Resistance Thermometers
About resistance thermometers
Resistance thermometers, which are
alternatively known as resistance-temperature devices (or RTDs), rely
on the principle that the resistance of a metal varies with
temperature according to the relationship
R = R0(1+a1T+a2T2+a3T3+
. . . +anTn)
This equation is non-linear and so is inconvenient for measurement
purposes. The equation becomes linear if all the terms in a2T2
and higher powers of T are negligible. This is approximately true for
some metals over a limited temperature range and, in such cases, the
resistance and temperature are related according to:
R ≈ R0(1+a1T)
Platinum is one such metal where the resistance-temperature
relationship is linear within ±0.4% over the temperature range between
-200°C and +40°C. Even at +1000°C, the quoted accuracy is ±1.2%.
Its good linearity and chemical inertness make platinum the first
choice for resistance thermometers in many applications. Platinum is
very expensive, however, and consequently the cheaper but less
accurate alternatives of nickel and copper are sometimes used. These
two metals are very susceptible to oxidation and corrosion and so the
range of applications where they can be used is strictly limited even
if their reduced accuracy is acceptable. Another metal, tungsten, is
also used in some circumstances, particularly for high-temperature
measurements. The working ranges of each of these four types of
resistance thermometer are as shown below:
|
Platinum: |
-270°C to + 1000°C (though use above
650°C is uncommon) |
|
Copper: |
-200°C to +260°C |
|
Nickel: |
-200°C to +430°C |
|
Tungsten: |
-270°C to +1100°C |
In the case of non-corrosive and non-conducting environments,
resistance thermometers are used without protection. In all other
applications, they are protected inside a sheath. As in the case of
thermocouples, such protection affects the speed of response of the
system to rapid changes in temperature. A typical time constant for a
sheathed platinum resistance thermometer is 0.4 seconds.
The resistance thermometer exists physically as a coil of resistance
wire of the chosen metal. Different instruments available have
resistance elements ranging from 10Ω right up to 25Ω. The devices with
high resistance have several operational advantages. The first of
these is that any connection resistances within the circuit become
negligible in their effect. The second advantage is that the
relatively high voltage output produced by these instruments makes any
induced e. m. f. s produced by thermoelectric behavior at the junction
with connection leads negligible in magnitude.
More
Temperature Measurements
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