Temperature controllers

hi

i am working on the plc implementation of a furnace.

i am facing problems with a temperature controller. The problem is that the temperature controller has automatic compensation for changing cold junction temperature but does not have any compensation for changing room temperature. due to this the temperature controller tends to lose calibration and needs to be calibrated daily.

could anybody suggest some solution or a better product that u know
of.

regards

pkansara

I am assuming thermocouple, why is the cold junction compensation not in the room that is changing temperature? What kind of cable are you using between t/c and controller?

Brian

Doesn't your controller have built-in CJ compensation?

The temperature is calculated on the difference between the 'hot' end and the 'cold' end (the junction at the controller).

One problem people have is not allowing their calibrator to stabilize to the same temperature as the controller before calibrating...this kind'a depends on your calibrator also.

For example, when the controller cabinet is closed, it's common to have temperatures 80 to 100 F. In a air conditioned room, when you open the cabinet, the controler is exposed to possibly 60 to 70 F. This is why you should give the controller & calibrator some time to stablize at the same temperature.

Just some thoughts....

On the assumption that you have something of an expandable PLC, use of an RTD card and a 4 wire RTD will compensate for any changes in temperature that occur outside of the measuring area. The last 2 wires act as a separate RTD to correct for the resistance of the wire run from the measuring point (RTD resistor) to the end of the cable. Omega manufacturing does some good writeups on measuring temperature, and a 4 wire RTD should give you what you are looking for.

Its also a good idea to leave the temperature controller on all the time, that way the instrument remains warm and stable. Many furnaces have seperate instrument power and control power switches for this reason.

The connecting wire needs to be thermocouple wire or thermocouple extension grade wire of the appropriate type to match the thermocouple. If you have a type K thermocouple and type J extension wire, or even just plain copper wire, you have just shifted the cold junction from the controller where it is compensated for out into the room where the incompatible junction is located.

Every wire junction that is not the same kind of material is a thermocouple. So if you connect a type K TC to type J wire, you get three thermocuples in series: A type K, a Iron/Chromel and a Copper-Nickel/Alumel, each adding its own EMF according to whatever temeprature it is at. With copper hook up wire you get type K, Copper/Chromel, and Copper/Alumel TCs in series. An intermediate material is allowed only when both ends of the material are at the exact same temperature, such as a small teriminal block.

Make sure that the TC lead connections are high integrity and that the leads are not grounding out somewhere as well, make sure that the lead insulation has the needed temperature rating.

hi

thank you all for ur help.

i am try out ur suggestions

thanks again

pkansara