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

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.

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.

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.

canyonbrewer said:

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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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.

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.

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.

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.

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.

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

Yes, you could switch between two RTDs on one PID. If you do care about your HLT temps, just put a single RTD in the HLT and call it a day, this plan sounds rather complicated.

Bachhus said:

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

Click to expand...

RTDs have three wires so that they can automatically compensate for the resistance added by lead wires, connectors, etc, etc.

One carries current out to the probe.

One wire is simply a return of that 1st wire - it does not go through the resistor in the probe - it just makes a U-turn and heads back to the PID.

The third wire is the return of current that actually went through the pobe.

By looking at those first two wires, the PID is able to compensate for the voltage drop that is caused by the leads and connectors.

I think, ideally, you would want to switch all three wires, but you might be able to get away with only switching 2.

The real issue is (as someone mentioned earlier) the fact that every probe has some amount of static error to it. With a single probe, you can just prigram the PID to account for that error and forget about it.

If you calibrate the PID for probe A, then probe B will have error on it when you switch to it. If you know the error, or if the error is small, you can probably just ignore it or manually compensate in your head.

I've had three different RTDs: cheap one I got at first that broke, good one that I use today, and another in my smoker. All three of those have different errors. One was -2*F, one was +1.5*F, and the other was +4*F.

I use my brewery control panel to control my smoker, and I manually swap which probe is connected to the control panel, which is like what your switch will be doing basically. I do not adjust the PID when I am using the smoker.... I just know that I have to set the PID to a temp of 216 if I want to smoke something at 220.

Thanks guys, I had sent a request to Auber asking if this could be done and here was there response, just FYI.

"The probe you have has 3 wires. If you are going to use two identical probes, you can just switch the white line. Otherwise, you'd better to switch the two red lines. The controller measures the resistance of the RTD sensor to determine the temperature, you need to make sure the resistance of the switch is less than 0.2 ohm. Otherwise, the measurement would not be accurate. 0.2 ohm resistance will cause 1F jump."

Sounds pretty easy honestly.

Mark

I'm sorry to repeat this again, but all of the probes come with a different degree of innaccuracy and you can only compensate for this on the PID for one of the probes, not both, so one will be accurate after you adjust the PID for it and the other will likely be off by a few degrees regardless of how the switch is done. If you don't mind compensating in your head for the one that's not calibrated then it's no big deal, but I just wanted to make sure that was understood.

Though they do mention 'identical probes'. They don't have matched probes do they, sort of like stereo microphones?

Bachhus said:

Thanks guys, I had sent a request to Auber asking if this could be done and here was there response, just FYI.

"The probe you have has 3 wires. If you are going to use two identical probes, you can just switch the white line. Otherwise, you'd better to switch the two red lines. The controller measures the resistance of the RTD sensor to determine the temperature, you need to make sure the resistance of the switch is less than 0.2 ohm. Otherwise, the measurement would not be accurate. 0.2 ohm resistance will cause 1F jump."

Sounds pretty easy honestly.

Mark

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I don't agree with their advice there w/ regards to only switching the white wire. The compensation part of the RTD will be all messed up because two different probes with different leads on them would be reporting back compensation info to the PID.

I think you could switch just one of the red wires, but you would need to make sure it was the red wire that was the actual current source on the PID.

That way you only send current out to one probe, so only one U-turn wire would be carrying compensation info back to the PID and only one white wire would be carrying return current back to the PID.

In regards to the resistance thing, I could be wrong (probably am), but it sounded like the PID uses the three wires to measure the resistance and adjust automatically.

I think if I go this route I will use a dpdt switch and switch both red wires. Problem I'm having is finding a 22mm rotary two position switch that matches the other ebay switches I have. I have a hard time reading the description the manufacturers are putting on the switches. Looks to me that they are all N/O-N/C.

Does anyone see any DPDT style 22mm rotary switches that would work for this?

Thanks
Mark

Bachhus said:

In regards to the resistance thing, I could be wrong (probably am), but it sounded like the PID uses the three wires to measure the resistance and adjust automatically.

I think if I go this route I will use a dpdt switch and switch both red wires. Problem I'm having is finding a 22mm rotary two position switch that matches the other ebay switches I have. I have a hard time reading the description the manufacturers are putting on the switches. Looks to me that they are all N/O-N/C.

Does anyone see any DPDT style 22mm rotary switches that would work for this?

Thanks
Mark

Click to expand...

Just use an on/off switch to pick and drop a relay.

cl330b said:

Just use an on/off switch to pick and drop a relay.

Click to expand...

You can use the n/o contacts for one sensor and the n/c contacts for the other. Just wire the moveable side of the relay to the PID.

Thanks, i was wondering if the rotory switches I picked up would work. I believe I purchased a couple extra of both the two position and three position ones so I should be able to come up with something that will match the rest of the pannel.

Mark