RIMS circulation temp overshoot

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Bru

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I have a 1500W low density RIMS tube with the temp probe installed on the wort "out" side.
My grain bed stays at the right temperature but wort circulating through the RIMS over shoots by about 2F.
For example if I set my desired temp to 151 the RIMS will turn on at 150.5 and turn off at 151 but the thermometer will continue to rise to about 153F before gradually falling back to 150.5 and the process starts over.

Considering the mash temp stays constant should I be concerned about the overshoot ?
Any ideas how to fix it ?
 
Sounds dialed in to me. I have a herms setup and need to re-circ a degree or two hotter than mash temps to maintain.
 
How are you controlling the element? Don't ask me to explain it, but with a BCS system the PID is adjustable (easy for some) to control any overshoot. You can watch it in action as see how the power to the element starts to throttle back as it reaches temperature. I'm not saying your finished product will show the 2F difference but it doesn't sound like something that can't be fixed with a setting or change in probe location.

I don't understand more than 2% of this http://en.wikipedia.org/w/index.php?title=PID_controller but it might help you.
 
Mine stays pretty much dead on what I have it set at. How fast is your pump flowing I think dialing in your wort flow will help stabilize your temp not to fast not to slow.;

Pat
 
I use a PID from Auber Instruments. Given my current calibration, I have found that I have to set my temp 2 degrees below my target, let it overshoot, then about 20 minutes into the mash as the temperature has stabilized I can slowly raise the PID up the final 2 degrees. That generally keeps me right at the temperature I am looking for. I originally set my probe at the inlet to my RIMS tube, but have moved to directly in the mash. I found that given the thermal mass of all the external equipment the RIMS tube temperature fluctuates much more slowly. It can take up to 10 minutes for the 2 location to agree on the current temperature. The mash will rise at almost 1 degree every 45 seconds when the heater is continuously on, but the RIMS tube measurement reflects a 1.5 to 2 minute degree changes.

Personally, I do not worry about the temperature of the wort as much as the grist. My theory being that I am more worried about the enzymes in the grain. Whatever is in the wort is already dissolved, and does not need as much conversion. I know that most HERMS brewers are concerned about killing the enzymes by running the wort past a heater element. I always see efficiencies of 75 - 80%, and have never scorched anything. I do keep the pump running at all times.
 
That makes sense. My probe is installed at the "out" side of the RIMS. Im tempted to move the probe to the MLT for reasons you state - more stability - but as its working Im going to leave it.
 
If you want to control overshoot, it may help to lower to proportional gain on your controller (the 'P' in 'PID'). Increasing the 'I' gain should get help rid of steady state error.
 
Sounds like a tunning issue to me. The probe is in the correct spot then. If it was in the mash you introduce a lot of lag to the response time, hence jmer's method.

A forward feed from the mash tun would allow you to reach your temp a little faster but, I haven't seen any economy priced PID controllers that support that.

If you're fluctuating 2 deg F at the RIMS tube by the time it reaches the mash it will average out rather quickly.

Oh, I bet I know what you're problem is! There is a time variable that you need to set some times. This is how long the output should stay on at a minimum. I think Auber PIDs default to 2 seconds. If you set that variable to 0 it goes down to 0.5 seconds minimum on time. This is for things like compressors so you dont short cycle the hell out of them. If you have a different PID read the manual and look for a minimum on time variable.

What happens is the overshoots but cant turn the element off because it hasnt been on long enough.
 
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