JATL INST OF STAND & TECH

L11D7 ElDfiOD

] Resistance Thermometer

Pressure Type Thermometer

Hermetically Sealed

Thermistors

Ti Reference

Temperature

Thermocouple

Bimetallic Thermometer

NBS BUILDING SCIENCE SERIES 153

Calibration of

Temperature Measurement Systems

ta"^ -'tailed in Buildings

435

U58

No. 153

193^

2.

ARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ot 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, M D, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
NBS BUILDING SCIENCE SERIES 153

Calibration of

KATIOnAL BUREAU OF STAIIDAHPS

Temperature Measurement Systems U6S

Installed in Buildings

C, X

C. Warren Hurley

Building Equipment Division

Center for Building Technology

James F. Schooley

Temperature and Pressure Division

Center for Basic Standards

National Bureau of Standards

Washington, DC 20234

Prepared for:

Naval Civil Engineering Laboratory

Port Hueneme, CA 93043

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

Issued January 1984
Library of Congress Catalog Card Number: 83-600622

National Bureau of Standards Building Science Series 153

Natl. Bur. Stand. (U.S.), BIdg. Sci. Ser. 153, 84 pages (Jan. 1984)

CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

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

Energy Management Control Systems (EMCS) cannot function properly or efficiently without accurate temperature measurements since temperature is one

of the fundamental measurements of any EMCS. This report was written for the

purpose of describing various methods of on-site calibration of temperature

sensing devices used in EMCS and to review the characteristics of these

devices that are directly related to calibration. The significance of

recording the results of each calibration is emphasized and the possible

effects of systematic errors in temperature monitoring systems is discussed.

Illustrative examples of the calibration of temperature monitoring systems are given.

Liquid-in-glass thermometers, pressure thermometers, resistance temperature

detectors (RTD), thermistors, integrated circuit temperature sensors,

thermocouples, and bimetallic thermometers are discussed in detail with

respect to their characteristics related to calibration.

Key Words

Averaging thermocouples; bimetallic thermometers; calibration techniques;

integrated circuit sensors; liquid-in-glass thermometers; pressure

thermometers; resistance temperature detectors (RTD); systematic errors;

thermistors; thermocouples; thermopiles.

ill

SI CONVERSIONS

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

the calibration of instrumentation monitoring the temperature of air, water,

and steam 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

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

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

Metric Conversion Factors

To convert from To Multiply by-'

Area

ft

in.

2 2 metre2 (1112) metre (m ) 9.290304E-02

6.451600E-0A

Energy

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

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

erg joule (J)

1.055056E+03

A.186800E+00

l.OOOOOOE-07

Force

pound-force (Ib^)

kilogram-force

Length

newton (N)

newt on (N)

4.4A8222E+00

9.806650E+00

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.

v
Metric Conversion Factors (cont.)

To convert from To Multiply by*

Mass per unit vol.

g/ cm kg per

3

m_ l.OOOOOOE+03

kg per

3

TM3 1.601846E+01

lb/in. kg per m 2.767991E+04

Pressure (force per unit area)

atmosphere pascal (Pa) 1.013250E+05

in. of mercury (60 °F) pascal (Pa) 3.376850E+03

mm of mercury (32 °F) pascal (Pa) 1.33322AE+02

in. of water (60 °F) pascal (Pa) 2.488400E+02

in. of water (39.2 °F) pascal (Pa) 2.490820E+02

Temperature

degree Fahrenheit degree Celsius (°C) subtract 32 and

divide by 1.8

degree Celsius degree Fahrenheit (°F) multiply by 1.8

and add 32

degree Fahrenheit kelvin (K) add 459.67 and

divide by 1.8

degree Celsius kelvin (K) add 273.15

degree Rankine kelvin (K) divide by 1.8

Velocity

f t/min

ft/s

in. /s

metre per second (m/s)

metre per second (m/s)

metre per second (m/s)

5.080000E-03

3.048000E-01

2.540000E-02

Volume

ft

in.

3 3 metre^ (m^)

metre (m ) 2.831685E-02

1.638706E-05

Volume per unit time

f t„/min

ff^/s

3 3 metre^ per second (m^/s)

metre per second (m /s)

4.719474E-04

2.831685E-02

*(see preceding page)

vi
TABLE OF CONTENTS

PAGE

ABSTRACT iii

LIST OF FIGURES viii

SI CONVERSIONS v

1. INTRODUCTION 1 2. DEFINITION OF TERMS USED IN TEMPERATURE MONITORING 2

3 . CALIBRATION TECHNIQUES 4

3.1 Definitions of Calibration and Standard 4

3.2 Calibration Of Temperature Sensors In-Place 5 3 .3 Calibration Of Temperature Sensors Retaoved

From the HVAC System 8

3 .4 Calibration Of Remote Temperature Monitoring

Systems 8

3.4.1 Calibration of the Sensor 10

3.4.2 Calibration of the Transmitting Means 10

3.4.3 Calibration of the Receiving Unit 11

3.5 Additional Factors To Be Considered During

Calibration 13

4. DESCRIPTION OF VARIOUS TEMPERATURE SENSORS AND

SUGGESTED CALIBRATION TECHNIQUES 15

4.1 Liquid-in-Glass Thermometers 15

4.2 Pressure-Type Thermometers 21

4.3 Resistance Temperature Detectors (RTDs) 27

4.4 Thermistors 33

4.5 Integrated Circuit Temperature Sensors 41

4.6 Thermoelectric Sensors (Thermocouples) 42

4.6.1 Thermocouples 46

4.6.2 Averaging Thermocouples 50

4.6.3 Thermopiles 52

4.7 Bimetall ic Thermometer s 54

5. CALIBRATION RECORDS OF THERMOMETERS, SENSORS, STANDARDS,

AND SYSTEMS 56

6. POSSIBLE EFFECTS OF SYSTEMATIC ERRORS 58

7 . ILLUSTRATIVE EXAMPLES OF CALIBRATING TEMPERATURE

SENSING SYSTEMS INSTALLED IN BUILDINGS 60

7.1 Thermistor Calibration 60

7.2 Thermocouple Calibration 66

7.3 Bimetallic Thermometer Calibration 72

References 74

Appendix A. Additional Definitions of Terms 75

vii
LIST OF FIGURES

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"Pr TX O" 1 1 f" ^ ^ U- jl c 3 . 1 FxATfinlp*? a f t'Vip in — nlacp cfllibTation

teclinioue.. ............................ ....... 6

F "i Q" 11 T P 3 .2 Examole of a mixinc device 7 Pii. X g

0" nU Ti. pc 3 .3 RpTHOtp tpmnpTAl'iiT'p TTionitorinc? svstPTn. 9J

TiL X1 ^

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t" li p TTTi nm p t" P T 16X Vl

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T?r TX O" 11 T" O ^ LL I. c: 4 .4

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F 1 O" 11 T P 4 .6 Rpci l-vv-OXOUClLl^\_ cf'anr'o/t'PTnriPT^xij'nt"*:^ / L.C UL L/CXuL-UXC fiiT^i^oc n.^LIXVCO XvJX i' rt r tii LiX^IN.CXj pVciI

r\ r\ 1^ ^ IT sanH tiI ;at"i'niiTn ^2 8o Ft Q" n t p 4 .7 RtiHc^p rivriiit*^; fnT* l?TTiQ 3 0

F i g ur 6 4 .8 An RTD in a MiipIIpt hTidpp 3 1 F 1X O" 11 1" p ^ U X c 4 .9 Rfici iVCoXOCdli^C Ql"anr*fi/f"PTnnf^T"i^f"iii"o i^iif^^fi fr^i^ a 1"Vi^iT*Tni ct'OT" / L-CUlpCi-dLuLC CUXVC L KJ L d. LllCXitlXoLLIX • 3 4

Fti> X ^o 11U tX pc 4 .10 Ri'iHo'fi Ui.XVXgC r'iT"pnif'Q ^XX^LiXLO X^X 'fnv linpflT'i7fiH XXliCdXX^CVi' t"VitfST"mic1"OT*G LilCXlllXoLL/XO« • • • 3 "i Ft O" 1 1 t* r X ^ ti X c 4 .11 T n o T* m 1 c t" r\ T" t.t Tf~ri a cl^iini" iracict'r%ir 3 7

T7r X1 Cr 1 1 1* O ^ li X t: 4 .12 XjXLi(::dXX^cU LwU dllU Lllicc LiiexTllXoLCJL CXxCUXLo* • J o Ft O" 1 1 t" i/ X g U X c 4 .13 Pocit*i"\7 XVJoXLXVC0 or^OTTi/^iont" CUCXXXCXCLIL oWXL^ilXil^ o t»t i i~r^riino i"V*oT* LilCXlUXoUUxo**** m i o t" o 40

Figure 4 .14 43

Figure 4 .15 45

Figure 4 .16 47

Figure 4 .17 Thermocouple circuit with reference junction.. 48

Figure 4 .18 51

Figure 4 .19 53

Figure 4 .20 55

Figure 7 .1 67

viil
1 . INTRODUCTION

The measurement and control of the temperatures of various components and

fluids in the mechanical systems serving a building and of the air within the

various areas of that building are often considered to be simple tasks. As a result, accurate temperature measurements are often neglected in an Energy

Management and Control System (EMCS). However, the laws of thermodynamics

demand careful control of the temperatures in both heating and cooling systems

if the equipment is expected to operate at optimum efficiencies. Also, human

nature demands that the temperature of the air in areas where people are working be maintained within a comfortable range to allow them to function

with optimum effficiency. Therefore, one of the primary objectives of an EMCS

of a heating, ventilation, air-conditioning (HVAC) system in a building is to

accurately measure and control temperatures.

To accurately measure and control temperatures, each temperature sensing

device in an EMCS must be properly calibrated and maintained in calibration

during the entire operation of the equipment. The number and various types of

devices used to monitor temperature in an EMCS will depend upon the size of

the building, the number of heating zones, and the design and complexity of

the control system. Since many different types of temperature monitoring

devices may be used in a single EMCS, the personnel responsible for the

operation and calibration of the monitoring components of the system must be

familiar with a variety of temperature measurement techniques and the

equipment required for calibration.

This report provides fundamental information pertaining to the on-site

calibration of temperature sensing devices such as the definitions of terms

used in the monitoring of temperatures, on-site calibration techniques used in

maintaining the accuracy of EMCS temperature monitoring systems, a description

of various temperature sensing devices used in EMCS and their characteristics

related to calibration, and procedures for logging the results of

calibrations for determining the scheduling of future calibrations. The

possible effects -of systematic errors in temperature sensing systems and

illustrative examples of calibrating temperature sensing systems installed in

buildings are also included.
2. DEFINITIONS OF TERMS USED IN TEMPERATURE MONITORING

This section contains a brief list of the major terms used in this report.

Other useful terms are defined in appendix A. Since this report is intended

for EMCS operators and managers, several of the defined terms list a second

definition of the term, as commonly used in the field of EMCS, which may

deviate slightly from the formal definition. The reader is encouraged to

review the definitions in this section and in appendix A whenever there is

doubt about the meaning of a particular term or instruction.

Inaccuracy and Accuracy - The lowest level at which the measurement of a particular parameter by a particular instrument agrees with the measurement

made by correct procedures with a calibrated instrument is properly called

"the inaccuracy"; however, the word "accuracy" is often used to express the

same idea.

Measurement - The act of using an instrument to obtain a value of a particular

parameter. Also, the value of the parameter thus obtained.

Resolut ion - The ability of an instrument to discriminate between one reading

and another. For example, "The liquid-in-glass thermometer No. 81-465 has a resolution of +^0.05 °C when read with a lOx telescope."

Imprecision, Precision, Irreproducibility, and Reproducibility - The lowest

level of measurement to which a given instrument repeats its reading when used

with a particular system at a given time by a particular observer is properly

called "the imprecision" or "the irreproducibility". The words "precision"

and "reproducibility" are also used to express the same idea. For example,

"The temperature of the ice-point bath was measured ten times by operator No.

243, using a liquid-in-glass thermometer in conjunction with a lOx telescope.

The average imprecision (or precision) was +^0.04 °C."

Thermometer - The true definition of a thermometer is a device used to obtain

the temperature of an object or system. However, in EMCS applications, a device to measure temperatures often consists of several components such as a sensor or transducer, transmission means and a receiver or readout device.

Since each of these components will be discussed separately for the various

types of temperature monitoring methods in this report, the term thermometer

will be limited to liquid-in-glass thermometers, bimetallic thermometers,

and pressure-type thermometers.

Temperature - The "hotness" of an object, usually expressed as a numerical

value on an agreed scale. It is important to remember that heat flows from an object whose temperature is higher to any contacting object whose temperature

is lower until they are separated or until their temperatures become equal as

a result of the energy exchange.

Heat . Heat Energy, and Thermal Energy - The capability of an object to perform

work as a result of its temperature is called its "heat energy" or its

"thermal energy". Stored thermal energy is known as "internal energy". The

transfer of this energy from one object to another is referred to as "heat" or

"the flow of heat". Heat flows as the internal energy of a hotter object is

2
dissipated in order to raise the temperature of a colder object in thermal

contact with it.

Thermal Contact - The capability of carrying heat from one object to another.

The three ways of establishing thermal contact in temperature monitoring

systems are by conduction, by convection, and by radiation.

Thermal Equilibrium - The state in which two or more objects have the same temperature, or in which a single object has the same temperature throughout

all of its parts. This state is commonly achieved by bringing the objects

into physical contact or contact through a convective gas and allowing

sufficient time to pass for the required amount of heat energy to flow from

the hotter objects to the colder ones until their temperatures are the same.

Temperature Scale - A reference or standard used to assign a number to an object to indicate its temperature. There are several temperature scales in use today. These include:

a) the basic scientific scale, the "Kelvin Thermodynamic Temperature Scale",

which runs from absolute zero to positive infinity in units of the kelvin (K);

b) the scale endorsed by the General Conferences on Weights and Measures and

called the "International Practical Temperature Scale of 1968", which has two

sets of numerical values, one running from absolute zero to positive infinity

in units of kelvins (K) and the other running from negative 273.15 through

zero at the melting point of ice and then onward to positive infinity in units

of degrees Celsius (°C).

c) the "Fahrenheit Scale" (used commonly in the United States but not

elsewhere), running from negative 459.67 through zero and on to positive

infinity. Zero on the Fahrenheit Scale does not occur at a common fixed point.

The melting point of ice is generally used to reference the positive value of

32 °F. The Fahrenheit and Celsius temperature scales are commonly used in

EMCS. The conversion factors are listed in the "SI Conversions" given in the

front of this report.

d) the "Rankine Scale" (the analog of the International Practical

Temperature Scale of 1968 in kelvin units), running from absolute zero through

491.67 at the melting point of ice to positive infinity.

Thermal Gradient - The existence of different temperatures in different parts

of an object or system. An example of thermal gradient is the difference

between the temperatures of the inner and outer surfaces of the wall of a boiler

.

Sensor/ Transducer - A sensor is defined as a device that receives and responds

to a signal or stimulus. A transducer is defined as a device which converts

one form of input energy into another form of output energy. In temperature

monitoring for EMCS and for the purposes of this report, these terms are used

interchangeably

.

3
3. CALIBRATION TECHNIQUES

Many different types of temperature monitoring systems are used in EMCS. In

addition, many combinations of temperature sensors and transducers, methods of

transmitting the output of the transducer to the point of termination, and

methods of reading and translating the received signal into an engineering

term exist. For these reasons, a single method of calibrating various systems

is seldom sufficient.

Throughout this report (especially in this section) the text may appear to

deviate from the direct subject of "calibration". However, in each case the

reader will be made aware of typical conditions that are found in EMCS and

reflect the results of the calibration of EMCS equipment. The fundamental

characteristics of the various temperature sensing devices found in EMCS will

be described in section 4. Special emphasis is placed on those

characteristics pertaining to calibration.

In this section, two fundamental terms are defined and several general

calibration techniques are described. The technique or techniques used in the

calibration of the EMCS temperature sensing device or system will depend upon

characteristics such as the complexity of the system utilized to obtain the

response of the sensor at a remote readout or control station, the type of

sensor, the method used in mounting the sensor in the medium it is monitoring,

etc. Since the majority of temperature sensing devices are mounted in EMCS

equipment for remote monitoring, this area is covered in more detail than the

simpler techniques. However, in many remote systems, the temperature sensing

device itself often can be calibrated by one of the simpler techniques

described. Therefore, all of the material presented is pertinent.

3 .1 Def initions of Calibration and Standard

The term "calibration" as used throughout this report refers to the comparison

of an indicated value of a temperature monitoring system or any part thereof,

to the value indicated by a "standard" device or a standard method of

generating a reference temperature. The calibration must also account for all

parameters which affect the final indication.

A "standard" is understood to be an instrument whose indications and

accuracies within the ranges it is being used have been determined and

recorded by a qualified laboratory. A standard may also be a technique which

produces a particular temperature within known error bounds when certain

transitions occur in pure materials. An example would be the melting point of

ice or boiling point of water of known purity.

For the purpose of this report, it must be emphasized at this point that there

is an important difference between maintenance of equipment and calibration of

equipment. Maintenance of equipment involves the tasks of keeping the

equipment running, while calibration of the same equipment involves

determining how well the equipment is doing its job.

4
3 .2 Calibration of Temperature Sensors In-Place

If the system is designed to allow a standard such as a laboratory-calibrated

liquid-in-glass thermometer, thermistor, thermocouple, or other suitable

standard to be placed in the system adjacent to the temperature sensing

component and if the temperature of the system can be varied over its normal

operating range, then calibration can be performed by comparing the

temperatures indicated by the standard and those indicated by the sensing

device. The values indicated by the device being calibrated must be those

used by the EMCS for monitoring and/or control.

In calibrating any sensing component while it is in place in a monitoring

system, extreme care must be used to avoid disturbing the natural environment

of the sensor being calibrated by positioning the standard being used for

calibration. Likewise, the environment of the standard in position in the

system must be the same as that of the sensing component. The characteristics

of the device being used as a standard as described in section 4 of this

report should be reviewed prior to use in any calibration method to avoid

exceeding the constraints of the sensor or the standard.

In many cases, it may be found advisable to install a removable section in

the system to allow a standard to be utilized as shown in figure 3.1. Using

this technique, the medium being monitored by the sensor being calibrated can

be monitored by a standard inserted in a second removable section similar to

that previously installed. The sections can be interchanged after the

calibration is completed. Care must be taken to avoid disturbing the normal

operation of the system while interchanging the removable sections. Figure

3.1 (b) shows one method (the use of valves) of utilizing the technique in

installed water and steam lines.

Often, mixing devices are required in systems to assure that the temperature at

the sensing device is representative of the average temperature of the medium

being monitored. Mixing is often required in air and in water distribution

systems upstream of the temperature sensor. The term "mixing" refers to

baffles or other means installed in the transporting enclosure (as shown in

figure 3.2) to generate turbulence in the medium and reduce any thermal

gradient that may be present. A temperature sensor or standard placed in a medium where thermal gradients are present will show significant errors in

the indication of the true mean temperature of the medium. These errors often

will vary as the velocity and/or the temperature of the medium are varied and

introduce additional problems for those attempting to calibrate the sensor.

It must be pointed out, however, that the installation of mixing devices in a system must be done with care since induced turbulence in a stream of air,

water, or steam may generate problems in the system monitoring the velocity of

the medium. Therefore, the location of the velocity-monitoring sensor should

be studied and given careful attention before arbitrarily installing mixing

devices to reduce thermal gradients. If a conflict in the two sensors arises,

it is generally wise to relocate the temperature sensor or to install a temperature sensor such as an averaging thermocouple which will reduce the error caused by thermal gradients to an acceptable level. Averaging

thermocouples are discussed in section 4.6.2. of this report.

5
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3 .3 Calibration of Temperature Sensors Removed From The HVAC System

When the complete working monitoring system can be removed from the building

system and calibrated directly at a primary calibration facility, or at a qualified calibration facility established at the site of the EMCS, this is

usually the most desirable method of calibration and should be utilized. An

example of this calibration technique would be the removal of the sensor and

its related components from the mechanical system and transferring it to a qualified laboratory utilizing a variable-temperature calibration bath with

appropriate standards to calibrate the system over its full operating range.

This technique avoids disturbing the operation of the mechanical system during

calibration.

At first glance, this may seem to be an unnecessary task, especially for the

more complex systems. However, the more complex the monitoring system, the

more susceptible it becomes to error. In addition, the more complex the

monitoring system, the more difficult the debugging of any internal electronic

problems that may be present in one or more of the components of the system.

A primary calibration facility or a qualified on-site calibration facility

should be equipped with the necessary test equipment to locate and correct

the majority of any internal problems in the system.

Manufacturers' representatives selling some of the more-complex temperature

systems are equipped with mobile equipment for testing and calibrating

temperature systems and their components without removing the complete system

from the EMCS. When such methods are used, it is advisable to recheck the

calibration and operation of the system by direct comparison with a standard

device before the mobile equipment leaves the site. The majority of the

personnel working with mobile equipment of this type will have access to

qualified standards for checking the components of a system. However, it is

not uncommon for a person who has been heavily involved in the debugging of

multiple problems in a complex system to neglect checking the calibration of an operating monitoring system that has just been repaired.

3 .4 Calibration Of Remote Temperature Monitoring Systems

The majority of EMCS temperature monitoring systems fall within this more

complex area since the sensors are located throughout the mechanical system of

a building or group of buildings and the central control unit for the system

can be located in any convenient place. The media being monitored, the size of

the ducts or pipes, the temperature ranges, etc. usually vary throughout the

system. In general, this is the primary reason calibration by removing the sensor from the system is often not practical for medium and large-size

systems. Therefore, methods of calibration of the system and its components

with a standard are required.

As shown in figure 3.3, each channel of a remote temperature monitoring system

consists of a temperature sensitive device usually referred to as the sensor

or transducer, a signal conditioning means for receiving the signal from the

transducer and "conditioning" the output signal of the transducer (by

amplifying, attenuating, impedance matching, etc.) to an acceptable level to

be received by the transmitting means for generating and transmitting the

output from the transducer to the multiplexer. Often the output of the

8

signal conditioning device is adequate to be connected directly to the

multiplexer without the use of a "transmitter".

The multiplexer ("MUX"), as shown in figure 3 J, is a device usually mounted

at each primary piece of mechanical equipment in the HVAC system. The

multiplexer contains a "multiplexing" device which continually scans the

inputs from each remote sensing station. The voltage (or current) received by

the multiplexer is transmitted to an A/D (analog to digital) converter to

translate the amplitude of the signal received from the individual remote

lines into a corresponding digital signal. The digitizing of the analog

signal allows the signal to be processed by microprocessors and retains the

signal until changed by the input from the sensor or the command for a readout

by the central control unit. The output signals from the "MUX" as shown in

the figure, are usually transmitted over a serial (two or three conductor)

cable to the central control unit. The frequency and the mode of transmission

will depend upon the basic design and programming of the "MUX" and central

control unit.

Since each component of a remote temperature monitoring system must function

properly to allow a signal analogous to the temperatures of the medium being

monitored by each sensing element to be received by the central control unit,

the calibration of a remote sensing system requires the calibration and

testing of each component for accuracy and proper operation throughout the

full range of temperatures and environmental conditions that will exist in the

actual operation. In addition, all the components as a system must be

calibrated

.

3.4.1 Calibration of the Sensor

The calibration of the sensing element or transducer can be made by either of

the methods described above, whichever is the more suitable for the type of sensor. One important point must be emphasized in this section on calibration

techniques: regardless of the available facilities for calibrating, the

manufacturers' instructions for calibration must be referred to and followed.

This applies to the calibration and the actual use of the sensor. Numerous

sensors and systems have been destroyed or improperly calibrated by the

people designing, installing, calibrating, etc. failing to spend a few minutes

to read the manufacturers' specifications! Still more temperature sensing

elements have been found to be malfunctioning for the same reason! Subjecting

a temperature sensing element to environments beyond its specified limits can result in costly replacements and, more often, in lengthy periods of down time

of an EMCS.

3 .4 .2 Calibration of the Transmitting Means

The transmitter can consist of something as simple as a serial cable (as

described above) or a capillary tube to a complex electronic system, depending

on the type of transducer. However, the calibration of the transmitter is

usually very straightf oward if the proper calibrating equipment is available.

In general, if the output of the transducer can be simulated and entered at

the transducer end of the transmitting means, the output at the receiving end

must be identical. Unfortunately, the input signal from the temperature

10
sensing unit is not always simple to reproduce. This signal must be reproduced

under the same conditions (impedance, voltage, current, etc.) as produced by

the transducer.

For the purpose of calibrating the transmitting means, consider the output of a simple thermistor which requires a pair of lead wires for transmitting a voltaic signal. Since thermistors used in EMCS are used in a voltaic mode,

the manufacturers' specifications may typically state that the output

impedance of the thermistor is in the order of 10 megohms. This high

impedance requires the leads to be shielded and that both ends of the leads as well as the main body of the leads be free from excessive electromagnetic

interference. Using care in routing the lead wires away from power lines,

electrically powered equipment, etc. will help to eliminate the excessive

interference from the main body of the leads. However, the ends of the lead

wires must be close together to minimize the electromagnetic fields between

them

.

The example given above for transmitting the output of a simple thermistor to

the multiplexer emphasizes the need for the calibration of the transmitting

means in any remote temperature sensing system. Other examples of possible

problems in the transmission of remote signals from temperature transducers

will be given in section 4 as they apply to the types of tranducers described.

3 .4 .3 Calibration of the Receiving Unit

The calibration procedure for the receiver or interpreting means of the remote

temperature monitoring system will vary depending upon the type of receiver

installed in the system. The typical receiver in a remote temperature

monitoring system will consist of a MUX, a FID (field interface device), and

a central control unit as shown in figure 3.3. Although numerous combinations

can be assembled to perform the tasks as they are shown in the figure, the

complexity will vary from system to system. However, each receiver of this

type will have the capability of converting an analog signal into a digital

one which is then converted into a decimal value which represents the

temperature of the medium the sensor is monitoring.

This conversion starts at the A/D converter previously described. As the

magnitude of the analog signal changes, the A/D converter changes it into a digital value in the base in which the computer is designed to function. In

general, the input to the A/D converter is presented in a magnitude (voltage)

directly proportional to the temperature of the sensor. In some cases, the

signal is transmitted in the form of electrical current which is translated

using a resistor of known value to voltage and is directly proportional to the

input. The electrical current method of transmission is often preferred if

line losses become significant. In some cases the response of the

sensor/transducer is non-linear, and the necessary manipulations to produce

the representative data are made by the central control unit. Thermocouples

are an example of a non-linear temperature sensing device.

Although the input signal to the A/D converter may be presented in an infinite

number of increments, the A/D converter will divide the range of the converter

into an integral number of increments depending upon its capability. For

example, an eight bit A/D will divide the full scale input range of the

11
converter into 256 parts while a twelve bit A/D will divide the full scale

input range into 4096 parts. If the person performing the calibration has the

option, the input range of the incoming signals should be "conditioned"

(amplified and/or offset) to cover most of the input range of the A/D

converter. This will reduce the error introduced by the conversion of an analog signal into a fixed number of increments as required to process the

data by a digital computer.

The method of transforming analog data into digital data is being described to

allow the person performing the actual calibration of the equipment to

understand why the temperature being measured by the sensing device, or an analog signal being applied to the input end of a transmission line, may not

be exactly analogous to the signal displayed by the receiving device. However,

the increments described above indicate the level of discrepancy that can be

caused by this conversion. When the A/D portion of the receiving component is

being calibrated, it should be calibrated over its full range, applying input

signals in both ascending and descending directions.

The actual calibration of the receiving unit is best accomplished by utilizing

the in-place comparison technique at the sensor and comparing the sensor output with the output displayed at the central control unit for the

temperature of the sensor being calibrated. If a discrepancy is noted, the

next step is finding the cause for the difference. This is often a difficult

task. However, by following some of the techniques noted above, the area of

the problem will soon surface. If the signals at all points in the remote

sensing system are found to be within an acceptable range yet the temperatures

displayed at the central control unit are not acceptable, the person

responsible for the software of the central control unit should be presented

all of the facts found in checking out the various parts of the system.

Although errors in the software for an operating system seldom exist, the

electronic components of the MUXs, FIDs, and the central control unit do not

have a lifetime warranty. The facts found in tracing the source of the

problem through the system will often lead the people responsible for the

operation of the MUXs, FIDs, and central control unit directly to the

solution.

Factors that affect the calibration of other types of receiving components

such as mechanical, pneumatic, and direct readouts will be discussed in

conjunction with the applicable descriptions of the various temperature

sensors

.

12
3 . ,5 Additional Factors To Be Considered During Calibration

Many additional factors reflecting on the calibration of temperature

monitoring equipment could be added to this section. However, this sub-section

will list a few items that are often overlooked by personnel performing the

task of "determining how well the equipment is doing its job"; i.e.

calibration.

Many Energy Management Control Systems are installed in existing mechanical

systems of buildings. Some of these mechanical systems may have been in

service for many years. The person designing the EMCS will often be working

from drawings of the mechanical system as it was originally designed, or was

installed, or was intended to be installed. Unfortunately, very few mechanical

systems are installed exactly as originally designed. Obstacles are frequently

generated during the constuction phase of a building which require revisions

of the original design. Even if the working drawings were revised to reflect

the changes in the original design, the majority of drawings available will

not reflect the modifications that have been made in the mechanical system

since its original installation.

This unfortunate (but typical) fact must be considered by those performing the

calibration of the EMCS. Unless all sensors are properly placed in the

mechanical system, the EMCS cannot function as designed. Checking the location

and method of installation of temperature sensors is part of the task of

calibration. If additional branch lines or ducts have been added to an air

system upstream of the temperature sensor and the sensor is assumed to reflect

the temperature of the supply air stream, the person responsible for the

calibration should record the modification and compare the temperature at the sensor with the actual temperature of the supply air. If a discrepancy is

noted, action should be taken to move the sensor to a more appropriate place.

This same example can be used to point out another typical error made in the

installation of sensors. If the sensor is intended to monitor the temperature

of the air flowing in the duct, does the cold (or hot) surface of the duct on which the sensor is mounted affect the output of the sensor? In section 4,

the reader will be reminded of the possible effects of the thermal

conductivity of the lead wires on several types of sensors. This example

points out the possible temperature effects of the surface on which the sensor

is mounted. In general, the person responsible for the calibration of the

system is not responsible for the original design of the system. However, if

the calibration is to determine how well the system is functioning, the person

performing the calibration must take responsibility for recording and

reporting any sensor found to be incorrectly monitoring a function of an EMCS.

Another item that is frequently overlooked in the mounting of sensors and the

use of standards for calibration is the thermal conductivity of the materials

used. For example, heat can be either carried to or from the bulb by the stem

of a liquid-in-glass thermometer being used as a standard for calibration

faster than the air flow in the duct can remove heat or add it. The stem

correction factor described in section 4 for liquid-in-glass thermometers will

further address this problem.

13
A further responsibility of the person performing the calibration is

monitoring the performance of the signal conditioning and transmitting means

of the temperature sensor. If the sensor is functioning properly and the

central control unit is not receiving the appropriate signal, the calibration

of the sensor is of little, if any, value. The signal for the temperature

measured by the standard at the location of the sensor must be the same as that received at the central controlling unit. Often relatively high- impedance

instrumentation amplifiers are required to amplify the signal from the sensor

to a level acceptable by the central control unit. Such amplifying circuits

are subject to drift and possible loss of linearity. In performing calibration

of temperature sensors in systems with signal conditioning circuits,

verification of the proper functioning of all related circuits is considered

to be part of the job of calibration of the temperature sensor; otherwise the

task of calibration is incomplete.

One of the most important factors that is always present in calibration and in

the use of a standard is the possible effect of direct radiation from another

source of thermal energy. It is the responsibility of the person performing

the calibration to determine whether direct radiation to or from a component

in the system is influencing the output of the sensor or the standard being

used in the calibration. Such effects usually can be avoided adequately by

shielding the sensor or moving the sensor and/or the standard to a position

where it does not receive direct radiation. Components such as intermittentacting electrical heating elements or pre-heat steam coils often are overlooked during the installation of the sensor and during calibration. When

a heating element or other high temperature source is activated, the direct

radiation from the source can heat the sensor and/or the standard. Radiation

pyrometers use such radiation effects to measure temperatures. However, they

are best suited for temperature ranges higher than those used in EMCS.

Therefore, they are not discussed in detail in this report. For further

information see reference 1.

14
4. DESCRIPTION OF VARIOUS TEMPERATURE SENSORS AND SUGGESTED CALIBRATION

TECHNIQUES

In this section typical temperature sensing devices used in EMCS temperature

monitoring systems will be described. The fundamentals of operation, typical

ranges, expected accuracy, precision, time constants, recommended areas of

application, advantages, disadvantages, etc. are listed for each type

described. Particular attention is given to those characteristics which

pertain directly to calibration.

4 .1 Liquid-In-Glass Thermometers

The liquid-in-glass thermometer is a well-known temperature measuring device

with a wide range of applications. The physical principle upon which these

thermometers operate is that of thermal expansion. It should be noted that

the reading of a liquid-in-glass thermometer depends upon the difference in

the thermal expansion between the glass envelope and the liquid enclosed

within it as shown in figure 4.1. Mercury is the most common liquid used at

intermediate and high temperatures, although its freezing point is -38 °F

(-38.9 °C) which limits its lower range. The upper limit for mercury is in

the region of 1000 °F (537.8 °C) and requires the use of special glass and an inert-gas fill (usually dried nitrogen) in the capillary space above the

mercury. The compression of the gas, which is inserted under pressure, helps

to prevent the separation of the mercury column and raises the normal boiling

point of the liquid metal. For lower temperatures, alcohol is usable to -80

°F (-62.2 °C), toluol (a commercial grade of toluene) to -130 °F (-90 °C) and

a mixture of propane and propylene giving the lower limit of -360 °F (-218

°C). These lower temperatures may not be directly applicable to EMCS;

they are presented to emphasize the broad range of temperatures that can be

measured by the liquid-in-glass thermometer.

For the mercury in a bulb made of borosilicate or other type of glass, the

bulb volume is approximately 6,222 times the volume of a one-degree length of

the capillary on the scale for Celsius thermometers and 11,200 times the

volume of a one degree length of capillary on the scale for Fahrenheit

thermometers. These relatively high ratios produce the high sensitivities

offered by liquid-in-glass thermometers. They will be discussed subsequently.

Liquid-in-glass thermometers normally are available in two types: total

immersion and partial immersion. The total immersion type is initially

calibrated with the liquid column completely immersed in the measured fluid.

Since this may obscure the reading, a small portion of the column may be

allowed to protrude, thereby inducing negligible error. Partial immersion

thermometers are calibrated initially to read correctly when immersed to a given depth (immersion line, see figure 4.1) and the exposed portion is held

at a definite temperature.

Corrections for total- and partial-immersion thermometers when used under

conditions other than those intended are shown in figure 4.2. The temperature

of the exposed portion of the stem is measured by a second thermometer and the

general correction may be calculated from the equation:

15
16
Correction = 0.00009 nCt^ai '^act^

°^

number of scale degrees equivalent to the portion of

the stem not immersed

air temperature (°F) at calibration

air temperature (°F) during measurement.

The bulb of the auxiliary thermometer should be located at the midpoint of the

exposed column of mercury. Note from figure 4.2 that when a total- immersion

thermometer is used at partial immersion, (t^,^^^ - t^^^) in the correction

equation is replaced by the temperature indicated by the main thermometer

minus the temperature indicated by the auxiliary thermometer. For Celsius

thermometers the constant 0.00009 becomes 0.00016. In the example shown in

figure 4.2, the significance of the corrections should be noted and compared.

The bulb of the auxiliary thermometer should be located at the midpoint of the

exposed column of mercury.

The expansion chamber noted in figure 4.1 is provided to prevent the buildup

of excessive pressures in gas-filled thermometers as the liquid advances to

the top of the scale. The contraction chamber, which is frequently located

just above the bulb, is an enlargement of the capillary column. It serves to

reduce a long length of capillary above the auxiliary scale.

Liquid-in-glass thermometers are relatively inexpensive. They serve as good

standards when properly used and their calibration is easily checked by a qualified laboratory facility. They are dependable, with a relatively long

life. More elaborate types are available for higher accuracy over smaller

temperature ranges.

The accuracies of the total immersion types are:

0.5 °F (0.28 °C) for the -68.8 °F (-56 °C) to 32 °F (0 °C) range,

0.05 °F (0.03 °C) for the 32 °F (0 °C) to 212 °F (100 °C) range,

0.4 °F (0.22 °C) for the 212 °F (100 °C) to 600 °F (315 °C) range, and

0.8 °F (0.44 °C) for the 600 °F (315 °C) to 950 °F (510 °C) range.

Errors in the partial-immersion type may be several times larger even after

corrections for air temperature have been made. The uncertainties in

precision are slightly less than those listed for accuracy. The response time

of the liquid-in-glass thermometer varies with the size of the instrument and

the medium being monitored—usually 5 to 20 seconds.

The required manual reading of the liquid-in-glass thermometer often induces errors far beyond those caused by the instrument itself. This type of

thermometer in EMCS applications is usually graduated in one degree

increments. Should the person using the instrument record the integer value

of the graduation which appears to be the closest to the top of the column,

the value of the accuracies previously listed are far exceeded by "human

error." This is often the case, and the effectiveness of the functioning EMCS

where

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17
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18
will suffer from this fact. Carefully reading the scale with a magnifying

glass will allow most typical liquid-in-glass thermometers to be read to

within +0.15 degrees.

Liquid-in-glass thermometers, although not especially fragile, are frequently

dropped and broken. Proper precautions should be used in collecting the

scattered mercury as soon as possible for safety reasons. An experienced EMCS

person using a liquid-in-glass thermometer will carry the instrument from

point to point in a protective tube that is supplied with each instrument.

The experienced person will also place the instrument in the tube while it is

not actually in use. Such action prevents many unnecessary accidents.

Liquid-in-glass thermometers installed in pipe lines are used for manually

monitoring the functioning of a supply system rather than specifically for

EMCS. Such units are assumed to have been calibrated prior to installation.

However, it is not unusual to observe an instrument of this type in a working

mechanical equipment system which is obviously indicating an incorrect

temperature or which has a cracked stem. When a thermometer is observed in

this condition, it should be noted in the calibration notebook and reported to

those responsible.

The calibration of liquid-in-glass thermometers to be used as standards in

EMCS work should begin by examining the instruments, using a 15-20X

microscope, for errors, chips, or irregular coatings in the graduations; glass

chips or debris in the capillary; non-uniform capillary; and cracks in the

glass. The actual calibration of the instrument then is best accomplished by

the technique described in section 3.2. In checking the calibration of the

instrument over its full range, it is easiest to check the performance at the

ice point, 32 °F (0 °C). If the range of the thermometer is above this

temperature and the instrument has an auxiliary scale as shown in figure 4.1,

most often the auxiliary scale will include the ice point.

If the ice point is to be used as a calibration point, the thermometer should

be immersed to the proper level in shaved or crushed ice. The ice should be

made from distilled water or obtained from the clear portion of block ice.

The ice should be contained in a Dewar flask to retard heat flow from the room. Distilled water should be added to the ice, but only enough to fill the

spaces between the ice particles. As the ice melts, more ice should be added

so that the thermometer bulb is never sitting only in water. A carefully

prepared ice bath will provide a 32 °F (0 °C) reference within +.02 °F (.01

A variable temperature calibration bath using another liquid-in-glass

thermometer or an RTD as a standard is best suited for checking the

calibration of the full range of the thermometer. In such baths, adequate

agitation is usually provided to maintain the homogeneity of the temperature

in the bath. However, it is advisable to keep the instrument being calibrated

and the standard far enough away from the heating or cooling coils in the bath

to avoid possible errors. The proper immersion for the type of thermometer

being calibrated must be observed.

Determining the corrections to be applied to the readings of a liquid-in-glass

thermometer is accomplished by comparing its readings with those of the

19
standard. The number of calibration points depends upon the accuracy required

and the behavior of the thermometer. If the corrections are similar in

magnitude throughout the thermometer range, or if the corrections vary slowly

with the temperature, only a few points are needed to characterize a given

thermometer

.

The bulb is the thinnest and therefore the weakest part of the thermometer.

When the thermometer is heated to high temperatures, the resulting pressure

causes the bulb to expand; subsequently, the thermometer will read low. With

time, glass tends to relax to its former condition, but until it does, all of

the readings of that thermometer will tend to be in error by the same amount.

Keep in mind the vast ratio of the volume of the bulb to that of the

capillary. Reference 2 fully describes the methods to be used in arriving at

corrections for emergent-stem temperatures.

During the calibration of a liquid-in-glass thermometer and when using it as a standard, care must be used in several areas. First, make sure that the bulb

is in the medium to be measured; e.g., if the temperature of the air flowing

in an air duct is to be determined, the bulb should project inside the duct

far enough to avoid the gradients in the medium caused by the hot or cold

surface of the duct wall. Second, avoid touching the stem of the thermometer

to the wall enclosing the fluid whose temperature is to be measured. Third,

the medium being measured must be "mixed" as described in section 3.2 to avoid

"hot" or "cold" areas of flow. Fourth, determine whether heat is being either

carried to or taken from the thermometer bulb faster than the flowing medium

can remove or add it. This last area presents a very general problem that

applies to all types of thermal sensing units. In the case of the liquid-inglass thermometer, the stem correction factor can generally be used as a solution

.

Recertif ication by a qualified laboratory is recommended for all standards . The frequency of recertif ication will be discussed further in section 5 of

this report.

20
4.2 Pressure-Type Thermometers

Pressure-type thermometers consist of a sensing bulb, an interconnecting

capillary tube, and a pressure measuring device such as a diaphragm, bellows,

or Bourdon tube as shown in figure 4.3. The typical system of this type is

completely filled with a liquid (mercury and xylene are common choices) or with gas under an initial pressure.

A less-popular vapor-pressure thermometer of this type utilizes a nonvolatile

liquid to fill the lower portion of the sensing bulb, the capillary tube, and

the pressure measuring device. A volatile liquid is used to fill the center

portion of the sensing bulb and the vapor from the volatile liquid fills the

upper portion of the sensing bulb as shown in figure 4.4. The end of the

capillary tube is always protruding into the nonvolatile liquid in the sensing

bulb.

For the liquid-filled system, the compressibility of the liquid usually is

small enough that the measurement is essentially one of determining the

changes in the volume caused by the expansion and contraction of the liquid.

Compensation is required for the expansion and contraction of the bulb itself

together with changes in the capillary and pressure measuring device volumes

caused by changes in the temperature in their respective environments. The reverse is true for the gas-filled systems. Here, the basic effect is one of

change in the gas pressure at a constant volume. If a pure gas is used, the

indicating pressure generally follows the ideal gas law:

PV=nRT.

See reference 4 for further details. Minimal compensation may be needed for

the expansion and contraction of the enclosing components.

The capillary tubes on liquid and gas filled systems can be as long as 200 ft

(61m) for remote measurement. However, at these capillary lengths,

temperature variations along the capillary and at the pressure measuring

device usually make compensation mandatory. The systems using a volatile

liquid generally will not require compensation since the pressure depends only

upon the temperature at the liquid's free surface located in the bulb. Again,

the capillary may be as long as 200 ft (61m) for the volatile-liquid type if

the end of the capillary is retained in the nonvolatile liquid in the bulb.

A common method used for compensation of the liquid-filled thermometer is the

attachment of an auxiliary pressure sensing device and capillary. The

movements of the auxiliary system are caused by the interfering effect only.

These components are mechanically attached to the primary measuring device to

subtract (or add) to the output normally indicated by the primary system as shown in figure 4.5. Bimetal elements sometimes are used to obtain

compensation for the temperature differences of the case enclosing the

measuring device and of a portion of the capillary tube. See reference 2 for

further details.

Systems filled with xylene, or a similar liquid, have a range of -150 °F (-101

°C) to 7 50 °F (399 °C). Systems filled with mercury have a range of -38 °F

21
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24
(-56 °C) to 1100 °F (593 °C). The response is essentially linear over ranges

up to 300 °F (149 °C) for xylene and 1000 °F (538 °C) for mercury. Gas-filled

systems generally operate over a range of -400 °F (-240 °C) to 1200 °F (649

°C). The response for the gas-filled systems is essentially linear up to 1000

°F (538 °C). The vapor-pressure systems are generally usable in the range of

-40 °F (-40 °C) to 600 °F (315 °C). However, the calibration of the vaporpressure thermometers is non- linear to such an extent that special linearizing

mechanical linkages are needed if a linear output is required.

Additional characteristics of pressure thermometers that may affect the

calibration include:

1. In the liquid-filled types, differences in the elevation of the bulb

with respect to the pressure sensor will cause slight errors in the

calibration

.

2. In the gas-filled types, errors caused by capillary temperature

variations are usually small. However, compensation for variations

in the temperature of the pressure sensor is required and generally

is accomplished by bimetal elements.

3. The response time of a pressure thermometer will depend upon the

medium it is monitoring. For example, relatively long periods (1 to

3 min) may be required for the massive bulb and its contents to

respond to a small change in the temperature of an air stream it is

monitoring

.

4. The accuracy of pressure thermometers under ideal conditions is

about 0.5 percent of the scale range. Adverse environmental

conditions in the area of the capillary or pressure sensing device

will increase this error considerably.

5. Capillary tubes without the typical bulb are also used as temperature sensing devices in EMCS. The characteristics of the

capillary discussed in this section are utilized and the capillary

is mounted in the fluid stream in a manner to produce a pressure

that is representative of the average temperature of the fluid.

Sensors of this type are normally found in pneumatically-controlled

systems. Although sensors of this type are often "factorycalibrated", provisions generally are provided for"^ on-site

calibration. The manufacturers' instructions must be consulted

before any adjustments are made.

Pressure thermometers often are installed for manual inspection of the

operation of the mechanical equipment system in a building. However, they

also are used for activating and deactivating controls in an EMCS. Therefore,

they are included in this report.

The calibration of the pressure thermometers while they are in place is most

likely to provide satisfactory results. Such calibrations can be performed

using a suitable standard. The level of accuracy of the pressure thermometer

is not likely to require unusual calibration techniques. Regardless of the

25
level of accuracy provided, a pressure thermometer used for direct control in an EMCS requires special attention in its operating ranges.

If the thermometer must be removed for calibration, the problem of temperature

gradients within the medium or along the capillary or the pressure sensor must

be considered. It is the responsibility of the person performing the

calibration to determine the magnitude of these effects. In remote systems

with long capillaries, it may be found advisable to temporarily dismount only

the sensing bulb and a small portion of the capillary from the mechanical

system to allow a variable-temperature bath to be used for calibration at the

location of the bulb. This will allow the majority of the capillary and the

pressure sensor to remain in their typical environments while the calibrator

varies the temperature of the bulb within its operating range and checks for

the response of the device that the pressure sensor is actuating.

Although many negative features have been pointed out concerning pressure

thermometers, they have been installed and have functioned properly in many

buildings for many years. If a contractor prefers to use these types of

actuators and they meet the general requirements of the mechanical equipment

system for the building, they are likely to be found. Therefore, it is the

responsibility of the person performing the calibration of the EMCS to

calibrate and test each such instrument for proper operation.

26
4.3 Resistance Temperature Detectors (RTDs)

A resistance temperature detector (RTD) operates on the principle of a change

in its electrical resistance as a function of temperature. In general, the

resistivity of all metals increases with an increase in temperature and yields

a positive resistance-temperature coefficient [2,4]. Platinum, copper, and

nickel are the typical metals used for RTDs. The resistance/temperature

curves for these three metals are shown in figure 4.6. Platinum is the most

desirable for use as a sensor in EMCS because of the linearity of the

resistance/temperature coefficient. However, copper and nickel are used for

RTDs where the linearity is not important, and in installations in EMCS when

the signal from the sensor is not being conditioned.

Platinum is linear within +0.3 percent from 0 °F (-17.8 °C) to 300 °F (149 °C)

and improves to +0.2 percent from 0 °F (-17.8 °C) to 200 °F (93.3 °C). At the

higher temperatures, 500 °F (260 °C) to 1500 °F (815.5 °C), platinum is linear

within +.1.2 percent. The ranges of sensitivity and linearity of copper and

nickel relative to platinum can be observed from figure 4.6.

RTDs acceptable as sensors for EMCS installation and standards are made by

winding very pure (usually 99.99 percent), annealed platinum wire about a strain-free core usually made from ceramic or glass. This assembly is then

hermetically sealed in a ceramic or glass capsule. RTDs are made with two,

three or four leads, depending upon the desire for temperature measurements

independent of changes in the electrical resistance of the leads. The

encapsulated assembly is often mounted in a stainless steel sheath to provide

protection against moisture, shock, and the medium being monitored.

Open-type RTDs expose the resistance winding directly to the fluid being

monitored and give a faster response. However, the fluid must be

noncorrosive ; a fluid that seldom exists in EMCS.

Various flat types of RTDs are also available for measuring surface

temperatures. Surface temperatures of bodies being monitored with RTDs

consisting of grid windings and thin deposited films of platinum may produce

erratic outputs because of interfering mechanical strains in the sensor caused

by differential thermal expansion stresses in the bodies. If the surface

temperature is relatively high and/or the temperature measurement is critical,

the thermal radiation from the surface of the body being monitored is

obstructed by the flat RTD and the temperature indicated by the RTD may exceed

that of the temperature of the surface without the obstruction. Open, flat

grid, and thin-film types of RTDs are not recommended for general use in EMCS

for these reasons.

The most sensitive areas in the installation and calibration of the RTD are changes in the resistance of the lead wires caused by temperature variations

and variations in contact resistance. In operation, the RTD is generally

electrically mounted as one leg of an electrical resistance bridge. The

bridge circuit can be operated in either a^ null or deflection mode . For EMCS

measurements, the deflection mode is generally used. However, when the RTD is

being used as a standard, the null mode is often preferred. If the null method

27
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28
is used in a simple bridge circuit such as that shown in figure 4.7(a), the

resistor is varied until the bridge is balanced. The value of R^ is the same as that of R^ excluding all errors. The bridge circuit shown in figure

4.7(b) is useful for measurements of high accuracy since the contact

resistance of the variable resistor does not influence the resistance of the

bridge legs. If the leads from the RTD are long and are subjected to varying

temperatures, the bridge circuit shown in figure 4.7(c) is recommended.

Errors caused by the resistance changes in the legs will cancel since one of

these leads is in each of the bridge legs R^ and R^. The effect of a resistance change in the third lead is negligible if the indicating instrument

requires minimal current for operation. After proper calibration, the

circuits shown in figures 4.7(b) and (c) can be used in the deflection mode.

These circuits are useful in the null mode when being used as a standard for

detecting a given temperature (as opposed to detecting a range of

temperatures)

.

The circuit in figure 4.7(d) is shown to emphasize that when Rj^ = R2 > lOR^,

good linearity of the RTD can be obtained if the bridge is balanced with R^ at

the middle of the temperature range. A typical platinum RTD with a 100 ohm

nominal resistance, will vary approximately 20 ohms over its operating range

in an EMCS. If the legs of the bridge were of equal value, severe nonlinearity would occur. The higher values of Rj^ and R2 greatly reduce this

problem area and offer a bridge circuit with good performance.

When properly designed, the RTD will make an excellent standard. The use of a four lead Mueller bridge such as that shown in figure 4.8 is recommended [6].

The resistances of the leads are compensated in a large measure by placing the

resistance of one lead in each of the measuring legs of the bridge, then

reversing the connections before making a new balance. Using the resistance

terms shown in figure 4.8, the balance equations are:

Normal connections R^^ ^c

~ ^x ^ ^t

Reverse connections ^t ~ ^x ^c

By adding these equations, the value of R^^ can be determined from the

resulting equation:

^^x " "^dl * ^d2-

The unknown resistance of the RTD (R^^) is simply the arithmetic average of the

normal and reverse readings.

AC bridges are seldom available with AC resistance standards for EMCS

calibration standards. However, if such bridges are available, they offer a fast and extremely sensitive method for calibration in comparison with the

Mueller bridge shown in figure 4.8 or the bridges shown in figure 4.7. The

accuracy level of AC bridges can be determined by the use of AC resistance

standards .

Measurements made by the RTD sensor or standard using different currents will

show the presence of the self-heating error. Off-on measurements will give an indication of the time response. Other ambient conditions to consider include

the presence of electromagnetic fields from inductors, high-current power

29
30
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lines or rectifiers; unwanted sources of heat near the installation; frayed

insulation on the leads; and internal inhomogeneities in the medium being

monitored as mentioned in section 3 of this report.

In general, RTDs are best calibrated using the in-place comparison technique

and a standard. Assuming the RTD in the EMCS has a stainless steel sheath, a variable temperature bath may be well suited when properly used. In all

cases, the thermal energy gains or losses via the sheath and leads must be

considered and compared with such gains or losses when the RTD is mounted in

the system.

The response time of a RTD which has been hermetically sealed (without sheath)

is approximately one second in flowing water and two seconds in moving air.

These are 90 percent response times. The addition of the stainless steel

sheath will, of course, increase these response times depending upon the

design of the sheath and the medium being monitored. The self-heating error in

the hermetically sealed RTD is less than 0.1 °C/mW in moving air (v=l m/s),

and less than 0.25 °C/mW in still air.

The name of the precious metal, platinum, has a tendency to frighten

designers, engineers, and EMCS management personnel away from considering the

RTD as a sensor or a standard. However, the price of a platinum RTD

hermetically sealed in ceramic or glass is generally less than that of a certified liquid-in-glass thermometer. The cost of a platinum RTD mounted in

a stainless steel sheath is in the same price range as that of a certified

liquid-in-glass thermometer.

32
4.4 Thermistors

Commercially available thermistors usually are fabricated in forms of beads,

discs, rods or flakes. With the exception of the bead type, they are generally composed of sintered particles of metal oxides bound between two

conductive surfaces with lead wires attached. In the bead type, the lead

wires are embedded in the oxide. The oxides of nickel, manganese, iron,

cobalt, copper, magnesium, titanium, and other metals are used. Thermistors

differ from RTDs in several fundamental ways. First, a thermistor usually has

a high initial resistance (1,000 - 10,000 ohms) relative to the low resistance

of the RTD. Because of the higher initial resistance, the resistance of the

lead wires is usually negligible. Second, thermistors generally possess a relatively large negative resistance/temperature coefficient. Some

thermistors possess a positive resistance/temperature coefficient and are generally used for switching, not temperature measurement (this type will be

discussed later in this text). A third difference is that the resistancetemperature relationship of a single thermistor is very non-linear as shown in

figure 4.9. There are techniques, however, for linearizing the resistancetemperature relationship of thermistors. These types are commonly found in

EMCS. The fourth major difference is that the thermistor resistance varies

inversely with the applied voltage since the resistance is decreasing with

temperature instead of increasing as it does with the RTD.

Recent research [7] has helped to develop thermistors that are quite stable

with time. Off-the-shelf bead-in-glass and glass-coated disc thermistors

which drift no more than a few thousandths of a degree Celsius per year are available at relatively low cost.

There are several recommendations for thermistors which are to be used in

EMCS. First, the thermistor must be hermetically sealed to prevent the

deterioration of the oxides from the typical environments of EMCS.

Thermistors encapsulated in ceramic materials are not recommended for direct

immersion in liquids, especially water found in EMCS environments. Glass

encapsulated thermistors generally are suitable for direct immersion if

specified by the manufacturer. Second, two or more oxides separately

encapsulated within an outer seal and utilizing specified related components

to complete the bridge circuit should be used to produce a linear output

within an acceptable range for EMCS. Figure 4.10 displays some typical

circuits used for linearized thermistors. Figure 4.10 (a) represents a dualelement thermistor from which a positive or negative temperature/voltage

output can be obtained. Figure 4.10 (b) represents a more practical circuit

for EMCS using the same thermistor. The leads shown in the figure can be as long as 300 ft (91 m) for temperature ranges from 3 2 °F (0 °C) to 100 °F

(37.8 °C). By extending the upper end of the range to 212 °F (100 °C), the

length of the leads must be limited to 100 ft (30.5 m) to retain the specified

linearity. One restriction in using a typical dual-element thermistor in this

fashion is that the resistor R2 must remain close to the environment of the

thermistor. The linear deviation of available dual units such as shown in

figure 4.10 is approximately +0.4 °F (0.2 °C) over the specified range. The error introduced by interchanging the components (Rj^, R2, and the thermistor)

which are supplied by the manufacturer is approximately +0.27 °F (0.15 °C).

Currently-available thermistors made from three elements and using their

33
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respective resistors are linear within +0.09 °F (0.05 °C) over an operating

range of 0 °C to 100 °C.

Several additional factors should be noted in the application and calibration

of thermistors. One of the most important factors is the power dissipation

constant (i.e. power in mW required to raise a thermistor 1 °C above the

surrounding temperature). A typical thermistor suspended by its leads in a stirred oil bath has a dissipation constant of 8 mW/°C, or 1 mW/°C in still

air. The time constant (i.e. the time required for a thermistor to indicate

63 percent of a change in temperature) is approximately 2.5 seconds in stirred

oil and 25 seconds in still air. These factors must be kept in mind during

the calibration of any thermistor. Long-term operation at temperatures above

those specified for the thermistor will cause the unit to exceed the specified

tolerances

.

Linearized thermistors operating in a self-heated mode can be a problem in

calibration. If the applied voltage is not adequate, the medium being

monitored will dissipate the thermal energy faster than it is supplied. In

this case, the thermistor will not produce an output meeting the

manufacturers' specifications. In each case, the reference potential should

be computed as recommended by the manufacturer for the medium being monitored.

Simple, hermetically-sealed, single-element thermistors may also be found in

EMCS. Such components follow the general non-linearity shown in figure 4.9.

However, by adding a resistor in parallel with the thermistor of a value

approximately that of the thermistor at the center of the range in which it is

to operate, the extremities of the non-linearity will be greatly reduced. If

the range is relatively small (e.g. 20 °C) the thermistor will tend to

function in a linear fashion as shown in figure 4.11. This figure points out

another factor concerning the calibration of thermistors used in EMCS.

Thermistors are often used in the "resistive" mode instead of the voltage

mode. When thermistors are used in the resistive mode, the thermistor and its

related components are generally used as a leg in a bridge circuit where the

output of the bridge is amplified and used to transmit the signal to the

receiving unit of the EMCS. Figure 4.12 (a) shows the linearized thermistor

shown in figure 4.10 being used in a resistive mode. Figure 4.12 (b) is a schematic diagram of a linearized composite unit utilizing three different

thermistors being used in the resistive mode. The resistors Rj^, and

are generally supplied with the encapsulated thermistors.

The details of the circuitry in figures 4.10, 4.11, and 4.12 are presented to

emphasize one major item related to the calibration of any thermistor. This

item is impedance . The impedance of the device being used to measure the

resistance or voltage of the sensor during calibration, or, the input

impedance of the amplifier used to amplify and/or condition the signal

suitable for EMCS is critical . Although the exact value for this impedance

may vary from one manufacturer to another, in general, it will be in the range

of ten megohms. By observing the nature of the circuits shown for

thermistors, it will become obvious that any significant load placed across

the thermistor will reflect upon its reading. Therefore, the proper

instrumentation must be used to obtain the proper calibration signals. The

impedance of any additional load placed in parallel across the thermistor.

36
(aj

(b)

u

1»

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1

4 1

'^out,(ohmsl

Encapsulated thermistor

Ri= R of the thermistor at center of operating range

Thermistor

Shunt, R

Output of shunted thermistor

20 70

Temperature, ""C

Figure 4.11 a) Schematic diagram of a single element thermistor

shunted by resistor R-^. b) Resistance versus temperature diagram

37
(Dual composites)

Encapsulated thermistor

(Three composites)

Figure A. 12 Schematic diagrams of: a) a linearized two thermistor

circuit used In a resistive mode, and b) a linearized

three thermistor circuit used In a resistive mode

38
such as an amplifier, must also be considered if the thermistor is being

calibrated in-situ.

Other related items of importance include the selection of the values for the

other legs of the bridge when the thermistor is being used in the resistive

mode. The values of the resistors used must be high enough to prevent

unwanted self-heating and to minimize calibration errors caused by excessive

current in the thermistor. In simple bridge circuits, the values of the other

legs of the bridge will depend upon the reference voltage applied, the values

of the resistance of the thermistor, and, most important, the manufacturers'

specifications

.

Another typical problem in the calibration of thermistors is broken lead wires

in the immediate area of the sensor. The thermal expansion and contraction of

the encapsulating material and other materials that form the thermistor

package often break the leads within the hermetically sealed package. A

simple continuity test using a digital volt/ohm meter will locate this problem

during calibration.

Positive resistance/temperature coefficient thermistors are often found in

EMCS. However, such thermistors are generally used for switching.

Resistance/temperature curves for three such thermistors are shown in figure

4.13. If a thermistor is being used in a switching circuit in the system

being calibrated, it is the responsibility of the person performing the

calibration to test each thermistor switching circuit for its proper switching

range.

The various details of the thermistors and their related circuitry have been

presented in this report because they are known to be responsible for problems

that develop during calibration. If an error is found in the design of a circuit which reflects in the calibration of the sensor, it is the

responsibility of the person performing the calibration to report the problem

and be sure it is corrected. The typical design "errors** that have been

discussed should assist the person performing the calibration in determining

if the problem is one of design, installation, or calibration.

A thermistor is best calibrated in-situ by direct comparison using a standard.

This technique will allow the thermistor to be calibrated in the medium it is

monitoring and will test the circuitry being utilized for amplifying and

transmitting the signal. If the thermistor must be removed for calibration,

the necessary precautions mentioned throughout this section must be taken;

especially if the thermistor must be disconnected from its original circuitry.

39
10^

0.1 I I I I _j I i_-

0 30 60 90 120 150 180

Temperature, ""C

Figure 4.13 Positive resistance/ temperature coefficient switching

thermistors. Three switching values are shown.

AO
4.5 Integrated Circuit Temperature Sensors

Integrated circuit (IC) temperature sensors are available from many

manufacturers in different forms. Many of these sensors resemble an integrated circuit chip in the form of a dual-in-line package (usually 8 pins) or a metal can-package resembling a transistor. This type of temperature

sensor uses the fundamental properties of the silicon transistor. Reference 8 is included for the reader who is interested in the details of the designs.

Several manufacturers produce integrated circuits that are designed to yield

approximately 10 mv/°C. These IC can be biased in one of several ways to

function within a desired temperature range from -67 °F (-55 °C) to 302 °F

(150 °C). The self-heating effect of the excitation current can reduce the

specified accuracy significantly. Therefore, the manufacturers'

specifications must be followed to apply the lowest current suitable for the

application. Manufacturers of these sensors generally recommend from one to

25 typical application circuits including features which make the self-heating

error proportional to the absolute temperature. Several IC temperature sensors are available with a reference voltage stablizer and an operational amplifier.

Such units are more expensive. However, the temperature rise of the sensor is

only approximately 2 °F (1.2 °C) in still air from the effects of selfheating. Heat sinks are often used to reduce self-heating effects in moving

air

.

Still other types are available for yielding an output current proportional to

the temperature. These sensors also function over the range of -67 °F (-55

°C) to 302, °F (150 °C) with specified accuracies of +1 °C. However, the

factory calibration error for the less elaborate models has been found to be

in the area of +.5 °C at 25 °C.

In general, integrated circuit temperature sensors are satisfactory for a limited number of EMCS applications only if they are properly installed and

have been calibrated as specified by the manufacturer. Care must be taken in

applying and calibrating such circuits in many EMCS where the environment

often changes from a temporary power outage, causing the unit to fall well

below the minimum requirements for any EMCS. The particulate matter found in

typical EMCS environments is also a negative factor in this type of sensor and

the effects of accumulation of this matter must be carefully examined by the

person responsible for the calibration.

The time constant of sensors of these types is approximately 5 seconds in

moving air with no heat sink. Although these units are relatively inexpensive

and easily mounted, their application is usually limited to monitoring the

temperature of well filtered moving air. The self-heating effect and limited

accuracy and precision of these types of sensors often make them inadequate

for EMCS use.

Because of the numerous types, manufacturers, and installation techniques, the

methods of calibration are not noted in detail in this report. The reader is

encouraged to contact the manufacturer for calibration details.

41
4.6 Thermoelectric Sensors (Thermocouples)

Advancements in solid state electronics and the demand for more accurate

temperature measurements have made thermoelectric sensors more popular in

EMCS. Before getting into the calibration techniques of the various

applications of thermoelectric sensors (commonly known as thermocouples), a brief review of the fundamentals of the thermocouple itself will be given.

When two wires of different materials, A and B, are connected in a circuit as shown in figure 4.14, with one junction at temperature Tj^ , and the other

junction at T2 , a relatively high- impedance voltmeter will register an electromotive force (volts) E. If a galvanometer is used to indicate current,

a current I is measured. If current is allowed to flow, electric power is

developed. In EMCS measurements, this thermoelectric effect is used as a temperature sensor known as a thermocouple. The electromotive force, E, is

generally used as the temperature-dependent parameter [2,5]. The overall

relationship between the voltage E, and the temperatures T| and T2 is the

basis of thermoelectric temperature measurement and is known as the Seebeck

effect

.

The Peltier effect occurs when an external current is passed through the

junctions between dissimilar metals. One junction will be heated and one junction will be cooled. These heating and cooling effects are proportional

to the current and, fortunately, are completely negligible when the current

that is produced by the thermocouple itself is only that required by a highimpedance instrumentation amplifier or a digital voltmeter.

The Thomson effect influences the temperature of the conductor between the

junctions rather than that of the junctions themselves. When current flows

through a conductor having a thermal gradient along its length, heat is

liberated at any point where the current flow is in the same direction as the

heat flow, while heat is absorbed at any point where these flows are opposite.

Since the Thomson effect also depends on current flow, this effect is also

negligible if the current required to drive the potentiometer or amplifier is

negligible. Again, this is the case in proper EMCS installation and

calibration of the thermocouple.

The fundamental effects described above are mentioned to emphasize the need

for proper instrumentation and the possible side effects of junctions of

dissimilar metals inadvertently placed in the lead wires of a thermocouple

(T/C). The term "dissimilar metals" includes sections of the T/C wire that

have been cold-worked to change the homogeneity of the material as well as a junction at the terminating ends of T/C leads. Both of these conditions

inadvertently create another T/C junction.

Before looking closer at T/C applications and their related calibration

techniques, five fundamental laws of behavior of the T/ C will also be reviewed

[2]. Each of these laws apply to the calibration of the T/C and must be

understood to avoid typical erroneous conclusions that are often reached

during the process of calibration. Referring to figure 4.15:

1. The thermal electromotive force of a T/C with the junctions at

42
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43
temperatures of Tj and T2 is totally unaffected by temperatures

elsev/here in the circuit assuming the two metals are homogeneous,

figure 4.15 (a). (Note that all of these laws refer to the junct ion

temperature and assum.e adequate care has been taken to maintain the

temperature of the junctions at the temperatures of interest.)

2. A third homogeneous metal, C, can be inserted into either A or B as shown in figure 4.15 (b) if the tv70 new thermojunctions are at equal

temperatures. The net electromotive force (emf) arising from the

temperatures of the thermojunctions of A and B is not changed

regardless of the temperature of C.

3. A third metal C, can be added between the legs of one junction and

the temperature distribution of wire C is immaterial at any point

away from the junctions of interest AC and BC. If the junctions AC

and BC are at the same temperature , the net emf is the same as if

the third metal C were not in the circuit. This law permits

junctions to be soldered or brazed without affecting the readings.

See figure 4.15 (c).

4. The emf produced by the difference in the temperature of the

junctions in figure 4.15 (d) (left), using metals A and C is E^^.

The emf produced by the same junction temperatures using metals B and C is E^^^. The emf produced by metals A and B is E^^ + E^^,

again at the same junction temperatures. This law shows that all

possible pairs of metals need not be calibrated since the individual

metals can be paired with one standard (platinum is generally used)

and calibrated. Any other combinations can then be calculated;

calibration is not necessary. This technique is often used in

producing a standard for the higher temperatures in EMCS.

5. If a T/C produces an emf of E^ when its junctions are at 1-^ and T2,

and E2 when its junctions are at T2 and T^ , it will produce an emf

of E^ + E2 when the junctions are at Tj^ and T^ as shown in figure

4.15 (e).

The fifth law allows a thermocouple to measure an unknown temperature if the

temperature of one of the thermojunctions (called the reference junction) is

known. A voltage measurement allows the temperature of the second junction to

be determined by a voltage measurement and calibration tables. Most

calibration tables are obtaiiied by maintaining the reference junction at 32 °F

(0 °C) and varying the temperature of the measured junction over the desired

range [9].

The fifth law also allows temperatures to be measured when the reference

junction is at temperatures other than "ice point". A calibration table may

be used providing the temperature of the reference junction is known. For

example, the fifth lav/ allows the use of standard reference tables as follov/s:

suppose the reference junction is known to be 80 "^F (26.7 °C) and the voltage

reading is 1.24 mv. From figure 4.15 (e), T^ = 32 °F (0 °C), T2 = 80 °F

(26.7 "^C), and T^ is the temperature being measured. Assuming the T/ C is of

type T (copper-constantan) , E-^ is 1.06 mv found by using the tables. Since

the measured voltage E2 is 1.24 mv, Et^ + E2 - 2.30 mv. The unknown

44

temperature is found by looking for the temperature value corresponding to

2.30 mv in the standard T/C tables. In this case, T3 = 133 °F (56 °C).

4.6.1 Thermocouples

Common thermocouples (T/C) are formed by welding, soldering, or pressing two

materials together. If the same type (combination of materials) is used,

identical voltages will be produced by the same junction temperatures Tj^ and

T^. The type of T/C to be used in EMCS is determined by several factors.

Figure 4.16 shows the temperature/voltage curves for several of the typical

combinations of materials used. Type T (copper-constantan) are used for

typical EMCS measurements. Types J and K (iron-constantan and chromel-alumel

,

respectively) may be found in monitoring systems for higher temperatures such

as stack temperatures.

A common T/C is normally applied as shown in figure 4.17. The reference

temperature may be produced by an ice bath or a commercially available ice

point reference junction system. Such systems are available with one to fifty

or more reference junctions and allow both leads going to the voltmeter or instrumentation amplifier to be of the same material, reducing the possibility

of introducing unwanted T/Cs within the leads. Because of the high output

impedance of the T/C signal and the relatively low level of the signal, the

two leads of each T/C must be maintained in close proximity to avoid the

unwanted effects of electromagnetic fields; especially those of 60 Hz power

lines and inductive sources. The leads, if separated, will act as an antenna

and amplify the noise source. When the leads are maintained in close

proximity (electrically insulated from each other), each lead will pick up the

same noise signal and the net result will approach zero. This factor is very

important during calibration since the leads of a digital voltmeter attached

to the T/C junction at the amplifier can produce erroneous results if they are not twisted or bound together to avoid excessive electromagnetic fields from

passing between them.

The accuracy of common thermocouples is generally determined in two different

ways. If standard thermocouple wire is used which has not been calibrated by

the manufacturer, the manufacturers' quality control is the basis for

deviations from the published tables. These tables give the average

characteristics; not those of a selected batch. In the type T thermocouple,

this error is +0.5 % or +.1.5 °F (0.8 °C), whichever is the larger. Greater

accuracies are achievable when the individual thermocouples are calibrated.

Also thermocouple wire which has been factory calibrated yields a higher

degree of accuracy. In general, a maximum deviation of +^1 °F (0.5 °C) at

temperatures up to 300 °F (149 °C) can be obtained.

If automatic ice point control references are used, the accuracy of these

units must be considered also. Such reference junctions that use the Peltier

cooling effect as a refrigerator are available with an accuracy of +^0.05 ° C

[10]. However, the accuracy of typical commercially-available multijunction

ice point reference units is better than +0.25 °C.

In-line cold junction reference units which operate from a small "button"

battery are less accurate and require additional compensation for ambient

temperatures. Such devices are not recommended for EMCS since the battery

46
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48
adds to the task of calibration and the accumulated cost exceeds that of

multijunction ice point reference units.

A simple dual- junction thermocouple such as that shown in figure 4.17 using

ice as previously described for a reference bath will make an excellent

standard after calibration with a certified standard. Several multijunction,

digital-readout units are also available with built-in references. Such units

are often used for determining the stratification in a duct of air across a heating or cooling coil. However, units of this type must be carefully

calibrated under typical ambient conditions prior to their use as a standard.

The use of a digital voltmeter can induce further errors if the clip-on

devices used at the ends of the leads are of different materials and are still

further amplified if their temperatures vary. The clip-on devices can be made

from any one metal. Although two new thermocouples are added to the circuit,

if the temperatures are equal, the net effect will be zero.

The time constants for thermocouples vary from several microseconds to several

seconds depending upon the gauge of the wire used to make the thermocouple and

the media being monitored. For example, in still air the time constant for

0.001-inch-diameter wire is 0.05 seconds, while for 0.03 2-inch-diameter wire

the time constant is approximately 40 seconds. In air moving at approximately

18 m/s, the time constant is one order of magnitude smaller. In general, the

response time is very fast for thermocouples used in EMCS applications.

49
4.6.2 Averaging Thermocouples

A parallel arrangement of thermocouples as shown in figure 4.18 is used to

determine the average temperature of the medium in which the junctions are mounted. An arrangement of this type is commonly found in an air duct where

the mixing devices described in section 3 are not practical. It must be noted

that figure 4.18 is only a simplified schematic diagram and does not show

several items of importance. First, the leads of the individual thermocouples

are shown separated in the schematic diagram. As previously emphasized, the

leads must remain in close proximity to avoid noise from electromagnetic

fields ever present in EMCS work. Next, when using a group of thermocouples

as shown in figure 4.18, all thermocouple leads must be of equal length to

avoid unequal IR drops in producing the average emf of all thermocouples in

the parallel circuit. In addition, equal lengths of the thermocouple leads

should be exposed to the medium being monitored to avoid unequal thermal

conductivity from the various junctions by the thermocouple leads to the air or mounting means outside the pipe or duct in which the averaging

thermocouples are mounted.

An averaging thermocouple requires a reference junction as shown in figure

4.18 to allow standard T/C tables to be used to translate the voltage measured

by the digital voltmeter or amplifier to the average temperature of the

medium

.

In general, the individual thermocouple junctions are mounted at the center of

each of N equal areas of the duct or pipe in which the grid of averaging

thermocouples is mounted. The number N depends upon the size and shape of the

pipe or duct in which the medium is flowing. References 11 and 12 give

further details.

Averaging thermocouples open unique areas for the person performing the

calibration of a system. For example, each thermocouple can be removed or connected in any desired order and allow the performance of each thermocouple

in the grid to be tested for proper operation. This is an important factor

since the malfunctioning of any one thermocouple will affect the output of the

group. Also, by obtaining the outputs of the individual thermocouples, the

variations in the temperature across the section of duct or pipe in which the

grid is mounted can be determined directly from the outputs.

50
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51
4.6.3 Thermopiles

Thermopiles are multiple- junction thermocouples electrically connected in

series as shown in figure 4.19. In EMCS applications, this configuration is

commonly found monitoring temperature differences in HVAC systems. The

thermopile has* the advantage of multiplying the output of a single

thermocouple by the number of thermocouples in one of the two areas being

monitored. The typical nonlinear characteristic of the thermocouple requires

a direct reading of the actual temperature in one of the two areas where the

temperature difference is being monitored.

The same precautions pertaining to the close proximity of the thermocouple

wires, the equal lengths in the medium being, monitored, and the thermal

conduction of the thermocouple wire must be considered with the thermopile as

it is for the averaging thermocouple and the individual thermocouple. The

advantage of multiplying the signal has the side effect of also multiplying

any noise induced by electromagnetic fields.

Field calibration of thermocouple-type installations of all types usually can

be accomplished using a linearized thermistor as a standard in the in-place

calibration approach discussed in section 3. The in-place calibration

approach with a direct readout from the standard and a readout from the output

of the system utilizing thermocouples in any fashion will include any errors

caused by the amplification of the analog signal, noise generated in the

system, improper extension wires, etc. If a digital voltmeter is used at the

thermocouple-type installation, a direct comparison can be made v/ith the

thermal emf-temperature tables. In the case of the thermopile, the

fundamental law number five must be followed to adjust the temperature

difference in accordance with the actual temperatures measured by the

standard

.

52
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53
4 .7 Bimetallic Thermometers

Bimetallic thermometers are temperature-sensing devices that utilize the

phenomenon of thermal expansion of metals. A bimetallic thermometer is made

by firmly bonding two strips of metal together which have different thermalexpansion coefficients. A temperature change causes differential expansion of

the bonded strip, and, depending upon the configuration of the strip, a deformation will occur if the strip is unrestrained. Figure 4.20 shows a limited number of the typical configurations used. Since there are no usable

metals with a negative thermal expansion coefficient, the B element, figure

4.20(a), is generally made of Invar (a nickel-steel with an extremely low

coefficient of expansion). Brass originally was used as the high-expansion

strip A. However, a variety of alloys are now being used depending on the

electrical and mechanical characteristics required.

Although bimetallic thermometers with dial readouts are available

commercially, they are not recommended for calibration purposes because of the

poor accuracy offered by those commonly found in the marketplace. High

quality types are relatively expensive but offer accuracies to +^1 % full

scale. Bimetallic thermometers are rugged and have a time constant ranging

from 5 to 20 minutes in still air.

In EMCS, bimetallic thermometers are generally used for on-off switching as shown in figure 4.20(b). Bimetallic-thermometer-activated switches still are commonly found in EMCS as overload cutouts in electrical equipment where

excessive current flow in the bimetal causes heating and the accompanying

expansion results in opening an electrical contact. These types of

thermometers are also found in EMCS as temperature limit controls to avoid

thermal damage to the heating system. The requirements for limit control

applications are not extremely critical and lower cost units usually are satisfactory. For more critical applications such as room thermostats, the

performance must be improved to that of the higher-quality types.

In EMCS calibration, each bimetallic thermometer must be tested for proper

operation at the predetermined temperature limits. A room thermometer

requires careful calibration of the on and off temperatures, the time

constants, the switching differential, the operation of the anticipator built

into the thermostat, and other items described in detail in reference 13.

The bimetal sections and linkages of those units used in EMCS for limit

switching are subject to failure after long periods at elevated temperatures.

Therefore, the calibration of such units is imperative to maintain a system

operating with adequate safety.

Calibration using the in-place method and a standard is generally best suited

for the calibration of bimetallic thermometers. Those units used for

electrical overload require the assistance of a plant engineer to perform the

necessary calibration safely. In general, checking the time constants for all

types of bimetallic thermometers is very important in calibration.

54

5. CALIBHATION RECORDS OF THERMOMETERS, SENSORS, STANDARDS, AND SYSTEMS

Maintaining records of calibration has been found to be one of the mostneglected tasks of personnel who manage and perform the calibrations of

thermometers, sensors, standards, and remote temperature-measuring systems in

EMCS. The value of such records is never realized until an energy-balance

computation is performed and found to be in error by an unacceptable amount,

or until a sensor or remote-sensing system is calibrated and found to be in error to an extent that is inconsistent with the excellent performance of the

equipment being controlled. In general, the value of good calibration records

on each temperature-sensing device and system cannot be over emphasized.

The date and source of the calibration or recertif ication of all standards

being used to calibrate the various temperature-sensing devices and systems in an EMCS is a good place to start such a record. The time interval for

returning the standard to a qualified laboratory for recertif ication or recalibration can generally be determined by reviewing the dates and the

status of the standard each time the calibration was checked at the on-site

facilities. All calibration records of standards should be made available to

the qualified laboratory performing subsequent recalibration or recertif ication of the standard. Complete records of the use and the

calibrations of a standard often reveal factors which require particular

attention in the calibration process.

Similar record-keeping is directly applicable to the on-site calibration of

each instrument in an EMCS. Calibration records of temperature-monitoring

instrumentation, including remote temperature-sensing methods, often reveal

the source of differences in individual temperature-sensing devices that are normally very difficult to locate by mechanical means. Likewise, such records

often explain the discrepancies found in the thermal computation of the heat

exchange of a system or a particular portion of a system. Often, expensive

components of a mechanical system are replaced only to find that the error was

directly related to the method or inaccuracy of the calibration of the

instrumentation

.

In this report, many areas which generate errors in calibration have been

called to the reader's attention. The most important items to be included in

the records of EMCS temperature-sensing devices are the possible effects of

the uncertainties in the temperatures being monitored by the in-situ

instruments and the standards being used. Ambient conditions, temperature

ranges, any questionable areas of each individual sensor, and any other

factors which may affect the calibration are included in this area. Systematic errors cannot be determined without this information.

Calibration records of a specific temperature-monitoring device, remote system

or standard including the response to a reference temperature such as an ice

bath will generally reveal the necessity for replacement or, in the case of

the standard, the need to be returned to a qualified laboratory for

recertif ication. During the process of calibration of a temperature-sensing

device, remote temperature-monitoring system, or a standard, the values

indicated by the item being calibrated and the standard being used for the

calibration often are recorded casually on a pad of paper while checking the

56
response as the temperature is increased and decreased over the normal

operation range. It requires only minimal effort to record these values in a bound notebook instead of the pad of paper which is usually destroyed when the

calibration is completed. The value of casual records is limited, whereas the

well-kept laboratory notebook can provide unlimited evidence of the measuring

equipment performance.

57
6 . POSSIBLE EFFECTS OF SYSTEMATIC ERRORS

This section is presented to emphasize the necessity of careful and frequent

calibration of all components in any system being used for EMCS. Each item

being calibrated and every standard used for calibration is subject to error

beyond the statistical uncertainty in its measurement. The exact value of the error is very seldom known for any of the items used for the calibration or in

the end product being calibrated.

For example, consider the error of a single readout from a series of

components of a typical remote temperature-monitoring system. If the

inaccuracy in the calibrated sensor/ transducer device is +^ 1 %, the

inaccuracy in the transmitting device is also +. 1 %, the inaccuracy in the

transmission means is +_.5 %, and the inaccuracy of the receiving device is

1 %, then the maximum error in the actual readout would be +3 1/2 %.

Since the direction of the error seldom falls in the same direction for each

component, the uncertainty of a measurement is often expressed using the rootsum-square (RSS) technique, also known as the statistical bounds technique

(2). The RSS technique states that when a given variable, s, is determined

from several measurements (say w, x, y, and z) according to the relation:

s=f (w,x,y , z)

,

the probable uncertainty which we may call Rg of the calculated value s, is

determined by the following equation:

R = / r^^ + r ^ + r-^ + r ^ (1)

s / t m 1 r ^

'

where

Rg is the uncertainty of the temperature readout with

respect to the actual temperature being monitored,

r^. is the uncertainty in the calibration of the

sensor/transducer,

is the uncertainty in the calibration of the

transmitting device,

r^ is the uncertainty in the calibration of the

transmission means, and,

Tj. is the uncertainty in the calibration of the

receiving device.

Inserting the values given in the above example into equation (1), we have:

58
.01^ + .01^ + .005^ + .01

R

s +..018 or +^1.80 % is the probable uncertainty in the

temperature reading.

The above example points out several items of particular interest. First, the

values inserted for the uncertainty of the individual components may have at

first appeared to have been somewhat "risky"; i.e. on the low side or the more

accurate side. However, the maximum error in the typical example resulted in a value of 3.5 % and the uncertainty computed by the statistical bounds or RSS

technique was 1.80 %. Regardless of which technique was used, one point is

clear: both values are likely to be too high for the sat isf actory calibration

of a_ remote- sensing system. Therefore, the inaccuracy in the calibration of

each component must be reduced.

Second, the temperature measurements in an EMCS are generally used in

conjunction with other measurements such as flow to calculate the engineering

values required for efficient operation of the mechanical systems of the

buildings. Any uncertainty introduced at the temperature-monitoring level will

always increase the uncertainty of the engineering value that is required.

Third, the calibration of temperature-monitoring systems in an EMCS are usually considered by management to be the least troublesome and, therefore,

are expected to be the most accurate of all EMCS measurements. In this

example, the values listed for the uncertainties for each component are too

high for a typical calibration of a remote temperature-sensing system. Each

component must be calibrated with greater care to improve the accuracy. The

emphasis should be given to the components having larger absolute error

In general, the effects of the accumulation of errors in any chain of events

that take place in a remote- or direct-sensing system can result in an error

in the end product beyond an acceptable value. See reference 2 for further

details on error analysis.

values

.

59
7 . ILLUSTRATIVE EXAMPLES OF CALIBRATING TEMPERATURE SENSING SYSTEMS

INSTALLED IN BUILDINGS

This section presents several examples of temperature-sensing-device

calibration to illustrate the application of the material presented in the

previous sections of this report. In each example, the type of sensor, the

method used in mounting the sensor, the purpose of the sensor, and other

typical features found in EMCS pertaining to calibration will be described.

Details presented in the first example which are applicable to succeeding

examples will not be repeated.

7 ,1 Thermistor Calibration

In this example, the person performing the calibration of the temperaturesensing devices of a remote monitoring system has located several linearized

thermistors in the hot and chilled water supply and return lines serving the

air-handlers in a large building. The thermistors are mounted in thermowells

placed in the water lines to monitor the temperature of the water entering and

leaving the heating and cooling coils of the air-handler. The thermowells

have been filled with a synthetic oil to improve the thermal conductivity from

the walls of the thermowells to the thermistors. Since the thermistors can be

removed and replaced in the wells without damaging the insulation on the

conductors or causing excessive mechanical fatigue to the wiring, the

calibration technique with the sensor removed from the HVAC will be used for

the thermistors. The balance of the system will be calibrated using the

techniques for remote systems.

A schematic diagram of the system indicated that the thermistors are being

used in a voltaic mode and that the signals are being amplified in a signal

conditioning circuit. The output of the amplifying circuit is wired directly

to the appropriate channel in the MUX near the air-handler where the water

temperatures are being monitored and controlled.

The operating ranges of each thermistor are not noted in the calibration

notebook. Therefore, the high and low temperatures that occur at each of the

monitoring points must be obtained from drawings, specifications, or files at

the plant. Often the plant engineer, the central control unit (CCU) operator,

and person in charge of the software are good sources for this information if

all others fail.

The characteristics of the analog-to-digital (A/D) conversion circuitry must

also be known. In this case, the person in charge of the software, MUXs,

FIDs, and CCU may be the best source. The characteristics of the A/D

circuitry include the maximum and minimum input voltages and the temperatures

these values represent in the final CCU readout device. Examples of typical

voltage ranges for an A/D converter are: -10 volts to +10 volts; -2.50 volts

to +2.50 volts; 0 volts to +5 volts; and 0 volts to +10 volts. It is not

uncommon to find different voltage ranges at different A/D converters within

the same building! The range is often set at the convenience of the person

originally designing and/or installing the EMCS. In some of the more modern

systems, individually-programmed sensitivities for each channel may be present

60
in which the software is used to determine the range of inputs to produce the

desired outputs. In such systems, the person responsible for the software is

usually the source for the desired information for calibration.

A search for the necessary information revealed that all A/D converters were programmed to operate in the 0 volts to +10 volts range. For the cooling

coils, the supply and return temperatures represented by the A/D voltage range

were identical; 32 °F (0 °C) to 90 °F (32.2 °C). For the heating coils, the

supply and return temperatures represented by the A/D voltage range were also

identical; 60 °F (15.5 °C) to 200 °F (93.3 °C). A review of the MUX software

specifications clearly indicated that the temperature change was directly

proportional to the change in the input voltage to the A/D converter. In this

simple example, the temperatures represented by the A/D input voltages would

be found as follows:

For the cooling coilsAT = 90 - 32 = 58 °F,

AV = 10 - 0 = 10 volts,

58

Scaling factor = — =5.8 °F per volt,

10

Offset = 32 °F,

T (°F) = 32 + 5.8 V, where V is the digital voltmeter

reading and T is the temperature of the thermistor.

For the heating coilsAT = 200 - 60 = 140 °F,

AV = 10 - 0 = 10 volts,

140

Scaling factor = = 14 °F per volt,

10

T (°F) =60+14 V, where V is the digital voltmeter

reading and T is the temperature of the thermistor.

Now that the maximum and minimum temperatures, the maximum and minimum

voltages and their respective temperature indications, and any characteristics

of the remote monitoring system and/or the thermistor that may affect the

calibration have been found and recorded in the calibration notebook, the

calibration task can begin.

The most important part of any calibration procedure is making sure that the

necessary personnel at the plant and at the CCU have been notified in advance

of the calibration, the date and time it is to take place, the limits of the

calibration, the identification of the channels to be calibrated and the

desired sequence, and any other information that may be requested. This

61
important initial step has two areas of significance. First, it will allow

the plant and CCU personnel to prevent the "false" signals received from the

remote channel to cause the EMCS from responding directly in a normal fashion

and create discomforts, hazards, etc. within the mechanical system of the

building. Second, since this is a remote system that is being calibrated by

removing the sensor from the HVAC, the person performing the calibration must

know the temperature readout received by the CCU relative to the temperature

of the bath in which the thermistor is immersed. Therefore, a close working

relationship with the plant and the CCU is always necessary. For medium and

large facilities, the notification of the calibration of any one or more

remote channels should be made at least one week in advance in writing to

avoid possible "misunderstandings".

In this example, hot and cold temperature baths are required. Since each

thermistor was mounted in an oil filled thermowell, the thermistor must be

calibrated in a liquid such as water, oil, ethylene glycol, etc. to avoid the

self-heating effects described in section 4.4. In addition, the temperature

baths must be equipped with adequate circulators to retain equal temperatures

within the bath at any given time. Another thermistor, a liquid-in-glass

thermometer, a thermocouple, or a RTD may be used as a standard if it has been

calibrated over the range to be used. In any case, the t ime constants for the

thermistor being calibrated and the standard must be considered during the

calibration.

The thermistors are calibrated, one at a time, by immersing them in a circulating liquid bath adjusted to a temperature equal to that of the maximum

temperature of the point in the system being monitored. The leads of a digital voltmeter are placed on the output of the signal conditioner. Care

must be taken in using the proper electrical ground for the reference ground

of the meter. In general, the electrical ground for the channel being

calibrated is used since the voltage from the output of the signal conditioner

to the electrical ground is the same as that at the A/D converter input. The

input impedance of a typical A/D converter is usually very high (10-20

Megohms). Therefore, any voltage drop in a typical EMCS line is negligible.

If a sheath of any type is used on the thermistor being calibrated or on the

standard being used to monitor the temperature of the bath, the time after

immersion should not be less than three minutes before a calibration reading

is taken. Care also must be taken to avoid situations such as the following:

1 . A rapid exchange of the thermistor from one bath to another should

be avoided. The thermal expansion and contraction of the leads

which are hermetically sealed by glass or ceramic material will

cause the leads to fracture within the seal and the electrical

conductivity to be broken. In this fashion, the thermistor is

"broken". Holding the thermistor in the air for one or two minutes

before immersing it into the bath of the opposite extreme

temperature generally will prevent this catastrophic result. Three

or more immersions in each bath are recommended before the

calibration is completed.

62
2. The thermistor and the standard should be retained in the "stream"

of the circulating liquid in the bath. If either is allowed to

touch or come close to the walls of the bath, the validity of the

calibration is highly questionable. In a similar fashion, the

standard and the thermistor must be retained at adequate depths in

the bath to allow the true temperature of the bath to be monitored.

The thermal conductivity of the leads or sheath of the thermistor or standard can also create incorrect calibrations by inadequate immersion.

3. Since the thermistors being calibrated in this example are mounted

in oil-filled thermowells, the self-heating of the thermistor in

still air must be kept in mind. If the thermistor must be removed

from one of the baths for more than four minutes, it should be

immersed in a container of v/ater at room temperature while the

necessary adjustments to the baths are being made.

4. If a RTD or a thermocouple is being used as a standard, the thermal

conductivity of their leads becomes still more critical. Review

sections 4,3 and 4.6 before using these types of sensors as standards

.

5. In this example, "linearized" thermistors are being calibrated.

However, the calibration of any t emperature sensor is not completed

by only checking the respective readouts with temperature baths at

the two extreme limits of the operating range. A third calibration

point should always be made at a temperature approximately equal to

the middle of the operating range. This step is imperative to check

the operation of the sensor, the signal conditioner, the transmitter

(if one is used), the A/D converter, and the CCU. By observing the

schematic diagrams of a "linearized" thermistor, it will be noted

that one of the metal oxide elements could drastically change its

characteristics and still allow the signal conditioner to be

adjusted to give the proper readings at the CCU for the limits of

the range of operation. The linear response could be in error at any

other temperature. In the case of the linearized thermistor, the

change in characteristics will usually be caused by a broken

conductor

.

6. The typical stability of a thermistor is supposedly accomplished by

the manufacturer through proper aging of the elements prior to sale.

However, experience and references 2, 5, and 7, point out that the

stability of a thermistor of any type may drift significantly during

the first year of operation. This factor alone indicates the

necessity of calibration of a thermistor which has been recently

installed or replaced at frequent intervals (e.g., once every month)

until the calibration records indicate adequate stability.

If a telephone line is not available from the area of calibration to the CCU

operator to obtain readouts at the various calibration points, two-way radios

are helpful. Otherwise in a large building using a remote system, obtaining

the readouts at the CCU becomes a major problem.

63
In this example, the temperature of the bath, the output of the signalconditioning circuit, and the CCU readout should be recorded for each step

until the necessary minor adjustments are made to the signal-conditioning

circuit to make these parameters concur. If the bath temperatures do not concur with the relative voltage output of the signal-conditioning circuit,

problems can be expected in the transmission lines to the MDX and the

thermistor. Depending upon the design of the remote system, it may be

feasible to connect a portable terminal at the MUX and trace the source of the

error. If the output voltages at the signal conditioner concur with the

respective temperatures of the baths and an error is noted at the CCU readout,

the input voltage at the MUX should be checked with the digital voltmeter. If

a difference is noted, transmission-line problems usually exist between the

thermistor and the MUX. One test that can be made at this point is to place

the digital voltmeter in an AC voltage mode. If an AC voltage is indicated,

transmission interference (usually from a 60 hertz source) can be expected.

The source usually becomes obvious in tracing the lines back to the

thermistor

.

The shield of the transmission line may be grounded to the electrical ground

or to the earth ground. If ground-fault interrupters are not installed

throughout a building, it may be advisable to connect the shields of the

transmission lines to the electrical ground to reduce AC interference. This

step assumes that the outside insulation on the transmission lines has been

inspected and found to be in good condition. Often both ends of the shield

will be connected to one ground or the other. Removing the connection of the

shield at the sensor end will often help. If simple steps such as these fail

to reduce the AC interference, it is advisable to connect an oscilloscope to

the MUX end of the sensor transmission line and observe the frequency, wave

form, etc. of the interference. The oscilloscope will generally give clues to

the source of the interference.

Often the shielded transmission line will be found routed too close to a highcurrent power line. If this is the case, the frequency indicated by the

oscilloscope will not necesssarily be 60 Hertz or have the form of a sine wave. High current rectifiers, electronic ignition devices, welders, etc.

will usually be found if the transmission line is routed too close to a power

line feeding such items.

Thermistors as well as thermocouples generate a high-frequency thermal

junction noise within the metal oxides of the thermistors and at the junctions

of thermocouples. This noise usually will range from 0.5 to 3 megahertz.

However, the intensity of this noise is very low; amplifiers originally used

for signal conditioning were not capable of responding to these higher

frequencies. In this manner, the amplifier served as its own filter. The

instrumentation amplifiers now available and used in EMCS respond to this

noise and often cause problems for the A/D converter. By placing passive

filters in the transmission lines ahead of the amplifier, this noise problem

can be controlled even if electronic gains of over 10,000 are required to meet

the range of the A/D converter. See reference 2 for further details.

When the calibration of each channel is completed, measure the temperature of

the oil in the thermowell using a standard. Replace the thermistor in the well

and request a last readout from the CCU. Unless the system is in a transient

64
state, the readout from the CCU should be close or equal to that temperature

measured by the standard. Always notify the plant and the CCU operator when

the calibration of each channel is completed.

65
7 .2 Thermocouple Calibration

The typical environments of HVAC systems require routine calibration of

thermocouple sensors. Characteristics such as the relatively low output

signal of the thermocouple, the numerous ways thermocouples are used, and the

relatively high input impedance required for the signal-conditioning

amplifier often make the person responsible for the calibration of the EMCS

instrumentation hesitate to perform routine calibrations. However,

temperature-sensing systems utilizing the thermocouple are often the simplest

to calibrate in a routine manner. This example covers several different types

of thermocouple sensors utilizing the fundamental information presented in

section 4.6 and proven techniques applicable to EMCS.

In this example, a medium-sized building is using thermocouples in the

form of single thermocouples with a multi-channel ice-point reference to

monitor the temperatures of the water entering a heating coil and a cooling

coil in an air-handler. The primary objective of these thermocouples is to

provide a base temperature measurement for the thermopiles which are monitoring the difference in the temperatures of the water entering and

leaving the coils in the air-handler. The temperature profile of the air in

the supply duct leaving the air-handler is not uniform because a small heat

exchanger has been installed in the duct upstream of the supply air monitoring

point. Therefore, an averaging thermocouple grid is installed to monitor the

most critical part of an air-handler; the temperature of the output or supply

air. The ice-point reference for the averaging thermocouples utilizes a third

channel of the unit used for the single thermocouples.

A diagram of the air-handler is shown in figure 7.1. The thermowells are designed to protrude deep into the supply and return lines. This feature

allows the individual thermocouples in the supply lines and all thermocouples

of the thermopiles to be at the bottom of the oil-filled wells and avoid

excessive heat losses through the conductors and the walls of the thermowells.

All thermocouples are Type T (copper-constantan).

The individual thermocouples are wired identically to that shown in figure

4.17 except the reference bath shown in the figure has been replaced by a commercial, multi- junction, ice-point reference. (This device provides builtin thermocouple reference junctions of the type being used and maintains these

junctions at 32 °F (0 °C); the equivalent error of these references is less

than 0.23 yV/°C ambient temperature). The thermopiles are wired identically

to that shown in figure 4.19 and the averaging thermocouples are represented

in figure 4.18. An important feature present in the installation of all

thermocouples is not represented in the diagrams shown in section 4. This

feature is that all thermocouple leads of each temperature monitoring channel

are retained in close proximity to avoid the unwanted effects of

electromagnetic fields as explained in section 4.6.1. In addition, all

thermocouples have been electrically insulated to avoid contact with each

other

.

Because the air-handler in this example is in a medimum-sized building, the

mechanical system is controlled by a MUX, a FID, and a small CCU. The MUX is

adjacent to the air-handler, the FID is in the central part of the attic

66
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67
serving other MUXs monitoring and controlling other air-handlers in the attic,

and the CCU is located in an office adjacent to the mechanical-equipment room

in the basement. Under these conditions, typical calibration techniques are

not applicable because of the discomforts that would be imposed on the

occupants of the building by fluctuating the temperature of the water in the

coils and the supply air over the full range of operation during a routine

calibration

.

The thermocouple wire that was used in producing all of the thermocouples at

this site was factory-calibrated wire which has a maximum deviation of +1 °F

(0.5 °C) at temperatures up to 300 °F (147 °C). Theref ore, a special

technique applicable to EMCS is used for the general calibration procedure.

Before describing this technique, it must be emphasized that follow-up

calibrations using a standard are mandatory. These follow-up calibrations are made as the seasons change requiring different temperatures in the water

flowing through the coils and the supply air.

The special technique referred to above is the removal of the copper leads

from the signal conditioning amplifier which originate at the ice-point

reference and applying potentials from a well-calibrated source to the signal

conditioner. The source of the potentials being applied must be of laboratory

quality and capable of adjustment to within +^1 yV, In the case of the

thermopile, the copper leads (A) that are removed from the signal conditioner

are those coming directly from the thermopile as shown in figure 4.19. Except

for the thermopiles, the potentials to be applied are taken from reference 9 or from the literature of one of many manufacturers who provide the same data

and make direct reference to the NBS Monograph 125.

Starting with the individual thermocouples, potentials are applied to the

signal-conditioning circuit starting with that potential corresponding to the

lowest temperature of the water in the coil. (The various temperature ranges

are found and recorded in the same manner described in the previous example.)

The amplifier in the signal-conditioning circuits is connected to the various

thermocouple-type temperature monitors in a differential mode. This is

typical and is recommended by the majority of manufacturers of instrumentation

amplifiers for thermocouples. Therefore, careful attention must be given to

polarity when connecting the leads from the reference voltage source. A

simple method of checking polarity is to connect a digital voltmeter (a 5 1/2

digit voltmeter capable of reading microvolt potentials is required for

thermocouple calibration) across the terminals before disconnecting the leads

from the thermocouple. This simple procedure serves several purposes in

addition to determining the polarity. First, the potential indicated by the

digital voltmeter must lie within a reasonable range of values with respect to

the estimated or measured temperature of the thermocouple. If it does not, a calibration of the ice-point reference should be made using an independent

thermocouple and a reference temperature bath with a standard. (All channels

of the ice-point reference should be calibrated in this fashion before

completing the calibration regardless of the initial readout when checking for

polarity .

)

Second, this simple test will immediately indicate any broken leads, leads or thermocouple junctions short-circuited to the mechanical system, thermocouples

that have fractured, etc. The temperature readout at the CCU can be

68
referenced to the type T temperature/voltage reference tables being used, and

3" init ial check on the remote transmission and A/D converting system is also

performed by this simple preliminary test.

The ice-point reference was found to be in proper working order and the

polarities of the amplifiers were determined and recorded. The leads from the

reference junction were removed and the reference voltage source applied and

the voltages were varied up and down throughout the range of voltages produced

by the temperatures of each thermocouple. It will be noted that the

thermocouple does not function linearly. Therefore, tables must be used when

comparing a readout at the CCU with the potential applied to the signal

conditioning-amplifier. In general, thermocouples are very stable

temperature-sensing devices. Therefore, only very minor adjustments, if any,

should be required. If other than very minor adjustments are required, the

channel should be examined for problems in the thermocouple itself, the icepoint reference, the junctions, the signal conditioner, transmission, etc. as described in the previous example.

Throughout all thermocouple calibrations including the superficial calibration

described above, care must be taken to avoid establishing additional

thermocouple junctions during the calibration procedure. Use like materials

for the leads and connecting means on the voltmeter and the reference voltage

source. Refer to the thermocouple laws given in section 4.6. Fortunately,

these laws make it very easy to avoid establishing additional thermocouple

junctions during the calibration procedure if they are followed.

By noting the schematic diagram shown in figure 4.18, one will observe that

the averaging thermocouples in the supply air duct are identical to the

individual thermocouples. The basic superficial calibration of the channel is

made in the same way as that used for the individual thermocouples. However,

one additional test is required assuming that the superficial tests indicated

everything to be functioning in a normal manner.

The leads of the averaging thermocouples are removed from the ice-point

reference and the leads of an independent type T thermocouple being used as a standard are then placed on the ice-point reference. The standard

thermocouple junction is carefully inserted into the supply air stream. A

small wooden dowel is used to support the standard. After placing the

voltmeter on the output of the signal-conditioning circuit, the output voltage

is observed and noted as the standard is moved to areas of the air stream

simulating the positions of the individual averaging thermocouples. The leads

of the standard are then removed from the ice-point reference and the leads of

each thermocouple are then connected to the ice-point reference and

independently tested. The output voltage resulting from the connection of

each individual pair of leads of the averaging thermocouples to the ice-point

reference is recorded. After each individual thermocouple has been tested and

recorded, the values are then compared to those obtained with the standard for

the various areas of the duct. If any one or more of the averaging

thermocouples fall more than 5 % outside of the value established by the

standard, the individual thermocouple should be checked for electrical

insulation damage, dirt collected on the thermocouple junction, sharp bends in

69
the leads, and other typical faults noted in section 4.6. Correct the fault

if it is a simple one. Otherwise replace the thermocouple using the

precautions given in section 4.6.1.

In the above example for the calibration of averaging thermocouples, it is

assumed that the temperature of the supply air is constant and that

alternative methods have been used to maintain this critical parameter.

As shown in figure 4.19, the thermopile does not use an ice-point reference.

The two copper leads "A" are connected directly to the signal conditioning

unit. The potential resulting from the differences of the temperatures of the

water entering and leaving the cooling coils is directly applied to the

amplifier. The software uses this potential and the temperature indicated by

the input from the single thermocouple monitoring the water entering the coil,

to compute the difference in the temperatures of the water entering and

leaving the coil. See reference 9 for further details.

Since the potentials needed for the signal-conditioning unit cannot be

determined directly from tables, the person most familiar with the software

utilized by the CCU is contacted and the technique of applying reference

potentials to the thermopile amplifiers is discussed. By referring to the

software files and the temperature/voltage conversion tables, the CCU operator

can produce the potentials to be applied to the input of the signal

conditioning unit to produce 0 volt and 10 volt output signals. Since the

thermopile is a multi- junction thermocouple sensor, the potentials that are applied to the signal conditioner to produce the maximum input to the A/D

converter will be higher than those used for the single thermocouple. The

values obtained were 0 millivolts for 0 volt output and 43.628 millivolts for

a 10 volt output.

After checking polarity, the connections from the thermopile are removed and

the voltages from the reference potential source are applied as previously

described. The output of the signal conditioner is monitored by the

voltmeter. Only very minor adjustments to the signal conditioning unit should

need to be made. Otherwise problems in the thermopile, transmission lines to

the signal conditioner, the signal conditioner, etc. probably exist. The

techniques of detecting noise from electromagnetic fields previously described

are used in addition to checking the thermopile leads for electrical contact

with the thermowells and other parts of the air-handler. If problems still

exist, the thermopile is checked for the proper functioning of each

thermocouple in the thermopile. Broken junctions and junctions making

electrical contact with other junctions or the thermowell are possible problem

areas. The conductivity of an improper liquid that has been placed in a thermowell also has been known to generate problems unless each thermocouple

has been hermetically sealed.

After the above calibration is completed, the actual temperatures are periodically measured at each thermowell using a standard as described for the

single thermocouple. The difference in the actual temperatures must be the same as that indicated by the CCU readout. Otherwise, the same types of

problems listed in the previous example may exist. In addition, the person

responsible for the software and/or the CCU should be approached with the

facts noted in the calibration notebook or described in the first example. In

70
general, finding and correcting such problems in the calibration of a thermopile is a joint effort shared by the person responsible for the

temperature sensing calibrations and the person operating the CCU.

71
7 .3 Bimetallic Thermometer Calibration

In this example, a bimetallic thermometer employed in a switching device on an oil furnace used to heat a remote building requires calibration. The device

functions to shut the furnace down if either of the following conditions

exist:

a) The temperature of the heat exchanger being monitored has not

reached 120 °F (49 °C) within 30 seconds after the furnace has been

turned on. (This is to detect a possible failure of the ignition

device in igniting the fuel.)

b) The temperature of the surface of the heat exchanger exceeds 300 °F

(149 °C).

If the furnace is shut down by the bimetal thermometer device for either of

these reasons, a manual reset is required.

Calibrating the thermometer in situ presents two problems. First, operating

the furnace long enough to deliver an accurate calibration of the thermometer

may cause discomfort to people working in the area, especially if the

calibration is performed during the summer months. Second, allowing the

temperature of the heat exchanger being monitored to rise high enough to shut

the furnace down will present a safety hazard if the control unit is not

functioning properly. For these reasons, the unit is removed from the HVAC for

calibration

.

It is necessary to remove the control unit from the furnace and replace it

with a tight fitting cover. Qualified plant personnel should be assigned to

manually monitor the furnace if its operation is required during the

calibration period. In any case, plant personnel must be notified of the date,

time and duration of the calibration period as was done in the first example.

The status of the unit is visually inspected for deposits caused by the

combination of heat and particulate matter. Any deposits which are found are removed with a soft wire brush or other suitable tool to avoid damaging the

bimetal thermometer. The repeated expansion and contraction of the bimetal

portion of the thermometer in conjunction with various deposits often cause rapid deterioration. If either of these conditions are found during

inspection, the thermometer is repaired or replaced.

Before beginning the calibration, the wiring diagram for the control unit is

studied. It is necessary to power the unit in the same way as is done in the

furnace. The locations of all the appropriate connections are noted. These and

all other calibration information are recorded in the calibration notebook.

A suitable variable-temperature electric oven is found and its rate of

increase in surface temperature is adjusted to reach 120 °F (49 °C) within 30

seconds. The thermometer portion of the unit is installed on the surface of

the oven and it is confirmed that the thermometer is shielded from the direct

radiat ion of the oven's heating elements. The appropriate power connections to

72
the control unit are made. A suitable standard is used to monitor the

temperature of the surface of the oven.

Continuity-detecting devices are connected to the appropriate switch contacts

and the status of each switch is noted. The power to the control unit and the oven is turned on and the time and temperature of the surface of the oven required for the low temperature timing switch to change status is recorded.

The surface temperature of the oven at which the high temperature relief

switch changes status is also noted. The results of the calibration are then

compared with the specified switching time and temperatures of the device. If

more than a 10% deviation from the specified values is found, the bimetal

control unit is repaired or replaced.

73
REFERENCES

1. Harrison, T.R., "Radiation Pyrometry and its Underlying Principles of

Radiant Heat Transfer", John Wiley and Sons, Inc., 1960

2. Doebelin, E.O., "Measurement Systems: Application and Design", McGraw-Hill

Book Co., 1975

3. Wise, JA., "Liquid-in-Glass Thermometry", NBS Monograph 150, National

Bureau of Standards, Jan. 1976

4. Keenan, J.H., "Thermodynamics", The M.I.T. Press, 1970

5. Lion, K.S., "Instrumentation in Scientific Research", McGraw Hill

Book Co., 1 959

6. Riddle, J.L.; Furukawa, G.T.; and Plumb, H.H., "Platinum Resistance

Thermometry", NBS Monograph 126, National Bureau of Standards, April 1973

7. Wood, S.D.; Mangum, B.W.; Filliben, J.J.; andTillett, S.B.,"An

Investigation of the Stability of Thermistors"; Journal of Research of

the National Bureau of Standards, Institute for Basic Standards, NBS,

Washington, D.C. , Vol.83, pp. 247-263, No. 3, May-June 1978

8. Sheingold, D.H., "Transducers Interfacing Handbook", Analog Devices,

Norwood, Massachusetts 1978

9. Powell, R.L.; Hall, W.J.; Hyink, C.H.; Sparks, L.L.; Burns, G.W.;

Scroger, Margaret G.; and Plumb, Harmon H. , "Thermocouple

Reference Tables Based On The IPTS-68", NBS Monograph 125, 1973

10. Feldman, C.L., "Automatic Ice-point Reference Junctions", Instrument

Control Systems, Jan. 196 5

11. "Methods of Testing for Rating Unitary Air Conditioning and Heat Pump

Equipment", ANSI/ASHRAE Standard 37-1978, ASHRAE, 1791 Tullie Circle N.E. Atlanta GA, 1978

12. "Standard Measurement Guide: Section on Temperature Measurements", ASHRAE

Standard 41.1-74, ASHRAE, 1791 Tullie Circle N.E. Atlanta GA, 1974

13. Kao, J.Y., "Low-voltage Room Thermostat Performance", NBS Building

Science Series 150, National Bureau of Standards, 1983

74
APPENDIX A

ADDITIONAL DEFINITIONS OF TERMS USED IN TEMPERATURE MONITORING

75
APPENDIX A. ADDITIONAL DEFINITION OF TERMS USED IN TEMPERATURE

MONITORING

TEMPERATURE FIXED POINT—A particular, precisely reproducible hotness suitable

for calibrating thermometers or serving as a marker on a temperature scale.

Temperature fixed points are commonly achieved by use of transitions occurring

in pure materials, including boiling points, melting or freezing points, and

triple points.

MELTING OR FREEZING POINT—The temperature at which the liquid and solid forms

of a substance are in thermal equilibrium with the vapor of that substance and

with air, usually at one atmosphere of pressure. Sometimes the temperature

achieved during a warming cycle (melting) is different from the temperature

achieved during a cooling cycle (freezing); this difference can arise from the

presence of impurities in the substance. Sometimes also a particular

substance "undercools" or "supercools" considerably before the onset of

freezing; in such a case one might choose to use the melting procedure to

define a fixed point of temperature.

TRIPLE POINT—The temperature at which the vapor of a particular one-component

substance is in thermal equilibrium with both the liquid and the solid forms

of that substance in the absence of air. According to the Gibbs Phase Rule,

only one unique temperature and one unique pressure can exist in such a system.

THERMAL EXPANSION—The property, common to most materials, of becoming larger

in physical dimensions as the temperature increases. All gases undergo

thermal expansion, more or less in accordance with the "Ideal Gas Law",

PV=nRT; thus their rate of expansion is about 1/273 of their room-temperature

volume per °C, if they are maintained at constant pressure. Many liquids

expand with increasing temperature, as, for example, liquid mercury does;

their behaviors are more complex, however, as evidenced by water, which

contracts in volume when its temperature is raised a few degrees above its

melting point, then expands as its temperature rises above about 4 °C. Solids

also show complicated thermal expansion behavior; their rates of expansion in

general are lower than those of liquids or gases, however.

THERMAL CONDUCTIVITY—The ability of a substance to carry heat under the

influence of a thermal gradient. Note that heat always flows from the hotter

part of a system or material to the colder part. Note also that, technically

speaking, it takes an infinite length of time for the two ends of a rod of

thermal conductivity "K" to reach the same temperature, if they started at

unequal temperatures; the heat energy per unit time, dQ/dt, carried from the

hotter end to the colder end can be calculated from the relation (dQ/dt)=K(Tj^-

T2). As 1-^ and T2 become more and more nearly the same, the heat flows less

and less rapidly. The thermal conductivities of different materials vary

greatly, from less than 0.01 W/(cm. °C) for substances such as glass to more

than 10 W/(cm. °C) for pure metals.

76
NBS-114A (REV. 2-ec) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PU BLICATION OR

REPORT NO. NBS BSS 153

2. Performing Organ. Report No. i. Publication Udie

January 1 98^^

4. TITLE AND SUBTITLE

Calibration of Temperature Measurement Systems Installed in Buildings

5. AUTHOR(S)

C. Warren Hurley and James F. Schooley

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

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

Naval Civil Engineering Laboratory

Port Hueneme, CA 93043

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 83-600622

Document describes a computer program; SF-185, PIPS Software Summary, Is attached.

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

bibliograpliy or literature survey, mention it here)

Energy Management Control Systems (EMCS) cannot function properly or efficiently

without accurate temperature measurements since temperature is one of the fundamental measurements of any EMCS. This report was written for the purpose of

describing various methods of on-site calibration of temperature sensing devices

used in EMCS and to review the characteristics of these devices that are directly

related to calibration. The significance of recording the results of each calibration is emphasized and the possible effects of systematic errors in temperature

monitoring systems is discussed. Illustrative examples of the calibration of

temperature monitoring systems are given.

Liquid-in-glass thermometers, pressure thermometers, resistance temperature

detectors (RTD) , thermistors, integrated circuit temperature sensors, thermocouples, and bimetallic thermometers are discussed in detail with respect to

their characteristics related to calibration.

12. KEY WORDS (Six to twelve entries; alpliabetical order; capitalize only proper names; and separate key words by semicolon s) averaging thermocouples; bimetallic thermometers; calibration techniques; integrated

circuit sensors; liquid-in-glass thermometers; pressure thermometers; resistance

temperature detectors (RTD); systematic errors; thermistors; thermocouples; thermopiles

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