Differential Temperature ( T) Measurement with Thermocouples
Thermocouples are often used for differential temperature ( T) measurement in industrial processes where the differential is of a span sufficient to generate the required signal for the associated instrumentation. Thermocouples provide a possible advantage over other sensor types where small size and ease of interchangeability are important.
The output of a given span is not linear for the common thermocouple calibrations. See Table
1. When the process requires temperatures, the non-linearity
T measurement over only a restricted range of working may not be significant. The combined limits of error in
degrees for two thermocouples in
T measurement is equal to the square root of the sum
of the squares of the individual thermocouple limits of error. For J and K calibrations, and standard limits of error for thermocouples below 530F, the combined thermocouple error is _+%(4)2+(4)2 = 5.640F. See Table 2. Intermediate connections should be minimized in the AT thermocouple circuit to lessen the introduction of parasitic EMF’s and the reduction of accuracy that can result. One, or both, of the thermocouples should be ungrounded.
Schematic Wiring for T with Thermocouples
When two thermocouples are connected for
T measurement either the positive or negative
legs of the thermocouples are connected together, with the remaining legs connected to the instrument. There is a reversal in polarity of the differential temperature signal when the thermocouple junctions change relationship regarding the “hot’ and “cold” designations. If a zero-centered span is not available on the associated instrument, then provisions must be made to reverse polarity at the instrument. Reference junction compensation is not utilized
for the individual thermocouples when they are connected for
A AT measurement is not absolute and, as such, will not give an indication of undesirable over-temperature condition in a process. For safety considerations, an absolute temperature measurement and/or indication may be necessary when measuring a AT condition. In any temperature measurement, attention should be provided to selection of sensors sufficiently rugged for the environment of the process, location of the sensors, and installation details.
One theromocouple grounded
Thermocouple Wire: A (Positive) B (Negative)
Table 1. Output (mV) for a 10F Span at Various Temperatures for Common Calibrations
Calibration J K T E
Table 2. Output (mV) for a 30F Span at Various Temperatures for Common Calibrations
Calibration J K T E
REFERENCE R.B. Crawford, “Industrial Applications of Temperature Difference Measurements~ Temperature, Its Measurement and Control in Science and Industry” (Reinhold Publishing Corporation, New York, 1962. Volume 3, Part 2, PP. 91 3-925.
Barber-Colman Company, Loves Park, IL.