What would cause an unstable reading on a temperature controller?
What would cause an unstable reading on a temperature controller?
(OP)
I have a temperature controller connecting to a heater mat, it takes the reading of the heater mat temperature and also controls steady state relay to switch on or off the heater mat.
The controller's set point is 90 degreeC, when the temperature reaches around 90 the reading starts to jump up and down a lot, one second it reads 90.6degreeC the next it reads 98degreeC. I doubt the heater mat will heat up or cool down 8 degree in just a couple of seconds?
The temperature controller is set to auto-tune. What I expect to see is the displayed temperature changing slowly and showing the true temperature of the mat.
Can you please help?
Thanks
Michelle
The controller's set point is 90 degreeC, when the temperature reaches around 90 the reading starts to jump up and down a lot, one second it reads 90.6degreeC the next it reads 98degreeC. I doubt the heater mat will heat up or cool down 8 degree in just a couple of seconds?
The temperature controller is set to auto-tune. What I expect to see is the displayed temperature changing slowly and showing the true temperature of the mat.
Can you please help?
Thanks
Michelle





RE: What would cause an unstable reading on a temperature controller?
The auto tune function does not work well where the heat capacity is low (fast changing) and on/off control is used. Look up previous threads on auto-tune.
RE: What would cause an unstable reading on a temperature controller?
Bill
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"Why not the best?"
Jimmy Carter
RE: What would cause an unstable reading on a temperature controller?
It's a PID controller, that's why it's set to auto-tune.
RE: What would cause an unstable reading on a temperature controller?
RE: What would cause an unstable reading on a temperature controller?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What would cause an unstable reading on a temperature controller?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What would cause an unstable reading on a temperature controller?
This has nothing to do with the auto-tune as auto-tune doesn't have anything to do with the display of the temperature, i.e. the reading of the temperature value.
What technology is the sensor?
How exactly is the sensor wire routed with respect to the heater power cable?
Keith Cress
kcress - http://www.flaminsystems.com
RE: What would cause an unstable reading on a temperature controller?
I will try everything that has been suggested. I checked controllers (with same configuration) on other plants, they were pretty stable.
I can't do anything just yet, because the plant is running.
They are K type thermocouples. The heater mat came with thermocouple cables and power cable. Normally we use helaglass sleeving to keep thermocouple cables and power cable together. That's how it's done on all the other plants, it seems I'm only having problems with this one. I might try to use a different heater mat.
RE: What would cause an unstable reading on a temperature controller?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What would cause an unstable reading on a temperature controller?
Is the supply voltage stable or is it being modulated by the controller? You will probably need a scope to see this although a fast meter might pick up fluctuation caused by an integral cycle controller. We recently had some serious problems with burner controls caused by the I.C. controllers in our gas chromatographs modulating the supply at the DB. Simple to fix once we knew what we were chasing but took a bit of finding.
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If we learn from our mistakes I'm getting a great education!