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Figure 153 J thermocouple circuit
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Figure 154 Cold-junction compensated thermocouple circuit
The use of a thermocouple requires special connections, because the junction of the thermocouple wires with other leads (such as voltmeter leads, for example) creates additional thermoelectric junctions that in effect act as additional thermocouples For example, in the J thermocouple circuit of Figure 153, junction J1 is exposed to the temperature to be measured, but junctions J2 and J3 also generate a thermoelectric voltage, which is dependent on the temperature at these junctions, that is, the temperature at the voltmeter connections One would therefore have to know the voltages at these junctions, as well, in order to determine the actual thermoelectric voltage at J1 To obviate this problem, a reference junction at known temperature can be employed; a traditional approach involves the use of a cold junction, so called because it often consists of an ice bath, one of the easiest means of obtaining a known reference temperature Figure 154 depicts a thermocouple measurement using an ice bath The voltage measured in Figure 154 is dependent on the temperature difference T1 Tref , where Tref = 0 C The connections to the voltmeter are made at an isothermal block, kept at a constant temperature; note that the same metal is used in both of the connections to the isothermal block Thus (still assuming a J thermocouple), there is no difference between the thermoelectric voltages at the two copper-iron junctions; these will add to zero at the voltmeter The voltmeter will therefore read a voltage proportional to T1 Tref An ice bath is not always a practical solution Other cold junction temperature compensation techniques employ an additional temperature sensor to determine the actual temperature of the junctions J2 and J3 of Figure 153
Resistance Temperature Detectors (RTDs)
A resistance temperature detector (RTD) is a variable-resistance device whose resistance is a function of temperature RTDs can be made with both positive and negative temperature coef cients and offer greater accuracy and stability than thermocouples Thermistors are part of the RTD family A characteristic of all RTDs is that they are passive devices, that is, they do not provide a useful output unless excited by an external source The change in resistance in an RTD is usually converted to a change in voltage by forcing a current to ow through the device An indirect result of this method is a self-heating error, caused by the i 2 R heating of the device Self-heating of an RTD is usually denoted by the amount of power that will raise the RTD temperature by 1 C Reducing the excitation current can clearly help reduce self-heating, but it also reduces the output voltage The RTD resistance has a fairly linear dependence on temperature; a common de nition of the temperature coef cient of an RTD is related to the change in
Part II
Electronics
resistance from 0 to 100 C Let R0 be the resistance of the device at 0 C and R100 the resistance at 100 C Then the temperature coef cient, , is de ned to be = R100 R0 100 0 C (152)
+ Vo
Iex rL RT rL
A more accurate representation of RTD temperature dependence can be obtained by using a nonlinear (cubic) equation and published tables of coef cients As an example, a platinum RTD could be described either by the temperature coef cient = 0003911, or by the equation RT = R0 (1 + AT BT CT )
= R0 (1 + 36962 10 3 T 58495 10 7 T 2 42325 10
(153)
Figure 155 Effect of connection leads on RTD temperature measurement
where the coef cient C is equal to zero for temperatures above 0 C Because RTDs have fairly low resistance, they are sensitive to error introduced by the added resistance of the lead wires connected to them; Figure 155 depicts the effect of the lead resistances, rL , on the RTD measurement Note that the measured voltage includes the resistance of the RTD as well as the resistance of the leads If the leads used are long (greater than 3 m is a good rule of thumb), then the measurement will have to be adjusted for this error Two possible solutions to the lead problems are the four-wire RTD measurement circuit and the three-wire Wheatstone bridge circuit, shown in Figure 156(a) and (b), respectively In the circuit of Figure 156(a), the resistance of the lead wires from the excitation, rL1 and rL4 , may be arbitrarily large, since the measurement is affected by the resistance of only the output lead wires, rL2 and rL3 , which can be usually kept small by making these leads short The circuit of Figure 156(b) takes advantage of the properties of the Wheatstone bridge to cancel out the unwanted effect of the lead wires while still producing an output dependent on the change in temperature
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