Compensating Cables

Average Temperature Measurement with a Thermocouple

1.How to Measure Average Temperature with a Thermocouple

As shown in the circuit below, insert a resistance for calibration, R = approx. 500 Ω, to the circuit at one side depending on the length of the thermocouple and the compensating cables. Actually, it is placed in a J.BOX in many cases.

How to Measure Average Temperature with a Thermocouple Image

For three points or more also, measuring is available with the same circuit.

2.Errors on Average Temperature Measurement

(1) The average temperature measurement circuit is replaced as shown below.

How to Measure Average Temperature with a Thermocouple Image

R1, r2 Line resistance including thermocouple and compensation cables
R1', R2': Inserted resistance
E1, E2: Thermo-electromotive force of a thermocouple

(2) Circuit Calculation

Circuit Calculation Image

When the error is calculated with the formula above,

Circuit Calculation Image

Therefore, since when R1 = R2 is satisfied, the error becomes zero,

R1 = r1 + R1' 、 R2 = r2 + R2'

With this relation, when sufficiently large R1’ and R2’ are inserted for r1 - r2, the difference of line resistance, the error of the average temperature measurement circuit can be decreased. Also, since the allowable line resistance is limited depending on the type of receiver, the insertion resistance value must be determined within the allowance.

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