Page 273 - Instrumentation Reference Book 3E

Measurement techniques: electrical 257

for a resistance thermometer material as resist- Rt = R,[1 +At + Bt? + C(t - 100) . t3]
ance values can be determined with a high degree (14.13)
of accuracy using suitable equipment and taking
adequate precautions. and for the range 0 "C to 850 "C:
Platinum, having the highest possible coefficient
of resistivity, is considered the best material for the Rt = Ro(l + At + Bt') (14.14)
construction of thermometers. A high value of this where A = 3.908 02 x 'e-', E = -5.802~
coefficient is an indication that the platinum is of lo-' 'Cp2, and C = -4.273 50 x 10-1'"C-4'.
high purity. The presence of impurities in resist- The temperature coefficient is given by:
ance thermometer material is undesirable, as diffu-
sion, segregation, and evaporation may occur in
service, resulting in a lack of stability of the thermo-
meter. The temperature coefficient of resistivity
is also sensitive to internal strains so that it is As indicated by equations (14.14) and (14.151, the
essential that the platinum should be annealed at value of a is not constant over the temperature
a temperature higher than the maximum tempera- range. Figure 14.16 shows the tolerances. in ohms
ture of service. The combination of purity and and degrees Celsius, over the specified tempera-
adequate annealing is shown by a high value of ture range. (Figure 14.16 is based on Figure 2 of
the ratio of the resistances at the steam and ice BS 104: 1984 and is reproduced with the permis-
points, To comply with the requirements of the sion of the BSI. Complete copies of the standard
International Practical Temperature Scale of may be obtained by post from BSI Sales, Linford
1968 this ratio must exceed 1.39250. Wood, Milton Keynes, MK14 6LE, UK.) For
It is essential that the platinum element is industrial applications class B thermometer sensors
mounted in such a way that it is not subject to are normally used. Class A sensors are available for
stress in service. greater precision. but they are specified over a
Platinum is used for resistance thermometry in more restricted temperature range. Although the
industry fix temperatures up to 800 "C. It does not standard specifies the temperature range down to
oxidize, but must be protected from contamin- -200 "C (73 K). class A sensors may be used for
ation. The commonest cause of contamination of temperatures down to 15 K. Resistance therm-
platinum resistance thermometers is contact with ometers are calibrated to IPTS-68 which is
silica, or silica-bearing refractories, in a reducing normally done by comparison with a standardized
atmosphere. In the presence of a reducing atmos- resistance thermometer. For industrial use, most
phere, silica is reduced to silicon, which alloys with thermometers are made to be 100n at 0"C,
platinum, making it brittle. Platinum resistance but 10 R thermometers are manufactured where
thermometers may be used for temperatures down particularly robust sensors are required. A wide
to about 20 K. range of sensor designs is available, the form used
For measuring temperatures between 1 K and depending upon the duty and the speed of
40 K doped germanium sensors are usually used, response required. Some typical forms of con-
while carbon resistors are used between 0.1 K and struction are illustrated in Figure 14.17. Figure
20 K. About 20 K platinum has a greater tem- 14.17(a) shows a high-temperature form in which
perature coefficient of resistivity and has a greater the spiral platinum coil is bonded at one edge of
stability. Between 0.35 K and 40 K a resistance each turn with high-temperature glass inside
thermometer material (0.5 atomic % iron-rhodium) cylindrical holes in a ceramic rod. In the high
is also used. accuracy type, used mainly for laboratory work.
the coil is not secured at each turn but is left free
to ensure a completely strain-free mounting,
Tenzyeratirrelresist~~~ice relationship of resistance Figure 14.17(b). Where a robust form. suitable
thermometers BS 1904: 1984 and its internation- for use in aircraft and missiles or any severe
ally harmonized equivalent standard IEC 75 1 : vibration condition is required, the ceramic is in
1983 specify the resistance versus temperature solid rod form and the bifilar wound platinum
characteristics of industrial platinum resistance coil is sealed to the rod by a glass coating as
thermometers. The standard provides tables of shown in Figurel4.17(c). Where the sensor is
resistance against temperature for 100 R resist- intended for use for measuring surface tempera-
ance thermometers over the temperature range tures. the form shown in Figure 14.171d) is used.
-200°C to 850°C. Two grades of thermometer In all forms, the ceramic formers are virtually
are specified. The equations are provided from silica-free and the resistance element is sealed in
which the tables are derived. with high temperature glass to form an imper-
Between -200 "C and 0 "C the resistance of the vious sheath which is unaffected by most gases
thermometer, R,, is given by: and hydrocarbons. The external leads, which are