Monday, August 30, 2010

Thermocouple Application Note

Unusual Thermocouple Application

Our customer's client had a large spa facility. The controls had been located near the spa, but the client felt they should be moved to a more benign environment, several hundred feet away. The system included four thermocouples. Sending thermocouple signals this distance is expensive, since thermocouple wire (not copper) must be used. Even more basic, sending weak millivolt signals hundreds of feet leaves them subject to interference and signal degradation.

To boost the signals, four standard thermocouple transmitters (our Model JH4130) were added near the spa. Their 4-20mA current signals easily traveled the distance with no degradation. The system designers opted not to change the control system, so it was necessary to convert the current signals back to their original thermocouple voltages. "Reverse" thermocouple transmitters, 4-20mA input, thermocouple output, were needed.

We provided them. We modified our Model JH4300 DC input, DC output transmitters to have the proper millivolt outputs. The most interesting challenge was, the millivolt outputs needed to be connected to thermocouple wires (not copper) for proper connection to the system's thermocouple inputs. This called for the reverse of cold junction compensation (we called it "cold junction un-compensation"). A temperature sensor embedded in the output terminals and some properly-designed circuitry solved the problem

Temperature Transmitters & Signal Conditioners - our web site:

1 comment:

  1. Ooooohhhh.... a spa?? I'd be willing to volunteer my services to go check out their services & make sure your electrical doohickies are properly working :)