Thermocouple Inputs for Control

Please check these T/C input cards are Multiplexing cards or not. I think these cards are multiplexing cards & so does not continuously tracks the field values. In case mV from thermocouples converted to 4-20 ma & taken as analog input, system can get continuous values & so can be used for more reliable control.

Hi there,

As stated mA signals are not as succint to interference as mV signals. Another factor is the cost of the compensating wire. It is far more expensive than normal instrument cable.

Another factor to take into account is the the two junctions (hot and cold) of the thermocouple - preferabely you would to try and minimilize the distance between the two as you can induce more error the longer they are.

Regards,

Craig

Another aspect you should consider becomes apparent when you consider what makes a thermocouple. A thermocouple happens when any two dissimilar metals touch. The small voltage created at this junction, tracks very linearly with temperature. Naturally different metal combinations produce different voltages for a given temperature. So wire A and wire B are fused together to make a thermocouple. Now wire A and wire B must get terminated a another terminal block made of another metal C. Now instead of one thermocouple made of an AB junction, you have three thermocouples in series when you include the AC and BC junctions. So, metal C gets chosen to produce the tiniest of thermocouple voltages with both metals A and B. Once a common metal carries this signal, a quick examination of transitions to other common metals shows pairs of thermocouple voltages that cancel each other, preserving the original voltage generated by the AB junction.

1. Design engineer's personal preference.
2. Lower installation costs. (depending on length of signal cable & Z/I Device used)
3. Less chance of induced noise in the measured signal. (the longer the cable/wiring length, the higher the chance of noise impingement on the mV signal)
4. Better resolution and manipulation of the measured signal for higher control accuracy.

All this being said, I have worked on several systems wherein the TC's report directly to TC input cards and they work very well if the wiring is routed correctly, is properly protected, and is terminated correctly. (properly sized, properly shielded, one-end, single point cable drain attachments to ground)