Resistance-Temperature Detectors

Overview

A resistance-temperature detector (RTD) is a temperature sensor that is based on the principle of metal resistance increasing with temperature. Metals used in these devices vary from platinum, which is very repeatable, quite sensitive, and very expensive, to nickel, which is not quite as repeatable, more sensitive, and less expensive.

Sensitivity

An estimate of RTD sensitivity can be noted from typical values of a_o, the linear fractional change in resistance with temperature. For platinum, this number is typically on the order of 0.004/°C, and for nickel a typical value is 0.005/°C. Thus, with platinum, for example, a change of only 0.4 W would be expected for a 100-W RTD if the temperature is changed by 1°C. Usually, a specification will provide calibration information either as a graph of resistance versus temperature or as a table of values from which the sensitivity can be determined. For the same materials, however, this number is relatively constant because it is a function of resistivity.

Response Time

In general, RTD has a response time of 0.5 to 5 seconds or more. The slowness of response is due principally to the slowness of thermal conductivity in bringing the device into thermal equilibrium with its environment. Generally, time constants are specified either for a "free air" condition (or its equivalent) or an "oil bath" condition (or its equivalent). In the former case, there is poor thermal contact and hence slow response, and in the latter, good thermal contact and fast response. These numbers yield a range of response times depending on the application.

Construction

An RTD, of course, is simply a length of wire whose resistance is to be monitored as a function of temperature. The construction is typically such that the wire is wound on a form (in a coil) to achieve small size and improve thermal conductivity to decrease response time. In many cases, the coil is protected from the environment by a sheath or protective tube that inevitably increases response time but may be necessary in hostile environments. A loosely applied standard sets the resistance at multiples of 100 W for a temperature of 0°C.

Signal Conditioning

In view of the very small fractional changes of resistance with temperature (0.4%), the RTD is generally used in a bridge circuit. Figure 4.4 illustrates the essential features of such a system. The compensation line in the R₃ leg of the bridge is required when the lead lengths are so long that thermal gradients along the RTD leg may cause changes in line resistance. These changes show up as false information, suggesting changes in RTD resistance By using the compensation line, the same resistance changes also appear on the R₃ side of the bridge and cause no net shift in the bridge null.

Because the RTD is a resistance, there is an I²R power dissipated by the device itself that causes a slight heating effect, a self-heating This may also cause an erroneous reading or even upset the environment in delicate measurement conditions. Thus, the current through the RTD must be kept sufficiently low and constant to avoid self-heating. Typically, a dissipation constant is provided in RTD specifications. This number relates the power required to raise the RTD temperature by one degree of temperature. Thus, a 25-mW/°C dissipation constant shows that if I²R power losses in the RTD equal 25 mW, then the RTD will be heated by 1°C.

The dissipation constant is usually specified under two conditions free air and a well-stirred oil bath. This is because of the difference in capacity of the medium to carry heat away from the device. The self-heating temperature rise can be found from the power dissipated by the RTD and the dissipation constant from where DT = temperature rise because of self-heating in °C
P = power dissipated in the RTD from the circuit in W
P_D = dissipation constant of the RTD in W/°C

EXAMPLE 4.7
An RTD has a _o = 0.005/°C, R = 500 W, and a dissipation constant of P_D = 30 mW/ °C at 20°C. The RTD is used in a bridge circuit such as that in Figure 4.4 with R₁= R₂ = 500W and R₃ a variable resistor used to null the bridge. If the supply is 10 volts and the RTD is placed in a bath at 0°C, find the value of R₃ to null the bridge.

Solution
First we find the value of the RTD resistance at 0°C without including the effects of dissipation. From Equation (4.9) we get


Except for the effects of self-heating, we would expect the bridge to null with R₃ equal to 45 W also. Let's see what self-heating does to this problem. First, we find the power dissipated in the RTD from the circuit assuming the resistance is still 450 W. The power is


and the current I to three significant figures is found from so that the power is

We get the temperature rise from Equation (4.13) Thus, the RTD is not actually at the bath temperature of 0°C but at a temperature of 1.8°C. We must find the RTD resistance from Equation (4.9) as


Thus, the bridge will null with R₃ = 454.5 W.

Range

The effective range of RTDs depends principally on the type of wire used as the active element. Thus, a typical platinum RTD may have a range of -100°C to 650°C, whereas an RTD constructed from nickel might typically have a specified range of -180°C to 300°C.

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Measuring Temperature with RTDs -- A Tutorial
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