Dumb question - temperature control


Thread Starter


This should probably seem like a dumb question. How do I simulate a thermocouple input to a temperature controller? I've got to verify a panel with a thermocouple turning on the output of a PID controller. That output in turn makes a PLC turn on a burner output. It's a cheap entry level PLC not capable of PID, and they want the ability to modify the setpoint, so wiring a cheap PID controller to the PLC was the easiest solution. Now we have to test the panel and I don't have access to their burner or tank.


Eric M. Klintworth

The "proper" way is to use a thermocouple calibrator. I found one quickly on the Transmation web site for $560, which is probably a typical price. The _cheap_ way is to get a short length of thermocouple wire, connect it to the PID controller, and twist the bare ends together (thus making a thermocouple). Then get a cup of ice water, a pot of boiling water, a cigarette lighter, propane torch, dry ice, or your favorite choice of hot or cold, and you are ready to test. Not precise (except the freezing and boiling water), but serviceable. Hope that helps, Eric M. Klintworth, PE Columbus, Ohio
CK - If you give us a quick mail by April 12 with the temperature F or C and the T/C type (letter) that you want the process at - we can give you an exact voltage to use (what V a T/C will produce at that temp) to simulate the T/C operation. Bob Hogg www.almegcontrols.com We are closed Apr. 12 to 20.
A more convenient way of testing is: Feed the receiving device with the mV (from a mV Generator, there are inexpensive ones in the market). Then from appropriate T/C table look for the corresponding mV.

Michael Griffin

You can simply connect a cheap thermocouple (thermocouple wire) and use any convenient heat source. I have found a pair of fingers to be quite satisfactory as a heat source. For testing purposes, the limits or setpoints in the temperature controller are arbitrary, and so can be set low enough to allow body temperature to produce the required response. Pinch the thermocouple between your fingers to heat it up, let go to allow it to cool down. This is easy, simple, and safe. ********************** Michael Griffin London, Ont. Canada [email protected] **********************

Meeuwis Bouw

pos neg working range U 100 (mV) Cu constantane -200 to +400 4.28 Fe constantane 0 to +750 5.40 NiCr Ni 0 to +1100 4.04 PtRh Pt 0 to +1500 0.64 Pt Ir up to +1500 0.65 W Mo * up to +1800 -0.35 C SiC * up to +2000 30 U 100 is thermovoltage for temperaturedifference between hot and cold junction of 100C with cold junction at 0C * is less stable ------------ Meeuwis Bouw Superfos Packaging
If I understood properly, you can use any standard mV injector inplace of thermocouple i/p. Use the appropriate table to know the correct mV to be fed to simulate the required temperature. (mV/Temp tables are to be used for the specific type - R,J,S &c - of thermocouples. Arunkumar

Bruce Durdle

CK, You can check any thermocouple-operated device by connecting a mV signal to the input in place of the thermocouple. You need access to tables for the appropriate thermocouple, and can calculate from these the mV corresponding to a given temperature. You also need to take into account ambient temperature corrections. (What is happening with your circuit to give cold junction compensation?) As an example, a Type T thermocouple gives 14.862 mV at 300 degC. If the ambient temperature is 25 degC, the equivalent mV is 0.992. So if you apply (14.862 - 0.992) = 13.87 mV to the controller input, that will be equivalent to a Type T T/C with hot junction at 300 degC. If all else fails, you can generate the correct mV with a couple of variable resistors in series, with a suitable high-resolution voltmeter to read the voltage. Bruce.
We have an overstock of Altek thermocouple and other process signal simulators. Some are demos that we will let go for a fraction of their list price. i was thinking of putting them on ebay, but i just haven't gotten around to it yet. the price is at least 50% off. [email protected]