Guide to Measurement and Instrumentation -- Varying Resistance Devices

Varying resistance devices rely on the physical principle of the variation of resistance with temperature. The devices are known as either resistance thermometers or thermistors according to whether the material used for their construction is a metal or a semiconductor, and both are common measuring devices. The normal method of measuring resistance is to use a DC bridge. The excitation voltage of the bridge has to be chosen very carefully the self-heating effect of high currents flowing in the temperature transducer creates an error by increasing the temperature of the device and so changing the resistance value.

Resistance Thermometers (Resistance Temperature Devices)

Resistance thermometers, which are alternatively known as resistance temperature devices, 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 nonlinear and so is inconvenient for measurement purposes. The equation becomes linear if all the terms in a2T2 and higher powers of T are negligible such that the resistance and temperature are related according to R _ R0 1 + a1T

This equation is approximately true over a limited temperature range for some metals, notably platinum, copper, and nickel, whose characteristics are summarized in Fgr. 8.

Platinum has the most linear resistance/temperature characteristic and also has good chemical inertness. It’s therefore far more common than copper or nickel thermocouples.

Its resistance-temperature relationship is linear within _0.4% over the temperature range between _200 and +40_ C. Even at +1000_ C, the quoted inaccuracy figure is only _1.2%. Platinum thermometers are made in three forms, as a film deposited on a ceramic substrate, as a coil mounted inside a glass or ceramic probe, or as a coil wound on a mandrel, although the last of these are now becoming rare. The nominal resistance at 0_ C is typically 100 or 1000 ohm, although 200 and 500 ohm versions also exist. Sensitivity is:

0.385 ohm/ _ C (100 ohm type) or 3.85 ohm/_ C (1000 ohm type).

A high nominal resistance is advantageous in terms of higher measurement sensitivity, and the resistance of connecting leads has less effect on measurement accuracy. However, cost goes up as the nominal resistance increases.

In addition to having a less linear characteristic, both nickel and copper are inferior to platinum in terms of their greater susceptibility to oxidation and corrosion. This seriously limits their accuracy and longevity. However, because platinum is very expensive compared to nickel and copper, the latter are used in resistance thermometers when cost is important. Another metal, tungsten, is also used in resistance thermometers in some circumstances, particularly for high temperature measurements. The working ranges of each of these four types of resistance thermometers are as shown here:

Platinum: _270 to +1000_ C (although use above 650_ C is uncommon), Copper: _200 to +260_ C, Nickel: _200 to +430_ C ,Tungsten: _270 to +1100_ C

In the case of noncorrosive and nonconducting 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 reduces 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. Moisture buildup within the sheath can also impair measurement accuracy.

The frequency at which a resistance thermometer should be calibrated depends on the material it’s made from and on the operating environment. Practical experimentation is therefore needed to determine the necessary frequency and this must be reviewed if the operating conditions change.

Thermistors

Thermistors are manufactured from beads of semiconductor material prepared from oxides of the iron group of metals such as chromium, cobalt, iron, manganese, and nickel. Normally, thermistors have a negative temperature coefficient, that is, resistance decreases as temperature increases, according to:

This relationship is illustrated. However, alternative forms of heavily doped thermistors are now available (at greater cost) that have a positive temperature coefficient.

The form of Equation (8) is such that it’s not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistor is very definitely a nonlinear sensor. However, the major advantages of thermistors are their relatively low cost and their small size. This size advantage means that the time constant of thermistors operated in sheaths is small, although the size reduction also decreases its heat dissipation capability and so makes the self-heating effect greater. In consequence, thermistors have to be operated at generally lower current levels than resistance thermometers and so the measurement sensitivity is less.

As in the case of resistance thermometers, some practical experimentation is needed to determine the necessary frequency at which a thermistor should be calibrated and this must be reviewed if the operating conditions change.

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