PID PV reads incorrectly

[Logan_01]

Just noticed an odd thing when doing a brew today. I have a REX-C100 PID controller, and it is the first time I have used my new setup.

I noticed that when the PV comes up to sp it stays there exactly. At first I thought, that's great it's keeping the temp perfectly. However once I put my element into manual, (overriding the PID controller), and gave it a bit of heat the PV stayed rock steady on the sp. after a while I thought its taking a long time... At some point I raised the sp by 10 deg out of boredom, and the PV jumped 4 deg C.

Very annoying when trying to get mash temps within a deg or so.

Has anyone else noticed this, or if you have the same controller, try it out and see what happens.

 

[glenos]

I don't have this controller but a couple of questions which spring to mind
What temp sensor do you have? how is it mounted in relation to the mash?
Are there any settings you can adjust in the controller? Has something been set-up incorrectly?

 

[Logan_01]

It's a k type thermocouple, mounted in an aluminium pipe acting as a thermowell. There are numerous settings but nothing relating to the displayed PV.

Anyway I have a workaround, by setting the sp away from the current PV it reads correctly... Something to be mindfully aware of though.

Seems like the manufactures have put this in to make it appear the controller is spot on sp, even when it's slightly off.

 

[Foxy74]

I had a similar issue with my temp probes. I had originally mounted them below my heater element. There seemed to be an 'inactive/cool zone' in that area. In the kettle this would lead to an aggressive boil and a lot of boil overs as it was always trying to heat up. In the HLT also it would be hotter than the displayed temp . I moved them both above the element and temp control has been easy ever since.

 

[trevgale]

I used the REX-C100 PID's when I setup my first prototype system and experienced the same problem. I eventually worked out it is was to do withe "Digital Filter Constant - dF" value. The default value is 1 sec which should be be ok but there seems to be a bug or they are just trying to make there PID look better at holding a temperature than they really are. Anyway by changing dF to 0 sec this solved the problem.

When I setup my current system I switched to Auber PID's and PT100 probes and couldn't be happier with them, especially using the ramp soak function for step mashing automatically.

Anyway, I hope this solves your problem.

 

[Logan_01]

Thanks, I will try changing that parameter next time I brew in a couple of weeks. In not that impressed with the controller anyway, mainly because it only shows 1deg increments, and there is no manual function for controlling the boil. I made a simple pwm for the manual part with a changeover switch, which worked well for me.

 

[trevgale]

Yeah, from memory it only allows 1deg increments for K-type probes. I'm pretty sure if you used a PT100 probe you can set it to 0.1deg increments.

 

[WoopWoop]

I have these same controllers (labelled rkc Rex c100), setup with pt100 rtds, and was experiencing the same glitch. Changing df to 0 fixed it. Thanks!!

Also I have 1 decimal place with these probes, but on another controller with a k thermocouple it has 0 dp.