Thermocouple With Conventional Connector
by Analog Services, Inc.
Conventional wisdom says that you can't bring thermocouple wiring
through a bulkhead or panel without using a feedthrough connector made with thermocouple
materials. But these connectors can be large, ugly, expensive, hard to find, or all
of the above. If, in addition, the cable is to be disposable it becomes important
that the connector halves be common and cheap. In some cases you may be able to keep
the temperature gradient across a conventional connector low enough that errors are
acceptable. But generally, the inside of an equipment cabinet or box may be 5
to 15 degrees higher than the temperature outside of it, which can lead to significant
temperature measurement error.
If you can place a second, dummy connector (with no external cable) near the first one, it is possible to cancel out the errors. In effect the second connector is used to measure the temperature difference across the first connector and cancel it. The beauty of the technique is that it is entirely passive. No active circuitry is involved in measuring temperatures in the dummy connector. The effectiveness of the method depends only on how closely you can make the thermal conditions of the two connectors alike. Even if you can't get exact duplication of the thermal gradients across the two connectors you may be able to keep the temperature measurement error within acceptable limits.
The method is illustrated in the figure. P1 and J1 are the plug and jack of the active connector. P2 and J2 form the dummy connector. Both connectors are 3-position with one pin removed (or not installed). In the dummy connector a thermocouple cable feeds through the hole of the missing pin. Once the thermocouple cable is installed inside the dummy connector, the connector halves are normally cemented together so that P2 cannot be removed. M1 and M2 are the two thermocouple materials (M1 = Chromel, M2 = Alumel for example). The four connector pins are identical, conventional gold-plated pins.
Figure -- Connector and Thermocouple Circuit Arrangement
There are a total of 7 thermocouple junctions. The voltage presented to the thermocouple conditioning circuit is the sum of all of these voltages or V1 + V2 + ... + V7. Assume that the junctions producing V2, V3, and V6 are all at the same temperature. Then, by the laws of thermocouple behavior, V6 + V2 + V3 = 0. Similarly, if the junctions producing V4, V5, and V7 are all at the same temperature, then V7 + V4 + V5 = 0. Then the voltage presented to the conditioning circuit reduces to just V1.
Best results are obtained when heat conduction through the thermocouple wires is minimized. This can be accomplished by using fine (small diameter) thermocouple wire; and by making a small coil of the thermocouple wire inside of P2 so that the thermocouple wires generating V6 are long.
In tests of this technique, the "inside" environment was created by using a temperature chamber that could be adjusted from 0 C to 50 C. The "outside" was ambient room temperature. The resulting measurement errors were typically less than 0.5 degree C.
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