PID Autotune Question

[Brewbien]

I have an Auber 2451 PID/Timer. After auto tuning, the temp probe always reads about 4 degrees hotter than the set point (verified with a separate thermometer). Any suggestions on fixing this?

 

[P-J]

Read item 4.8 in the PID user manual.

 

[Brewbien]

Read item 4.8 in the PID user manual.

Thanks PJ. Quick question on that. If I adjust the offset, am I assuming that the pid is right and probe is wrong? Or am I telling the pid that it needs to compensate 4 degrees lower?

 

[P-J]

Thanks PJ. Quick question on that. If I adjust the offset, am I assuming that the pid is right and probe is wrong? Or am I telling the pid that it needs to compensate 4 degrees lower?

You are telling the PID to compensate.

P-J

 

[Brewbien]

Now the PID reads the right temp but my thermometer still reads 4 degrees high. Do I just have to adjust the setpoint of the pid to a lower temp or is there a way to make it it accurate? If it helps, my system is a recirculating e-BIAB.

PS. Thanks for your help. I wouldn't have been able to build my system without reading through all the help you've given others.

 

[P-J]

Now the PID reads the right temp but my thermometer still reads 4 degrees high. Do I just have to adjust the setpoint of the pid to a lower temp or is there a way to make it it accurate? If it helps, my system is a recirculating e-BIAB.

PS. Thanks for your help. I wouldn't have been able to build my system without reading through all the help you've given others.

Now I do not understand what you are saying:

First you said:

temp probe always reads about 4 degrees hotter than the set point (verified with a separate thermometer)

Then you said:

Now the PID reads the right temp but my thermometer still reads 4 degrees high.

Which is it??? You initially based your info of the thermometer as you reference. Now what????

 

[MX1]

P-J, I am right there with you on this, as in confused.....

BrewB,

if it was me, this is what I would do.

1. Calibrate thermometer using the ice bath and again using boiling water. Be sure to know waht temp you water boils at your elevation.

2. after calibrating the thermometer, if you can adjust it, do so. If not make note of how far it is off.

3. Set your PID to temp mode, and place probe in boiling water.

4. Check against your calibrated thermometer (apply correction if needed)

5. Make adjustment to PID to read the correct temp.

Again, make sure that you are using the temp of boiling water at your elevation as your High Side base line.

Boil Temp at Altitude

Tim

 

[Brewbien]

Now I do not understand what you are saying:

First you said:
Then you said:
Which is it??? You initially based your info of the thermometer as you reference. Now what????

Sorry for the confusion. Ill try stating my issue a different way to see if that helps.

Originally I would set my pid for 152 and when it got up to temp, both my probe thats connected to the pid and a seperate thermometer would read about 156. So the PID was overshooting the temp by 4 degrees. You said to change the offset by 4 degrees which I did and the pid readout changed to 152 but the thermometer still reads 156. So that just changes the number that is displayed by 4 degrees, it doesnt tell the pid to compensate for the difference in actual temperature. So I need to figure out how to get the pid to heat to 152 when i set it to 152 instead of heating to 156 when i set it to 152.

Does that help?

 

[MX1]

set it for 148

 

[jeffmeh]

I think you need to reset the PID so the probe agrees with the correct temperature, based upon your reference thermometer (assuming that it is correct). Then run the autotune again.

Setting the offset to -4 would be the appropriate procedure if your probe were reading 4 degrees above correct temperature. Rather, if you are overshooting the set point by 4 degrees (and the probe is reading accurately), your PID parameters are not correct for your system.

Here is a good write-up of the PID setup procedure. http://www.theelectricbrewery.com/control-panel-setup