC applications.

Since the response time is slower than some other hygrometers, it may not be

suitable for space dew point monitoring where the humidifier response needs

to be fast, such as for the dry steam type humidifier. Frequent maintenance

is required.

3.2. Impedance Type Hygrometer

Operating Principles . There are various arrangements and materials in

construction of this kind of hygrometer. Some are based on the principle

that the electrical resistances and/or capacitances of certain hygroscopic

materials change with the concentrations of the material. Others are based

on the fact that the electrical resistances and/or capacitances of the

surfaces of certain materials vary with the moisture they absorb. Since

7
the moisture absorption and adsorption are functions of the relative humidity

of the space, the electrical impedance is measured to indicate the space

relative humidity. The effect of temperature must be corrected for in some

transducers, because temperature variations usually also change the electrical

impedance. However, some sensors are practically independent of temperature

in the temperature range of HVAC application. Multiple sensors covering

different ranges and wide range single sensors are available. Alternating

current circuitry is often used in the measurement to avoid polarization of

the sensors.

Performance . The performances vary widely among the sensors of this type

because of the diversified arrangements and materials used. Individual

hygrometer performances should be examined for each application. Sensors

covering the general range of 10% to 99% rh and in the HVAC application

range of dry bulb temperature are numerous. The response time is much

shorter than the salt-phase transition type hygrometer, ranging from a few

seconds to less than a minute (63% change) with an adequate ventilation

rate at the sensor. Dunmore type (electrolytic) hygrometers may be used in

the range of 2% to 99% rh and have accuracies of + 1.5% rh. Aluminum oxide

(surface resistance and capacitance) hygrometers may be used between 5%

and 100% rh with accuracies of +3% rh. These sensors are not significantly

affected by temperature. Dielectric film (capacitance) hygrometers are

usually in the range of 20% to 80% rh. The accuracy may be no better

than +5% rh. The errors in impedance type hygrometers are due mainly to

repeatability and hysteresis.

Calibration and Maintenance . Performance of these types of hygrometers is

determined empirically, and they must be calibrated individually to ensure

their accuracy. As with the salt-phase transition type hygrometers, the

recalibration period depends on the air quality and sensor handling

techniques. A proper calibration schedule should be established by
observing the drift characteristics with short calibration intervals at

the beginning. Calibration in laboratories can be done by: (a) generating

humidity at known levels, such as two-pressure systems; or (b) using saturated

salt solution chambers where the humidity level is known for a certain temperature.

Calibration services for transfer standards are available from the National Bureau

of Standards and other standards laboratories. Field calibration, which may be

more suitable for HVAC systems , may be performed by using a recently laboratory

calibrated hygrometer as a transfer standard. Some hygrometers have temperature

sensors for correcting temperature effects. They should be calibrated according to their requirements. Other than calibration and maintenance of any filter

equipment, these sensors require little maintenance.

Installation Precautions . Humidity sensors should be handled carefully

so that they will not be damaged or contaminated. Chemical contamination

by acids, sulfur compounds, or certain solvents may damage the sensors or

shift their calibrations. In locating hygrometers for outdoor air humidity

sensing, locations near cooling towers, boiler stacks or other chemical

exhausts should be avoided. Since condensation or water may shift the

calibration of some sensors of this type, rain protection should be provided.

Failure Modes . Total failure is not frequent but drifting of readings due

to sensing element contamination is common.

Building HVAC Suitability . Depending on the humidity range, some sensors

are not suitable for outdoor use, such as the dielectric film capacitance

type. The wide-range Dunmore type can be used in either outdoor, duct, or

space sensing. The fast response of these hygrometers allows them to be used

for any type of building humidification control.

9
Some operating characteristics of often-used impedance- type hygrometers

are detailed below.

3.2.1. Dunmore Relative Humidity Sensor

This hygrometer is often made up of several humidity sensing elements. Each

element has electrodes and is painted with hygroscopic material such as lithium

chloride. The hygroscopic material responds to the change of the ambient humidity and changes the electrical resistance. Since each element can cover only

a narrow range of humidity, the elements are usually combined and electrically

conditioned to cover a wide humidity range to suit its specific application.

The electrical resistance of the sensors are usually also sensitive to ambient

temperature change, some hygrometers have temperature compensation arrangements

to reduce temperature influence. Hygrometers of this kind may be found to

cover the humidity range between 2% rh and just below saturation and in a temperature range of the entire HVAC applications. The accuracy is + 1.5®rh and

is subject to the direction and magnitude of the ambient air humidity changes.

The calibration may change 1 to 2% rh per year operating in a clean environment.

This drift may be corrected by recalibration and adjusting of transducers. If

the sensor is calibrated with a psychrometer , agreement should not be expected

to be less than + 3%. Temperature fluctuations in the dry or wet bulb, and

humidity gradients (stratifications) can result in variations of 10% rh. A

secondary transfer standard should be used for calibration accuracy less than

+ 2%. . Lead wires should be included with the hygrometer during calibration.

In moving air the response time (63% change) to a step input in rh at 25®C is

approximately 40 seconds. The response time increases with decreasing temperature and is approximately 3-5 minutes at 5°C or 10 minutes at -18“ C. This

10
type of sensor will lose its accuracy if subject to saturated humidities.

Therefore, applications which may subject the sensor to condensation should

be avoided.

3.2.2 Aluminum Oxide Sensor

A porous metal surface absorbs and desorbs water molecules when the ambient

vapor pressure changes, and this in turn changes the electrical capacitance

and resistance of the metal surface. This principle may be used to measure

the ambient air dew point. Aluminiim oxide sensor uses a strip of anodized

aluminum where the poroxis surface is created. One manufacturing process is

to deposit a thin film of gold over the oxide layer, thus the aluminum core

and the gold film form the two electrodes. When water vapor diffuses through

the gold film into or out of the aluminum oxide, the impedance is changed and

this output is calibrated to indicate the air dew point.

The performance varies between manufacturing processes and makes. Generally,

these hygrometers may be used below a dew point of 70“C to much lower (-100°C

frost point). The response time (63%) is on the order of seconds. At high

relative humiditiy (say over 90% rh) , the response is slower. The user should

consult manufacturers on the details of other properties

3.3. Dimensional Change Hygrometer

Operating Principles . Many hygroscopic materials change length or volume

when responding to the relative humidity level of the air. Examples are hair,

wood, paper, and cellulose.

11
In one group of hygrometers, using a coating of carbon particles or

impregnating carbon particles in such materials, a change in the

electrical resistance of the sensor (corresponding to the change of its

dimensions) results with a change in humidity. Since the electric conductivity

is due to electronic rather than electrolytic effects, there is little

polarization of the sensor and either AC or DC excitation may be used.

The following discussion concerns this type of carbon sensor.

Performance . The sensitivity is low at low relative humidity. It increases

gradually after approximately 40% rtu Tlie useful range is somewhere between

20% and 90% rh. Hysteresis averages about + 3% rh with higher values

in the lower humidity range. Typical accuracy is no better than + 5% rh

and the response time (63% of change is typically less than one minute.

Calibration and Maintenance . Calibration should be performed frequently,

perhaps every three to six months. Since the resistance of the carbon sensor

is also temperature dependent to some extent, the hygrometer should be calibrated

for the existing site temperature range. No other maintenance is required.

Installation Precautions . General installation precautions for all humidity

sensors should be observed, i.e., avoiding locations where temperature

variation is large and air quality is poor.

Failure Modes . Similar to other type of hygrometers in that total failure

of the hygrometer is not frequent but drifting of readings due to sensing

element contamination, such as from dust, is common.

Building HVAC Suitability . This type of hygrometer is suitable for space

and return duct installations because of its fast response and relatively

maintenance free (except for calibration) characteristics.

12
A. Sensor Performance

4.1. Factors Affecting Performance

How close a given moisture-in-air level can be measured and maintained

depends upon how good the existing instrumentation techniques are. In

general, moisture is measured by wet and dry bulb temperatures, dew point

temperature or relative humidity. Instrumentation accuracy depends on the

method employed and varies v/ith the inherent accuracy of the measurement

of temperature, position, and/or electrical resistance or capacitance.

The accuracy of the humdity sensor system is a combination of the inherent

accuracy of the method and the operational error, i.e., drift of readout

instrument, sensor history, temperature/humidity range, excitation voltage

changes, noise, contaminant effects, air flow, and geometric parameters

(heat and mass transfer) . A convenient classification of these variables

is shown in table 1. Figure 1 shows a typical sensor system from the

sensor to the indicator and all its intermediate stages.

To aid in the identification of sensor development areas for research, the

characteristics of an "ideal" sensor are listed in table 2 (partly adapted

from reference [11]). Currently there is no commercial sensor which ran

satisfy all of these requirements.

4.2. Definition of Performance Terms

In assessing the performance of hygrometers, it is useful to state some

import2int definitions that relate to their characteristics [12].

13
Accuracy - The degree of confonnity of the measurement to a primary

standard reference. An estimate of accuracy is usually made by

summing the known sources of error such as drift, precision and

reproducibility measurements.

Calibration Precision - The variance of calibration data about the

best fit calibration curve.

Calibration Reproducibility - The variance between calibration data

obtained at different times during the instrument operational

period.

Drift - A gradual deviation from a set position.

Drift Rate - The time rate of change of a signal from a set position.

Effective Range - The instrviment range over which a single calibration

curve gives sufficient calibration precision.

Error - The difference between the instrximent reading and the reading

of a calibrating standard.

Non-Linearity - The maximum deviation between an actual instrument reading

and the reading predicted by a straight line calibration drawn

over the extent of its range.

Precision - The degree of exactness of the instr^lment expressed as the

reproducibility that can be demonstrated by repeated measurement

of the same sample. Specifically the degree of agreement between

repeated measurement of the same sample expressed as the standard

deviation of the single results from the mean.

Range - The maximum and minimum mensuration limits of humidity.

14
Reproducibility - The ability to obtain the same output for a fixed

input measured at intervals over a period of time. Estimates of

. reproducibility are influenced by the reproducibility of the

calibration.

Response Time - The time for an instrument to reach a new equilibrium

or steady-state value.

Sensitivity - The instrument output per unit input performance.

Span Drift - The change in instrument sensitivity at full range over a

stated period of time normally determined together with zero drift.

Stability - A measurement of the instriiment drift over a short time

or long time period.

Temperature Coefficient - The change in instrximent ouput per unit

temperature change.

Tolerance - Permissible error that is acceptable, normally specified

as a percent of the full scale reading.

Zero Drift - The change in instrument output at zero input over a

stated time period obtained through unadjusted continuous operation

or by a comparison of successive calibration data.

^•3. Measured Performance of Lithium Chloride Self-Regulating Salt-Phase

Transition Dew Point Sensors

In order to obtain more application information on hygrometers used in HVAC,

two of the commonly used sensors (LiCl self-regulating , salt-phase-transition,

designated as sensor numbers 1 and 2) was studied to determine typical values

of the parameters affecting performance. The results of these measurements are

discussed in this section.

15
4.3.1 Calibration Procedure

A self-regulating, lithium chloride dew-point sensor (sensor

number 1) was calibrated in the NBS two-pressure generator [6]

Table 3 shows the dry-bulb temperatures, relative humidities » and

corresponding dew points in the calibration chamber, and the millivolt

outputs of the sensing bobbin cavity temperature as measured by a type T

thermocouple. Table 4 shows a second calibration of the same sensor after

55 days of exposure to various static humidity conditions and low levels of

sulfur dioxide and ammonia contamination, which will be discussed later.

The second calibration was also performed using the same method. Comparison

of tables 3 and 4 indicates that the sensor output has decreased after the

55-day exposure period. Figure 2 shows the bobbin cavity temperature and

dew point curves for these calibrations. The thermocouple sensing junction

was located at 3.8 cm from the bottom of the bobbin cavity. This thermocouple

location gave the highest temperature readings within the cavity. More

discussion of the cavity temperature measurement is given in the next

paragraph and in paragraph 4.3.4.

4.3.2. Temperature Sensing Location Test

As discussed previously, the self-regulating lithium chloride dew point sensor

is based on the principle that the water vapor pressure in the ambient air

balances the water vapor pressure of the salt solution reflected by the

temperature of the salt. Therefore, it is important to measure the salt

solution temperature as close possible. The most feasible location for

the temperature measurement, of course, is inside the bobbin cavity. It

should be noted that there is a temperature gradient along the longitudinal

16
H

axis of the cavity ranging between the close-to-salt temperature and the

atmospheric temperature. This temperature difference may reach 50° C when

the -atmospheric termperature is high and close to saturation. A test was

performed by using two thermocouples to compare the temperature differences

at the bobbin bottom wall and 0.038 m distance from the bottom inside the

bobbin cavity. This 0.038 m- distance was determined by preliminary tests

to locate the highest temperature location inside the bobbin. Figure 3

shows the millivolt output differences of the thermocouples between these

two locations as compared to atmospheric dew points at a relative humidity

of approximately 70%.

4.3.3 Static Humidity Drift Test

To measure sensor stability and response time in clean and contaminated

enviroimients, a static h.umidity apparatus (figures 4, 5, 6) was constructed.

Various saturated salt solutions (tables 5 and 6) were agitated by a

magnetic stirrer and maintained at a constant temperature [13] . Two identical

bell jars with different salt solutions were used to provide step changes

in humidity to determine response times. The static humidity test apparatus

was placed in an environmental chamber with temperature and humidity controls

to minimize the effect of temperature and humidity conditions between the

inside and outside of the bell jars. The humidity level, exposure time,

air temperature inside the bell jar, saturated salt solution temperature,

and environmental temperature and humidity were recorded. The static test

apparatus included an injection port for introducing sulfur dioxide or

amonia as contaminants. The results of the tests are discussed in

paragraph 4.3.4.

17
4.3.4. Results and Discussion of Sensor Tests

The sensor characteristics determined from the NBS calibrations, the

temperature- sensing location test, and the static humidity tests are presented

in terms of the sensor parameters.

(a) Temperature Effect and Accuracy

There should be no marked temperature effect on self-regulating lithium

chloride sensors as the vapor pressure of the atmospheric air determines

the salt solution temperature. The apparent temperature effect shown

in figure 2 was probably caused by failure to fine-tune the precise

location of the cavity temperature-sensing location and by the thermocouple

wire conduction loss. When the bobbin was new (first calibration),

figure 2 indicates that the indicated dew point temperature was within

1°C of the calibration curve.

(b) Hysteresis

No clear trend of hysteresis may be concluded from the test data. As

shown in tables 3 and 4, the cavity temperature of the new sensor was

slightly higher for decreasing humidity than for increasing humidity,

and the trend was reversed when the sensor were contaminated, but the

magnitude of the differences was well within the experimental error.

(c) Response Time

Response times (63%) in still air were determined by moving the sensor

from one bell jar to another at different humidity levels. The response

times are summarized in tables 7, 8, and 9, which show that at different

dry-bulb temperatures and relative humidity the response time are much

scattered. The average values of rising humidity changes were 2.65 minutes

for sensor number 1 and 4.01 minutes for sensor number 2, and the average

values of falling humidity changes were 3.41 and 6.93 minutes, respectively.

18
In most cases, the response time for decreasing humidity was longer than

for rising humidity change. The considerable response-time difference

'between the two sensors tested was probably caused by the bobbin retreating

procedures. Comparing data of table 7 and 9, it does appear that the

response times were altered considerably by the introduction of contaminants. It should be mentioned that the effect on the response time by

exposing the sensors at room humidity for a very short time between the

two jar humidity levels was not investigated.

(d) Drift

Figure 2 shows the difference between the two calibrations for sensor

number 1 which was exposed to 55 hours of cycling humidities of 11 to

97% rh and less than 5 ppm sulfur dioxide and ammonia. The slope of

the calibration curve changed in such a way that higher errors occurred

at higher dew-point temperatures. The drifts were approximately 1.7°C,

2.3°C, and 3.0°C at 10“C, 20°C, and 30°C dew-point temperature, respectively.

Assuming that this drift is uniformly distributed over the 55 days, a

drift rate of 0.07°C/day at 20°C dew point is expected. This result

would be considered a short-range drift. The sensor exposure times are

shown in table 10.

(e) Location of Temperature Sensing

Figure 3 indicates that at approximately 7Q% rh and 26.7“C dew-point

temperature, the difference in millivolt output of the microvoltmeter

between the two thermocouple junctions (one at the bobbin bottom wall

and the other at 3.8 cm from the bottom) was .46 mv, which was equivalent

to 11.2®C in cavity temperature and 7.6°C in dew point. Resistance

thermometers and filled bulbs are also commonly used for these temperature

19
measurements. These higher thermal mass devices average the cavity

temperature over a larger space, thus reducing the importance of the

"temperature—sensor location. On the other hand, they compromise the

measurement accuracy and response time.

A .4 . Temperature Measurement

Many humidity sensors have a temperature output which is related to humidity.

For example, an optical dew-point hygrometer indicates the condensation

temperature; a psychrometer indicates wet and dry-bulb temperatures; a

self-regulating lithium chloride sensor uses a thermocouple or a resistance

thermometer for cavity temperature measurement as discussed above. Thus,

the errors associated with temperature measurement are coupled to the errors

in hnmidity measurement. Discussion of these errors may be found in

many temperature measurement publications and should be consulted for the

evaluation of humidity sensors having temperature as output.

5 . Sensor Selection and Maintenance

In selecting a sensor for HVAC applications, the required accuracy, initial

purchase cost, maintenance, and calibration costs must be considered. For

example, optical condensation hygrometers have a high initial cost of

approximately $2,000 - $4,000, but provide high accuracy. Only a costeffective study can determine the correct sensor alternative for the

application. Most selections are often based on the initial cost alone

(usually related to degree of accuracy) and do not consider the cost of energy,

maintenance and calibration. Because the life-cycle cost difference can

easily be greater than the purchased cost difference, the correct sensor

should be chosen by a benefit/cost analysis. With this principle as a basis.

20
a sensitivity analysis should be performed to evaluate the energy consumption

difference caused by possible sensor errors. Although energy consumption of

HVAC systems are building, system and climate dependent, it is probably true

that better quality sensors should be used for humidifying and dehumidifying

controls. On the other hand, humidity sensors used for enthalpy control may

be of lesser quality, since the yearly energy effect of these sensors is

generally less than those for humidifying controls.

Psychrometers are usually the lowest cost instruments, followed by

saturated-salt dew-point hygrometers, electrical rh sensors, aluminum oxide,

electrolytic, optical condensation hygrometers, and finally infrared, which

is the highest cost.

All humidity sensors require routine maintenance, the periodicity depends on

the environment and the type of the device. For example, a psychrometer needs

wick changes periodically and a routine inspection of the water supply system.

Otherwise, contaminated wick gives higher wet-bulb readings. Aluminum oxide,

electrical rh sensors (Dunmore, Pope) must be periodically recalibrated or

have the elements replaced. Most lithium chloride cells lose their calibration after exposure to high humidity or excessive contaminants. Some sensors

can be cleaned and resensitized. Since the performance of most of the

humidity sensors is affected by the amount of particulate and pollutants in

the local air where the sensors are located, it is important that an estimate

of the air quality be made in establishing initial maintenance and calibration

schedules. Permanent maintenance schedules may be detenained when a calibration

history is established.

21
6. Conclusions

A literature search and a laboratory investigation have been conducted to

provide information for the application of humidity sensors for HVAC systems.

The following conclusions, specifically with regard to a self-regulating

lithium chloride dew-point sensor, can be stated:

A. The laboratory tests indicated that between approximately

-10" C and 30“C dew-point, the self-regulating lithium chloride dewpoint sensors exhibited errors of + 1°C when new and over + 3.0°C

after 55 days of operation and exposure to low level of contamination. There was no clear trend on hysteresis. The average response

time (63%) of the sensors tested in still air was about four

minutes

B. The location of temperature sensors in the bobbin cavity of

self-regulating lithium chloride dew point sensors is very important. Error of over 7°C of dew-point may result if the temperature

sensor is not appropriately located.

C. Humidity sensor selections should be based on life-cycle costing,

including accuracy and energy sensitivity analysis.

D. It is important that humidity sensors be calibrated and

maintained periodically to reduce possible errors.

22
REFERENCES

[1] Humidity and Moisture: Measurement and Control in Science and Industry.

' 4 Volumes. A. Wexler, Editor-in-Chief . New York, NY: Reinhold

Publishing Corp.; 1963.

[2] ASHRAE Brochure on Psychometry. Prepared by ASHRAE Technical Committee

on Psy chrometrics . New York, NY: American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc.; 1977.

[3] Humidity Measurement, Proceedings of the Symposium on humidity Measurement

at the semiannual meeting of the American Society of Heating, Refrigerating

and Air Conditioning Engineers; 1968 February 5-8; Columbus, Ohio. New

York, NY: American Society of Heating, Refrigerating and Air Conditioning

Engineers, Inc.; 1968.

14] Wexler, A. Electric Hygrometers. National Bureau of Standards (U.S.)

Circular 586; 1957 September.

[5] Wexler, A.; Hyland, R. W. The NBS Standard Hygrometer. National Bureau

of Standards. (U.S.) Monograph 73; 1964 May.

[6] Hasegawa, S.; Little, J.W. The NBS Two-Pressure Htimidity Generator, Mark 2.

Journal of Research National Bureau of Standards (U.S.). 81A(1) ; 1977

January-Feb rxia ry

[7] Hasegawa, S. ; Greenspan, L. ; Little, J.W. ; Wexler, A. A Laboratory Study

of Some Performance Characteristics of an Aluminum Oxide Humidity Sensor.

National Bureau of Standards (U.S.) Technical Note 824; 1974 March.

[8] Lion, K. S. Instrumentation in Scientific Research. New York, NY:

McGraw-Hill Book Co., Inc.; 1959.

[9] Middelhock, S. ; Angell, J. B.; Novrlag, D, J. W. Microprocessors Get

Integrated Sensors. IEEE Spectrum 17(2): 42-46; New York, NY: Institute

of Electrical and Electronic Engineers, Inc.; 1980 February.

[10] Norton, H. N. Handbook of Transducers for Electronic Measurement of

Systems. Englewood Cliffs, New Jersey: Prentice-Hall, Inc.; 1969.

[11] Showalter, A. K. S tate-of-the-Art Survey on the Application of Hygrometry

to Meteorology. Wexler, A. Editor-in-Chief, Humidity and Moisture:

Measurement and Control in Science and Industry, Voliome 2 441-446. New

York, NY: Reinhold Publishing Corp.; 1963.

[12] Ballard, L. F. Instrximentation for Measurement of Moisture. National

Cooperative Highway Research Program Report 138; Washington, D.C.:

Highway Research Board, National Academy of Sciences; 1973.

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

Journal of Research National Bureau of Standards (U.S.) 81a (1) ; 1977

January-Feb ruary

23
Table 1. Classification of Factors Affecting

Humidity Sensor Performance

Intrinsic

Response Time

Temperature Coefficient

Hysteresis

Drift

Age

Operational

Internal Heat and Mass Transfer

Air Flow

Maintenance

Sensor Orientation

System Geometry

Temperature

Pressure

Humidity

Time (History)

Direction of Change in Operational Variables

Excitation Level

Calibration Method

Type of Output Device, Signal Conditioning, and Transmission

Environmental

Vibration and Shock (thermal and mechanical)

Contamination

Condensation

24
Table 2. An "Ideal" Humidity Sensor

1. 'Error of less than the equivalent of 0.3°C in dew point temperature.

2. Stability (remain in calibration) for several months.

3. Immune to common atmospheric contaminants.

4. Continued unattended operation for months.

5. Lag independent of temperature,

6. Output

a. Directly in terms of dew point temperature

b. Independent or insensitive of temperature and pressure changes

c. Satisfactory output form for data acquisition

7. Fast response.

8. Dew point range +30 to -65°C at temperatures of +50 to -60°C.

9. Few or no moving parts and complete temperature compensation.

10. Low weight and power requirements (less than 2 watts)

11. Packaging should not be moisture sensitive.

12. Long lifetime, easy maintenance, replacement and recalibration.

13. Remote capability.

14. Compatible with control strategy or objective function of control system.

15. Easy verification or debug system.

16. Insensitive to vibration.

17. Low Cost

25
Table 3

First Calibration of Self-Regulating

Temp

“C

RH

%

32.8 18.8

32.8 32.2

33.0 54.8

33.1 74.2

33.1 94.8

32.3 93.7

33.2 74.1

33.1 54.6

32.9 32.2

25.7 33.3

25.8 54.7

25.9 73.8

25.8 95.0

25.9 75.4

25.9 57.4

25.7 33.3

25.7 33.2

25.5 33.6

Chloride Dew-Point Sensor Number 1

Pt.

C

Output

Reading

mv

.0 1.690

.1 2.130

.7 2.657

.9 3.127

.1 3.494

.1 3.497

.9 3.138

.7 2.678

.1 2.140

.3 1.780

.0 2.235

.9 2.643

.9 3.007

.2 2.668

.8 2.352

.3 1.785

.3 1.784

.3 1.776

Li thium

Dew

O

6

14

22

27

32

32

27

22

14

8

16

20

24

21

16

8

8

8

26
Table 4

Second Calibration of Self-Regulating Lithium Chloride Dew-Point Sensor Number 1

(55 Days After First Calibration)

Temp.

^C

RH Dew Pt,

*C

Reading

mv

5.10 19.43 -16.39 .427

5.08 33.61 - 9.64 .685

5.10 75.36 1.11 1.254

5.10 98.49 4.89 1.505

5.10 75.55 1.14 1.258

5.09 33.63 - 9.62 .693

5.10 11.22 -22.79 .291

25.06 19.31 0.06 1.291

25.06 32.66 7.53 1.619

25.06 32.69 7.54 1.622

25.06 57.62 16.12 2.109

25.06 75.16 20.36 2.381

25.06 97.40 24.62 2.693

25.06 75.19 20.36 2.379

25.06 57.61 16.12 2.105

25.06 32.66 7.53 1.617

25.06 11.10 - 7.32 1.077

32.10 19.21 5.75 1.600

32.10 32.16 13.42 1.950

32.10 55.27 22.01 2.476

32.10 74.78 27.06 2.807

32.11 96.68 31.52 3.128

32.11 75.04 27.12 2.789

32.10 55.30 22.02 2.472

32.10 32.12 13.40 1.949

27
Table 5

Equilibrixim Relative Humidity of Saturated

Aqueous Salt Solutions (? 25°C

SALT % RH

LiCl 11.3

KF 30.9

Mg(N03)2 52.9

NaCl 75.3

KCl 84.3

^2^°4 97.3

Table 6

Typical Static Humidity Test Conditions

Air Salt Type Relative

TempTemp

of Humidity

°C °C Salt %

31.1 32.5 LiCl 11.3

28.4 26.2 KF 32.4

24.5 30.1 Mg(N03)2 51.4

26.3 24.9 NaCl 75.3

28.2 33.3 KCl 83.2

28.2 34.2 K^SO^ 96.8

28
Table 7

Response Time Summary for Static Humidity Test - Sensor //I before Exposure to Sulfur Dioxide or Ammonia

Step Change in %

Relative Humidity

Response Time (min) at Dry--Bulb Temperature

From To 5°C 25°C 35°C

11 53 3.32

53 11 4.98

31 53 1.89*

53 31 2.75*

53 75 3.99

75 53 8.24 7.40

75 84 1.71

84 75 1.88 2.30

31 75 1.75* 1.87

75 31 2.60* 1.99

11 75 7.73

75 97 2.44* 1.89*

97 75 2.73 2.03*

* Average of two tests.

29
Table 8

Response Time Summary for Static Humidity Test - Sensor #2

Step Change in % Response Time (min) at Dry-Bulb Temperature of

Relative Humidity

From To 5°C 35°C

11 75 7.73

75 11 14.87

31 75 9.58 1.67

75 31 9.28 3.56

53 75 4.16 0.48

75 53 8.24 3.22

75 97 2.39 2.07

97 75

Table 9

2.39

Response Time

after

Summary for Static Humidity Test - Sensor #1

Exposure to Sulfur Dioxide or Ammonia

Step Change in %

Relative Humidity

Response Time (min)

Dry-Bulb Temperature

at

of

From To 35°C Contaminants

75 97 1.77 5 ppm SO

2

97 75 2.14 5 ppm SO

2

97 75 1.82 5 ppm NH^

30
Table 10

Sensor #1 History in Hours of Exposure in Static Humidity Test

Air Temperature Approximate % RH

11% 30% 50% 52% 75% 84% 97% Contaminants

25*0 0 310 21 8 44 1 4 No

33“C 0 1 0 1 22 1 1 No

10°C 2 1 0 1 22 0 1 No

25°C 85 SO

2

33°C 25

33°C 2 NH_

3

Notes

Contaminates were less than 5 ppm.

During the remaining time of 55 days, the sensor was stored at room temperature

at approximately 50% rh.

31
Figure 1. Block diagram of a sensor system

32
•m

”r

30 40

~T

o

MO

MO

140

120

K»

W

10

40

20

-20

coif KMT TEMPERATMtE,

^

• M 20

T

-20 8) 40 to 10

lEW POIKT TEMPERATURE, °F

100

90

- 70

- 60

uO

50

40

30

20

10

Figure 2. Sensor no. 1 calibration

33

CAVITY

TEMPERATURE.
CAVITY

fiurm

KfTOtWCE,

AwV

mpmi memAm£, ^

9 ^ a) »

Figure 3. Temperature sensor location effect

34
Figure 4. Schematic of the static humidity drift test apparatus

35
Figure

5.

Static

humidity

drift

test

apparatus
37

Figure

6.

Instrumentation

for

static

drift

test
i .i \
NBS-714A (REV. 2-ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET fSee instructions)

4. TITLE AND SUBTITLE

1, PUBLICATION OR

REPORT NO.

NBSIR 81-2460

2. Performing Organ. Report No. 3. PubI i cation D ate

January 1982

Application Information on Typical Hygrometers Used in Heating, Ventilating

and Air Conditioning (HVAC) Systems

5. AUTHOR(S)

James Y. Kao and William J. Snyder

6. PERFORMING ORGANIZATION (If joint or other than MBS. see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

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

Civil Engineering Laboratory

Naval Construction Battalion Center

Port Hueneme, CA 93010

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

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

bi bliography or literature survey, mention it here)

This report provides hygrometer selection information for application in heating,

ventilating and air-conditioning (HVAC) systems. A general review of hygrometer

literature has been provided and the most commonly used ones for HVAC are discussed. Typical hygrometer parameters are listed to indicate the type of

performance that can be expected. Laboratory test results of self-regulating,

salt-phase transition hygrometers are presented and discussed in detail.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Building energy monitoring; heating, ventilating and air-conditioning controls:

humidity; humidity control; humidity measurement; humidity sensor; hygrometer

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