I have a controller board which is interpreting a 68ºC output from a J-Type Thermocouple as 62ºC and as a result will not let my system run. We do not have any documentation available to us for the controller board. If I am correct, I am needing to boost the signal from the Thermocouple by .0534mV / degree or a total of .3204mV total. Are there any simple Amplifiers readily purchased that are capable of adjusting over a range of 0.1 to 0.5mV.

 

hi vyyper,
Welcome to AAC.
At 62C what is the mV output of the J type?

This signal can be amplified to give the mV required mV out, equal to 68C
E

 

Is there any calibration adjustment on that controller?? Or is the problem that there are a lot of adjustments and you do not know which one to adjust?? The problem with changing the gain to increase the voltage is that it will alter the calibration over the entire temperature range, with the error increasing with the temperature. So IF the temperature you are measuring is actually 68 degrees, then it may only require an offset adjustment.


One cheating choice will be to add a small additional thermocouple in series to boost the voltage just tat tiny bit. That is an "offset " change.
Another trick will be to substitute a different thermocouple type!

OR just add a bit of heat to the thermocouple

 

@vyyper23 Are you sure it is not designed for 4-20ma loop system ?
https://www.predig.com/indicatorpag...FJyO5DPjrWERPg1ODdWkLXTp5AvtKZSl9JZkFc4OhG7tc

 

Equipment was purchased many years back, and we don't have any documentation on the Controller Board. System essentially works like this...
At point X in the process, we call for Heated water - that call turns on heaters and water starts being heated and cycled through system, Controller reads the J-type thermal couple inserted in the water line. The Thermal Couple does indeed read 68ºC as verified by 2 calibrated Thermal Meters. However, the Controller board is interpreting the signal received from the Thermal couple as being 62ºC (which triggers a low Temp. alarm) and halts the system. 2 spare boards we have are toast and am trying to get us by until a replacement piece of equipment can be received and installed.
Hence my thought of an amplifier to boost the signal from the Thermal couple by 0.32mV.

 

Hence my thought of an amplifier to boost the signal from the Thermal couple by 0.32mV.

Hi vyyper,
Yes it can be done, are you asking for someone to design and post a working amplifier design?
Or do you have a draft design circuit you can post?
E

 

Though I have used this circuit with type K thermocouples but you should be aware that this circuit does not pay attention to the Seebeck effect which causes thermal drift. The gain as shown is X1,000 so each millivolt of input results in 1 volt on the output.

 

Once again I will remind the TS and others that adding an amplifier will change the slope of the temperature indicated versus the thermocouple voltage. That means that NONE of the readings will be correct!!
BUT, if you add an offset voltage, OR succeed in adjusting the offset in the controller, then the error will be removed. So all that you really need is a "unity gain" amplifier with adjustable offset.
BUT, now I am guessing that it may be a connection issue the the TC wiring that is causing the offset.
That may be as simple as a connection with non-similar metals in one of the leads.
I had that issue in one engine test stand where the customer demanded all wires go thru terminals on entering the controls cabinet. The solution was for our installation techs to twist the TC wires to each other before putting them into the screw terminal. So there was no external TC junction changing the voltage.
That interesting Seebeck effect ,which causes thermal drift ( Seebeck effect) which causes thermal drift is very real, and accidental thermocouples are also quite real, and sometimes well hidden.

 

I have a controller board which is interpreting a 68ºC output from a J-Type Thermocouple as 62ºC and as a result will not let my system run

Hi,
The thermocouple output to the Display is according to the TS correct at 68°C.

An OPA will be used to directly amplify the thermocouple output signal so that it gives the correct 68C to the controller unit.

E

 

As the signal increases the amplifier gain will also increase the signal and so the input to the controller will be greater than the actual temperature by an increasing amount. THAT may cause a problem.

 

What sort of problem????

These are two simple options, all the TS needs to do is to adjust the Controller OPA gain.

 

Perhaps this will best represent what is going on. In above photo the only thing that has changed is the controller board for the system out of necessity. As I previously stated we have almost zero documentation / schematics for the controller board for the system. Previous Controller board that had to be pulled had no issues reading the Thermocouple. Connecting to either of the shown Hand Digital Thermal meters (both Calibrated annually) - returns expected temperature of 68ºC. We just need an accurate and appropriate way to "trick" the Controller board to be at 68 and not the 62 it thinks the water temperature is when the water temperature is actually at 68ºC. This is not a permanent solution for us but rather a band-aid till replacement system arrives and can be installed.

Everyone's feedback thus far is greatly appreciated. If there is an off the shelf solution someone could recommend that would be fantastic. If not a circuit design would be hugely beneficial. Am I off-base with my assumption that i need only a 0.3mV boost to the controller board to represent a 6º increase?

 

What sort of problem????

These are two simple options, all the TS needs to do is to adjust the Controller OPA gain.


View attachment 341909

IF the TS had the information to make that adjustment on the controller they would have done it. I quote from post #1:" We do not have any documentation available to us for the controller board. " That is the reason for requesting assistance. See also my response in post #3. The controller is a legacy system with no available information and nobody left who understands how to adjust it. And evidently nobody present able and willing to try adjusting it.

That does happen when the more experienced folks get old and retire, or are released because of mandated reductions, often starting with the older more experienced and thus expensive staff members.

 

If it is indeed a 4-20ma loop system as is typical for thermocouple circuits, my first test would be of the power supply for the loop. could well be something as simple as low voltage problem.

 

If it is indeed a 4-20ma loop system as is typical for thermocouple circuits, my first test would be of the power supply for the loop. could well be something as simple as low voltage problem.

Given that in post #5 the TS is asking to boost the TC signal by 0.32mV. " Hence my thought of an amplifier to boost the signal from the Thermal couple by 0.32mV. "
Based on that request it seems like it is a direct thermocouple level signal connection. In addition, that amount of difference can probably come from the existence of an accidental thermocouple having been created someplace in the connections of the TC cable. Consider that 0.32 millivolts is not very much. It might even be that the controller is set for a different TC than type "K". How would a type "J" TC work??? Anyone with a set of TV voltage versus temperature charts/tables can evaluate that possibility.

 

ALL,
Thank you so much for your inputs on this issue. As stated initially little to no documentation available. After a long and painful search, the responsible tech I was trying to help was able to back track the equipment process flow and found that the indications being given by the system itself sent us down a rabbit hole chasing a ghost. We faked the correct temperature by feeding a 68ºC source from another heat source and system started up then failed for a low O3 pressure alarm. Both checks happen simultaneously in the sequence and system didn't specify which was the actual cause of system stoppage. Now on to chasing down a slow O3 leak.

AGAIN MANY THANKS TO ALL.