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Figure 5. Typical Thermocouple
Measurement
The voltage measured by the voltmeter is proponional to the difference be-
tween temperature T, and T2. To determine T,, T2 the thermoelectric proper-
ties of the thermocouple are needed. Data is available to determine the voltage
versus temperature relationship based on a reference temperature IT*) of 0°C.
Thus, if the thermocouple-to-copper junctions were maintained at O°C it
would be possible to determine T, directly by referring to thermocouple
reference tables*, which give temperature as a function of the meter reading
V,. If these junctions are different from 0°C a voltage E2 will be introduced,
where V, = E,-E,.
2.6 MEASUREMENT
PROCEDURE
In the Model 7057A the reference temperatwre T is measured by a dual ther-
mistor in a bridge circuit. The temperature of a thermocouple connected to
Channels 2 through 10 is determined by the following procedure:
1. Measure the voltage for the reference junction V and calculate Tz. T2 =
30-w,
x 1031.
2. Determine the reference junction voltage, E2, either from thermocouple
reference tables' or by the calculation**:
5 = a0 + A, T + a,p + aaT + a,,+ where T = T2 as determined in step 1
above. Q is expressed in microvolts. The constant a0 through a4 are a
function of the thermocouple type connected to the channel. The coeff-
CiWntS,
aO, a,, a?, a3 and a4 are dependent on the thermocouple type and
temperature range under consideration.
3. Measure the thermocouple voltage V, at the channel output. Convert to
microvolts.
4. Add b to V, to determine E,. E, =V, +E, (where V, is expressed in
microvoltsl.
5. Determine the thermocouple temperature, T,, either from thermocouple
reference tables* or by calculation**:
TI = a, + %EI. + %E12r + e,E? + %EI+ (where E, is expressed in microvoltsl.
6. For example, consider a type J iron-constantan thermocouple at 300°C
(T,I. The reference junction temperature will be 6 to 10°C above ambient,
say 36% IT2).
A. The bridge output ichannel II will be -.006V. Since the bridge output is
zero when T, = 30°C this indicates T2 = 36OC.
B. Using NBS Monograph 125 to determine the reference voltage for a type
J thermocouple gives b 4 1649,O@V. Using the most accurate quark
approximation formula (Table 21 for this range gives lB49.085@,
C. The voltage at Channel 1, which is V,, will be 14476,OpV. IV, = E, -b).
D. Add 6 to V, and get E, = 16325.0@,
6
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