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Old 11-17-2011, 11:03 PM   #1
Bachhus
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Default PID for Herms/Rims?

So I have the majority of my components and was laying them out. I origianlly was planning on going the RIMS. My original plan was to have two PID's one for HLT control, one for RIMS control and a PWM setup for the boil. I have been thinking about going HERMS instead of RIMS.

My question to the group is, If I run a HERMS style heating system with the coil running through the HLT then don't I really only need one PID to control the element for the HLT tank? Could I just run two thermocouplers on a two or three position switch? One monitoring the internal water temp for HLT function and the second monitoring the output of the HERMS coil.

Thanks for the help
Mark

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Old 11-18-2011, 12:36 AM   #2
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i am doing e-herms. I have 1 PID controlling the HLT temp.

This same PID will also control the boil on manual mode.

You don't even need the PWM.

just keep in mind you may still want to view the MT temp...although in most rigs, that is close to HLT +-3 degrees.

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Old 11-18-2011, 01:02 AM   #3
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I want the ease of turning a knob to control the boil and I already have the PMW board designed, made and put together.

I said thermocoupler earlier but I actually have a RTD sensor. Since the RTD uses an electrical signal from what I can tell I beleive I can just put two RTD's on a switched line to toggle between which one I want to monitor and have them feed the PID. The PID will only be seeing one RTD at a time and it will be controlling one element (HLT). This way I can have it heat the strike water to a specific temp when on the HLT RTD sensor. Then when I'm ready to recirculate the MLT liquid I can switch it over to a RTD mounted in line with the HERMS coil output and have the PID bring the HLT temp up to make the output of the HERMS what I want.

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Old 11-18-2011, 02:16 AM   #4
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In my experience, if you monitor the herms output temp, you are more likely to overshoot. My buddies and I have had the best experience monitoring the HLT water itself. It is usually 3 degrees above mash temp.

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Old 11-18-2011, 02:23 AM   #5
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Quote:
Originally Posted by canyonbrewer View Post
In my experience, if you monitor the herms output temp, you are more likely to overshoot. My buddies and I have had the best experience monitoring the HLT water itself. It is usually 3 degrees above mash temp.
I'm going to disagree on this one.

I monitor HERMS output and my temps are always spot on. No overshoot at all, and no offset to account for. Set PID to my mash temp, and my mash is exactly at that temp.

When I was building my HERMS system I was trying to decide where to monitor, so I posted a poll on here with many options for people to state where they had their probe, as well as if they used to have it somewhere else and eventually moved it.

There were some people monitoring the HLT.
There were some people monitoring the HERMS output.

The more interesting data was that there were a number of people who used to monitor HLT and moved their probe to monitor the HERMS output because it worked better for them.

There was nobody who had moved from HERMS output to HLT because they found it to work better.

I'm trying to understand how you even could overheat the mash by monitoring the HERMS output. Monitoring there will esure that the liquid going back into your MLT is never over your mash temp, so it should be really really hard to overshoot.
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Old 11-18-2011, 02:50 AM   #6
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This is of interest to me as I'm converting my electric to a HERMS right now. I've already mounted the probe in the HLT so I'm going to give it a go there first. I guess I could always plug the hole and move it if needed.

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Old 11-18-2011, 02:57 AM   #7
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i am not trying to bad mouth the idea of putting the probe in the HLT. Lots of people do it and they seem to have no real issues other than needing to account for a small temp difference between the HLT temp and the actual mash temp.

I just like the WYSIWYG behavior of the HERMS output monitoring.

I think the only thing to stay away from is monitoring the mash temp to drive the control, or monitoring the output of the MLT to drive the control. I found a lot of anecdotal evidence that these two places will result in overshoot, and that make sense.

If you are monitoring the temp of wort coming out of the MLT, and it's low, you will start heating wort up as it passes through the HERMS and back into the MLT, but you won't see the effects of that heating until that heated wort makes it back through the grain and comes out of the spigot again. You will have a MLT full of recently heated wort before you see any of the effects on the temp probe.

So.. just don't use that method.

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Old 11-18-2011, 03:47 AM   #8
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Thanks Walker for that, I'll keep it in mind.

To the OP, every probe has a unique degree of inaccuracy so switching them would mean that only the one you've calibrated is going to be accurate. I'm tired and a little drunk so maybe someone else can explain how to do what you want in the best way.

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Old 11-18-2011, 04:18 AM   #9
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Ok, here's the deal with PIDs and overshooting: Properly tuned, regardless if the probe is on the outlet or in the HLT, you'll have no overshoot.

That's why you see some people like the temp probe in the HLT and others like it on the outlet. Properly tuned, they both work fine. With it in the HLT you have a more stable reading, and less chance of over shoot. With the probe in the outlet you have a greater chance for overshoot, but that's what the 'D' in the PID is for, it'll aggressively cut the heat when approaching the SP, and aggressively apply heat when falling towards the SP. The outlet reading obviously gives you a more accurate wort temp reading.

So there it is. Either placement works fine, all the eHERMS brewers out there are evidence to it. Personally i use a small HEX for my HERMS(1.8gal), and I monitor the outlet temp, but my HEX can quickly react to temp changes.

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Old 11-18-2011, 07:01 AM   #10
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Ok I did not intent this thread to turn into a debate on the proper or recomended location for monitoring the mash temp, lol.

I was basically trying to see if anyone had any experience using two different RTD's on a single PID by using a dpdt or tptt switch. I checked out my RTD and it has three wires coming out of it. Can one use two RTD's on one PID and use a switch to change which RTD feeds info to the PID? If so which wires have to be feed tot he PID, I am assuming the two reds and the white is ground on the RTD?

Mark

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