# Rex C-100 w PT-100 RTD jumping to set point



## takai (22/5/15)

Bit of an odd one here. I have just built a new PID controller for a mate using the pretty standard combo of a Rex C100 and a PT-100 RTD sensor. My controller is almost identical, and it measures fine throughout the range with relative accuracy.

However this one tends to jump or latch to the Set point when the PV is closeish. So if you have the urn cooling from mash and using the PID for just temperature monitoring (urn is unplugged) when the urn gets close to the SV the PV will tend to jump to it. For example i still had the controller set to 78deg from mash out and the urn was happily cooling away with the immersion cooler and got to 79.6deg with sampling dropping roughly 0.3deg/s and then the PV suddenly dropped to 78.0deg and stayed there before it jumped on to 75.4deg and continued with the 0.3deg/s ish rate from there on.

I cant duplicate it with my other two PID setups, but they both have a separate RTD sensor that is bolted to the urn rather than an RTD that sits in a thermowell.

Any thoughts?


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## Camo6 (22/5/15)

Are you using thermal paste in the thermowell and are you recircing while chilling?
Might be worth swapping the PID with one of your own to rule out a dud unit.


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## TSMill (22/5/15)

I've seen the same with my Rex, it will happily sit rock solid with PV = SV, but as soon as I change my SV the PV jumps by a couple of degrees.

I think perhaps some inbuilt code to misrepresent the degree of control being provided.


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## MastersBrewery (22/5/15)

My problem with the Rex C100's was a while back it was mentioned here that there were cheap knockoffs about ebay and the software on these was inferior. Can't be 100% sure who it was but do remember they went with sestos or Auber. I hope this is not the problem.


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## takai (22/5/15)

I suspect it is the software on the PID. Will have to check it out and compare with my existing units.


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## sponge (22/5/15)

I have an auber PID but know that different values of setpoints allow for certain functions, such as jumping to setpoints, sounding alarms, finish and hold temp, etc. Is there a chance you've accidentally set up something like that and is causing some issues?


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## takai (22/5/15)

sponge said:


> I have an auber PID but know that different values of setpoints allow for certain functions, such as jumping to setpoints, sounding alarms, finish and hold temp, etc. Is there a chance you've accidentally set up something like that and is causing some issues?


This is one of the issues with the Rex C100s, it is quite possible that there is something like that set... but who knows how to figure it out. I cant read any Chinese.


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## anthonyUK (22/5/15)

You can see the issue with the C100 in this yootoob

https://www.youtube.com/watch?v=kbVs1Bs1di8

How can changing the setpoint (SP) affect the measured temp (PV)??

There is a comment in the vid that changing the dF setting from 1 to 0 in the menu helped.


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## takai (22/5/15)

anthonyUK said:


> You can see the issue with the C100 in this yootoob
> 
> https://www.youtube.com/watch?v=kbVs1Bs1di8
> 
> ...


Ahah, that is exactly the problem. Ill have a look at the dF setting next time i fire it up.


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## TSMill (23/5/15)

http://aussiehomebrewer.com/topic/80896-pid-pv-reads-incorrectly

Covered here as well


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## takai (24/5/15)

Ah, i didnt find that one on the search, probably didnt share the search terms.

I changed the dF value when using it today and it is now working the same as my other C-100s. WIN.


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