Page 274 - Instrumentation Reference Book 3E

258 Temperature measurement

silver or platinum of a diameter much larger than wound element of the conventional resistance
the wire of the resistance element, are welded to thermometer by a metallized film track laid down
the fine platinum wire wholly inside the glass seal. on a glass or ceramic substrate. These thermometer
The inductance and capacitance of elements are elements are made by similar techniques to those
made as low as possible in order to allow their use used for making hybrid integrated electronic
with a.c. measuring instruments. Typically the circuits. After the laying down of the metallized
elements shown will have self-inductance of 2 pH film the film is trimmed by a laser to achieve the
per 100 fl and the element self-capacitance will not required parameters. These metal film devices can
exceed 5 pF. The current passed through a resist- be very robust and can be manufactured to a high
ance thermometer to measure the resistance must degree of accuracy.
be limited to minimize errors by self-heating of the
resistance element. Typical maximum acceptable
current is lOmA €or a 100 0 thermometer. 14.4.1.2 Nickel resistance thermometers
The rate of response of a resistance thermometer Nickel forms an inexpensive alternative to plati-
is a function of its construction and encapsulation. num for resistance thermometers. The usable
A heavy industrial type may have a response of one range is restricted to -200°C to +350"C. But
or two minutes when plunged into water while a the temperature coefficient of resistivity of nickel
naked type, like that shown in Figure 14.17(e), will is 50 percent higher than that of platinum which
be only a few milliseconds under the same condi- is an advantage in some instruments. Nickel resist-
tions. Figure 14.17(f) shows the cross-section of a ance thermometers find wide use in water-heating
resistance thermometer encapsulated in a metal and air-conditioning systems.
tube. Figure 14.18 shows a range of typical indus- As mentioned above, the current through a
trial resistance thermometers. resistance thermometer sensor must be kept low
A more recent development of resistance therm- enough to limit self-heating. However, in some
ometers has been the replacement of the wire- applications, such as flowmeters, anemometers,