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Thermocouple signal simulation
Application Questions and Problems topic
Posted by Linda Sellers on 13 May, 2002 - 1:27 pm
Be nice, I'm not an engineer!

I've been tasked with building a test panel for some new compressor control PLC's which we are installing at our plant.

I was told to wire up a pot and meter (0-100%) for simulation of the thermocouple signals. I'm not sure exactly what I need to purchase and how to hook it up. Can one of you explain, in fairly simple terms?

Thanks in advance,
Linda


Posted by jeff on 13 May, 2002 - 3:25 pm
Assuming that you are simulating T/C signals into a PLC thermocouple input module, or into a thermocouple input temperature transmitter, your best bet would be to purchase/rent or otherwise acquire a proper thermocouple signal simulator. Thermocouple-level signals are millivolt-signals, not easily or accurately controlled with a DC power supply and a pot. A T/C signal simulator will allow you to dial in a precise temperature you want to simulate,, for a given type of thermocouple, and will send a signal in millivolts precisely representing that temp. to your input (PLC or transmitter). You can then check the response of the PLC logic, transmitter output or whatever over the entire operating range very quickly. Check out "www.transcat.com/catalog":http://www.transcat.com/catalog and look under the "calibrator" section...or web-search thermocouple calibrators...there are a lot of choices from inexpensive to $$$.


Posted by Rich W. on 13 May, 2002 - 3:28 pm
Linda,

To simulate a thermocouple input, you need to simulate a DC millivoltage. You could do this very simply by connecting a battery (a 1.5 volt cell for instance) across the two outer terminals of a pot (10k for example)and then taking your millivolt "signal" from the wiper and the neagtive battery terminal. The value of the pot and the battery voltage are inconsequential, provided you get the millivolt output you desire and you don't kill your battery with too low of a resistance in the mean time. Put a good millivolt meter or digital meter with a high input impedance across the signal while the load is connected (to the imput card) to see what it is you are putting out to the card. If you need to know how the voltage correlates to temperature, you would have to consult a table or use a conversion factor for the particular themrocouple type.
Rich W.


Posted by RVissing on 13 May, 2002 - 4:44 pm
The millivolt signal from a thermocouple is non-linear so rigging your own can get confusing. Fluke makes a basic handheld T/C signal generator that already has the M/V curves pre-programmed for various T/C types.

"www.fluke.com/products/home.asp?SID=13&AGID=9&PID=170 7":http://www.fluke.com/products/home.asp?SID=13&AGID=9&PID= 1707


Posted by Rick Daniel on 14 May, 2002 - 10:10 am
Linda,

You can do a reasonable simulation with a 0-50mv dc signal -- that's the range most thermocouples operate in. If you want to get fancier, you can buy reasonably low cost thermocouple simulators from Omega Engineering.

Rick Daniel


Posted by Hakan Ozevin on 15 May, 2002 - 10:45 am
A TC produces DV voltage with temperature change.
There are some devices called "calibrator"s. If you want to simulate a TC signal acuurately, you have to use them, since TC signals are non-linear (i.e. a 10% change in the DC voltage does not mean a 10% change in the temperature).

But, if you want to look at a few points, you can use mV voltage source and use tables where u can see what mV means what temperature.

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Fortune
Please, won't somebody tell me what diddie-wa-diddie means?
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