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TitleManual on the Use of Thermocouple in Temperature Measurement
TagsThermocouple Temperature Quantity
File Size13.4 MB
Total Pages312
Table of Contents
                            Foreword
Contents
Chapter 1 —Introduction
Chapter 2—Principles of Thermoelectric Thermometry
Chapter 3—Thermocouple Materials
Chapter 4—Typical Thermocouple Designs
Chapter 5—Sheathed, Compacted, Ceramic-Insulated Thermocouples
Chapter 6—Thermocouple Output Measurements
Chapter 7—Reference Junctions
Chapter 8—Calibration of Thermocouples
Chapter 9—Application Considerations
Chapter 10—Reference Tables for Thermocouples
Chapter 11 —Cryogenics
Chapter 12—Temperature Measurement Uncertainty
Chapter 13—Terminology
List of ASTM Standards Pertaining to Thermocouples
The International Temperature Scale of 1990 (ITS-90)
Index
                        
Document Text Contents
Page 2

MANUAL ON THE USE
OF THERMOCOUPLES IN
TEMPERATURE
MEASUREMENT
Fourth Edition

Sponsored by
ASTM Committee E20 on
Temperature Measurement

ASTM Manual Series: MNL 12
Revision of Special Technical Publication
(STP) 470B
ASTM Publication Code No. (PCN):

Page 156

1 3 4 MANUAL O N THE USE OF THERMOCOUPLES IN TEMPERATURE MEASUREMENT

TC •

GAS-TIGHT SE«L

THIN-WALL CCASS HlfiCi, NOT LESS
THAN 13 MM I0.» IN I raOM WALL
OF OEWAR, IMMERSION AT LEAST
120 MM (4 S IN I FROM TOP OF
SLOSM TO TOP OF MERCURY

MERCURY O E P T H , 20 MM (0.7S IN I
TO 25 MM ( I IN I

SLUSH OF SHAVED OR FINELY CRACKED
CLEAR ICE TO BOTTOM OF DC WAR
WHEN BATH IS MADE . KEEP A
MINIMUM OF 25 MM ( I IN I OF ICE
BELOW TUBES.

COPPER LEADS TO MEASURING
INSTRUMENT MAXIMUM DIAMETER
0.8 MMI20CA B a S I

TC - MAXIMUM DIAMETER I 6 MM
(14 CA B a S I FOR IRON- OR NiCKEL-BASE
ALLOYS, OB MM DIAMETER (20GA.BaSI
FOR COPPER AND NOBLE METALS

CORK OR OTHER SUITABLE STOPPER

UPPER SURFACE OF SLUSH

WIRES INSULATED TO SURFACE,
CLEAN UNDER MERCURY

WlOE-MOUTH DEWAR FLASK,
I LITRE ( lOTI OR GREATER
CAPACITY, MAXIMUM OF SIX
TUBES IN I-LITRE FLASK

FIG. 7.2—Recommended ice bath for reference junction.

13]? Moisture-proof insulation should extend beneath the surface of the oil
to the ends of the wires where they may be connected by any means which
ensures a low resistance contact (for example, welding, soldering, crimping,
etc.). If the glass tubes are immersed 200 mm (8 in.) in the ice-water mixture
and the oil extends to within a few millimeters of the ice-water surface, the
immersion error will be negligible. Sutton [11] plots the error as a function
of the wire material, diameter, and immersion depth.

If the ice bath is used improperly, serious errors can result. The largest
error which is Ukely to occur arises due to melting of the ice at the bottom
of a bath until the reference junctions are below the ice level and surrounded
by water alone. This water may be as much as 4°C above the ice point. While
an ice bath is being used excess water should be removed periodically and

^Care should be taken to prevent oil contamination of that part of the thermocouples which
will be exposed to high temperatures.

Page 157

CHAPTER 7 O N REFERENCE JUNCTIONS 1 3 5

more ice added, so that the ice level is maintained safely below the reference
junctions [8,12].

If the ice used to prepare the bath has been stored in a freezer at a tem-
perature below 0°C, it may freeze the surrounding water and remain at a
temperature below 0°C for some time. To avoid the condition, the ice should
be shaved rather than crushed and thoroughly wetted with water before plac-
ing it in the vacuum bottle [lJ-13].

If appreciable concentrations of salts are present in the water used to make
the ice bath, the melting point can be affected. McElroy [14] investigated
various combinations of tap and distilled water to determine the error intro-
duced. He observed bath temperatures of +0.013°C using distilled water
with tap ice and — 0.006°C using tap water and tap ice. It is probable that
the proper use of tap water will not introduce significant errors unless accu-
racies better than 0.0 TC are required [9].

Another possible source of error is galvanic action which is discussed in
Section 7.3.2.

Although the ice bath is an easy way of achieving a convenient fixed point
for the reference junctions in the laboratory, the need for constant attention
makes it unsuitable for industrial application where some form of automatic
reference junction is desirable.

7.2.1.3 Automatic Ice Point—The development of the thermoelectric
refrigerator (Peltier cooler) has enabled the production of practical devices
in which an equilibrium between ice and water is constantly maintained
[15,16].

This device can provide a reference medium which is maintained at a pre-
cise temperature, but careful design is necessary if this precision is to be uti-
lized fully. The system is subject to immersion error (Section 7.3.1) and gal-
vanic error (7.3.2) due to condensation.

Some coinmercially available devices provide wells into which the user
may insert reference junctions formed from calibrated wire. Others are pro-
vided with many reference junction pairs brought out to terminals which the
user may connect into his system. The latter type are subject to the wire
matching error (Section 7.3.3).

If the potential errors have been successfully overcome, the error intro-
duced into a system by these devices can be less than 0.1 °C.

Automatic ice points can be used to further advantage in conjunction
with a "zone box" (see 7.2.2.1).

7.2.1.4 Constant Temperature Ovens—A thermostatically controlled
oven provides a way of holding a reference junction at an approximately
constant temperature. The major advantage of this method is that one oven
can be used with a large number of circuits while maintaining isolation
between the circuits without the need for providing separate power supphes
for each circuit.

Page 311

290 MANUAL ON THE USE OF THERMOCOUPLES IN TEMPERATURE MEASUREMENT

temperature measurement {see
Temperature measurement
uncertainty)

Uncertainty envelope method, 160,
162-163

Ungrounded junction, 121
Unsheathed thermocouples, accuracy,

238t

Validate, 256
Variance, 235-236
Verify, 256
Virtual junction, 15, 256
Voltage, 256

references, 129-180

W

Water, triple point, 133, 256, 269-
270

Wire {see also Extension wires)
annealing, 144-145
cryogenics, 238-240
homogeneity, calibration, 146-147
matching error, reference junctions,

139
sheathed, compacted, ceramic-

insulated thermocouples, 112

Zinc, freezing point, 271
Zone box, reference junctions, 137
Zone plate, 257

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