help with temp probe and PID on recirc system

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ben frazer

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Hi All,

Whilst I normally use a 3V 60 litre system for my house brews or splitting with mates, I wanted something a bit smaller for experimental brews and easy week night brewing. So I have gone down the recirc BIAB concept similar to others on this site, using a 40 litre electric urn and one of the little brown pumps, which returns the liquid to the middle of the mash though the top of the urn, and also to the bottom, across the element to reduce scorching risk.

The system is temperature controlled with a probe in a inserted in a 1/2" T section and is controlled by an old REX PID.

I am having two problems with this.

The first is that when I calibrate the temp probe in a bowl of water it reads perfectly (well close to) against a calibrated probe. However when I insert the probe in the T section and start recirculating the liquid, the temp probe consistently reads about 7 degrees higher than the calibrated reading. I have tried the probe in position before the pump, and also just before the return to the top of the tank. It makes no difference, the reading remains higher than the calibrated reading. Has anybody has this problem and can help with a solution?

Second problem is with my PID control. I have tried reading up on this but it is all a bit complicated for me to get my head around. Currently my PID seems to cycle the element on and off and climb its way towards the set temperature quite slowly. What I was expecting it to do was to keep the element on to move from cold to somewhere close to set point reasonably quickly, then start cycling as it got closer to the set point so as to not overshoot. Is there some thing in my PID settings that might not be right. The PID does have autotune capability, should I use this somehow?

Thanks for any help with this. I have trawled through various threads but can't find specific answers to these specific problems.
 
The first is that when I calibrate the temp probe in a bowl of water it reads perfectly (well close to) against a calibrated probe. However when I insert the probe in the T section and start recirculating the liquid, the temp probe consistently reads about 7 degrees higher than the calibrated reading. I have tried the probe in position before the pump, and also just before the return to the top of the tank. It makes no difference, the reading remains higher than the calibrated reading.
Apologies if I dont quite get what you mean....... but what your saying is your pid might be set at 65C but the measured value at the probe is 7C warmer? Have you checked this with your calibrated thermometer at the same position as the probe? You might find that the wort is in fact 7C higher as its just flowed past the element and heated up. But the pid should compensate for this by shutting down the element.

Which you say your pid seems to cycle the element on and off to reach your set temp quit slowly. Im not sure about these Rex pids but with the auber ramp soak pids if you set it to climb to a certain temp in a certain time it will take that enitre time to get there. So if you ramp from say 55c to 65C in 10 mins but your element can heat up at a rate of 2C a minute it will still draw the temp increase out to 10 mins by cycling the element on and off.

Once again my apologies if I dont quite get what you mean.
 
Which probe are you using: thermistor, thermocouple or RTD?
 
Hi Truman, thanks for your reply. I mean that the temp probe linked to the PID will be reading for example 67deg but the temp of the water in the kettle when measured with a calibrated thermometer is only 60deg. This is when the temp prob is in the recirc line. If I take the temp probe out of the recirc line and place it in the kettle then it reads the same as the calibrated thermometer. Therefore problem seemds to be that it reads higher than actual temp when it is in the recirc line.

Pretty sure the probe is RTD, but I will check tonight. The control box was put together by one of my work sparkies, but he has since left so I cant seek his advice.
 
Use the autotune function on a brew or something that mimics the conditions of a brew (ie right volume, pump running etc) and that will make things run better. The issue of the pid reading higher when the pump is on doesn't sound right.

Could you give us specific details about how you are measuring this.
 
It could be possible that the fluid loses 7c in transition through the pump and then back through the lines, you have it mounted in a t-piece, where is the t-piece, attached directly to urn outlet or at the top where it dumps back into urn?

Either way, best place to put the probe for BIAB is directly in the wort in the urn itself. For HERMS/RIMS it is at the outlet of the HEX/heat tube.

The comment above about ramp times is spot on, not sure what the REX pid provides, but the auber one allows programming of ramp rate.
 
If you have the probe close to the element it is less likely to overshoot and means the wort going over the element is less likely to scorch. I say "less likely" but to be honest I don't know how likely it would be that you would scorch anyway.

If you have the probe in the mash tun the wort going over the element will be hotter as your element will probably ramp to 100% output pretty quickly, and is then likely to overshoot.

The location of the probe will govern the behaviour of the controller. Unless you have two probes you will have some sort of uncertainty. If you control from close to the element IMO you should not use ramp rate as it will slow down the mash ramp rate.

Either way your actual mash ramp rate will be governed by the pump recirculation flow AND the element power/controller output.

EDIT : To summarise the above, the 'best' control is to have the PID control in the mash with a high pump flow (don't stick the mash) to a ramp rate, with the element physically as close to the mash as possible. Having the element a long way from the mash increases the chances of scorching and overshoot.
 
deleted post

Brew Rig.jpg
 
kiwi b said:
Hi Truman, thanks for your reply. I mean that the temp probe linked to the PID will be reading for example 67deg but the temp of the water in the kettle when measured with a calibrated thermometer is only 60deg. This is when the temp prob is in the recirc line. If I take the temp probe out of the recirc line and place it in the kettle then it reads the same as the calibrated thermometer. Therefore problem seemds to be that it reads higher than actual temp when it is in the recirc line.

Pretty sure the probe is RTD, but I will check tonight. The control box was put together by one of my work sparkies, but he has since left so I cant seek his advice.
Ok but if you put the calibrated thermometer at the outlet of the return back to the kettle (So right next to the temp probe) what does that read? im guessing it will be 67C also. Also when exacly are you measuring this? The water in your kettle will always be cooler than the water coming out of the return, but if you let it recirculate for awhile then what reading do you get?

As Black_labb said autotune your pid and see if that makes a difference. I know with my auber pid in certain circumstances (which I cant recall now what they are) my temp reading will be higher than my set value and the element stays on.
 
Truman said:
I know with my auber pid in certain circumstances (which I cant recall now what they are) my temp reading will be higher than my set value and the element stays on.
This is 'overshoot'. You probably still have your integral term keeping it in on from all the time it was below your set point. It should help to reduce your integral time (I believe it is time in seconds for the Auber units) a fair bit.
 
Looking at your pic is the probe mounted in the sight glass tee ?
Could ? This be part of the problem,could a hot pocket of water be trapped in the probe tee,with the probe not being subject to direct flow over it be giving a "false" reading.
Assuming that the probe is not long,as in protruding into the pot.
More questions not answers but.....
As for the PID,sorry as I have yet to sort mine out.
 
Hey Spog,

Thats a pic of my rig, not the OP's. I deleted my response because I misunderstood the OP's original question.

I'm not sure why the the recirc liquid would be 7c hotter on its way back into the kettle than it is down near the element. Maybe some pics of kiwis rig would shed some light on the problem. Need pics of the inlet and outlet on the urn. If the pickup is really close to the element in the urn it may cause the false reading.

I have calibrated my PID to several digital thermometers and the temp at the top and bottom of my mash while recirculating is the same.

Cheers,
 
When you first change your set point, eg from 56C to 66C and your PID controller probe is in the bottom of the mash, you could see a massive temperature difference between the element and the probe - could be 20 degrees or more at the very start, and then will reduce bit by bit.

There is merit in putting the controller probe right next to the element of you have a high power (3600/4800W) element or a lot of volume between the element and the measurement point as you can also use it to dial in your initial strike temp as well as avoid scorching during recirc . In lieu of hav ing a probe in the mash as well, once you set your set point you could then just wait for the controller output to drop to zero then back the recirc off and you have your next rest.

Many ways to skin a cat...
 
My rig is very similar to that posted by Brewologist, other than that my probe is in a T Joint straight after the outlet valve at the bottom of the tun, before the pump. So what I get is a reading from the Probe which is 7 degrees higher than the reading from the calibrated thermometer that is reading from the top 3rd of the mash. It could be that the water being drawn from the bottom of the tank is 7 degrees higher than the water in the top of the tank. I have however tried installing the probe at the position that Brewologist has his analogue probe and this has made no difference. when I did this, I put the calibrated thermometer in outlet of the return stream and still the PID probe was reading higher than the calibrated thermometer.

What I cant understand is when I take a reading of static water, the PID probe and the calibrated thermometer read within a degree of each other, and infact the PID probe reads slightly lower than the calibrated thermometer. When I put the probe inline with flow then it reads 7 degrees higher.

A few people have mentioned that the best place for the probe is in the mash rather than the recirc line. This could be the easiest solution to my problem. Aside from the idea shown in Brewologists rig of inserting it with the level tube, what other methods do people have for putting the probe in the mash, given that this is a biab set-up and I don't want the issue of the bag catching on the probe when the bag gets removed?
 
Is the 7 degrees constant? Have you tried measuring it 5, 10, 15min later?
 
It has been talked about before when calibrating probes, especially probes of differing types, calibrate at or close to the temperature they're going to be used, now that may only solve a small portion of your problem but should get you a little closer.


MB
 
Looking at the picture this is probably a pretty bad place for the PID probe, depending on how long it is. If it is short, there will be no flow across it and it may not detect the real temperature of either the mash or the wort flowing through the pump and into the grain bed.

I'd be looking at placing it either where you have your dial thermometer or where the wort enters the pump, both those locations would be pretty good.
 
What I cant understand is when I take a reading of static water, the PID probe and the calibrated thermometer read within a degree of each other, and infact the PID probe reads slightly lower than the calibrated thermometer. When I put the probe inline with flow then it reads 7 degrees higher.
Thats because it possibly is 7 degrees higher. As I said in my last post compare them side by side. Calibrated thermometer and pid probe right next to each other, just like when you do it in water. There is no point putting the pid at your vessel outlet before the pump and your thermometer in the mash and say there is a 7 C diference. Of course there will be. Once compared side by side if there is still a difference you have a problem with your pid. And you should maybe recalibrate it at mash temps as Adr_0 suggested. (But I think you will find they are fairly close to each other.)

Dont bother trying to measure the mash bed as it will always lag behind.

There is merit in putting the controller probe right next to the element of you have a high power (3600/4800W) element or a lot of volume between the element and the measurement point as you can also use it to dial in your initial strike temp as well as avoid scorching during recirc . In lieu of hav ing a probe in the mash as well, once you set your set point you could then just wait for the controller output to drop to zero then back the recirc off and you have your next rest.
Do what adr_0 says above and dont worry about what your mash bed temp is. As long as your measuring the temp of the wort as it has passed the element thats all you need to worry about. The mash will always lag behind.

My probe in my 3v herms system is right at the top of the herms in a T.
 
OK, so if the temperature of the wort that is being recirculated is more important than the mash bed temperature, then am I better to measure as it immediately leaves the vessel on its recirc path, prior to the pump, or just before it re-enters the mash, or does it make no difference? Perhaps because I am splitting my return between return into the top (into the mash) and return through the bottom, across the element, I should measure as it leaves the urn, before it reaches the pump.

I will have another play with it to see if I can better understand the temperatures in the system
 
Perhaps because I am splitting my return between return into the top (into the mash) and return through the bottom, across the element, I should measure as it leaves the urn, before it reaches the pump.
If you have wort at say 67C coming out of your pump and then going straight back across the element and past the probe at around 67C your pid will shut the element off and your mash will never rise to temp. You need to have your flow going to the top of the mash only.
I know your trying to eliminate scorching the wort but providing your getting good recirculation you shouldnt have any problems.

Not sure where the best place is to put your probe but in a rims the probe is just past the element so I would think where you have it now is probably ok, at the bottom outlet. At least then its measuring the wort temp past the element and the pid will cycle the element on and off as it cools and heats etc which will also help stop scorching.
 
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