PID control TC placement

OK, so I have what should be a fairly simple question. Of all the HERMS systems I've seen it seems that everyone places the TC for the PID temp. controller in the HLT is there some reason I'm missing that it isn't in the MLT in a constant recirc system? It would seem to me to make more sense to place it here as this is the temp. that is of concern.

I believe the reason you are looking for is that the temp of the HLT will be higher then what you want you wort to be. The heat transfer will have some loss along the way. So you will want the PID to hold a temp higher then what you will be reading in the MLT.

When I am maintaining 155 in my MLT. I keep the water in my HLT about 165. I have a bit of a long copper run and some silicon between my HLT and MLT so I get a bit more loss then I think most might.

I also have a TC in my MLT just for monitoring the temp.

Thank you for the response. I understand that there will be a difference in temperature between the HLT and MLT but, if the TC is placed in the MLT and the mash is recirculated during the whole process wouldn't it (with the proper SouF and Hy settings) take the guesswork out temp control. As I understand it in a system with the TC in the HLT you set HLT temp to some value above what you want the MLT to be. In your example 165F for a mash temp of 155F to account for thermal losses both to atmosphere in your plumbing and something less than 100% heat transfer in the HERMs coil. Where as in a system with the TC in the MLT the controller would do that for you (assuming of course the proper settings to minimize overshoot). I just didn't know if there was a process specific reason for not doing this.

How would you get your mash water to the correct temp before mash in if you don't have the TC in the HLT? Will you direct fire your MLT?

No, the PID would still control the SSR->heating element in the HLT but , I would pre-fill and recirculated the mash water to 165F or whatever I found nec. for dough in to acheive 150-155F after the room temp grains are added. Just so we are on the same page I haven't built this stand yet (still finishing a Reverse flow off-set firebox smoker before I start the brewstand) but have almost all the components either on hand or in the air to me. I'm just trying to get all of my concerns addressed before I start building to speed things along as much possible = brewing again sooner.

Yeah I guess that would be fine then.

I think you will find that it might be more cumbersome doing that way instead of just directly heating and then transferring. I usually just set and forget while doing other thing. You will still need to do a calculation to compensate for grain temps so there would not be much advantage to getting the temp in the mlt exact. You can just compensate for the extra loss from transfer.

It sounds to me like you should consider a RIMS system rather then a HERMS. Then your TC will be in the RIMS tube and you will be getting the temps you want at output.

If you do put the TC in the MLT I am interested in hearing how well it works.

Each of my 3 kegs are going to have couplings with sight glasses and either a brewmometer or the TC for the PID so moving it really isnt an issue. With that said I will probably try it both ways and see which seems to work out better.

Hi CW, I'm building a HERMS and had similar questions. Digging through the various posts on this subject, it's my impression that measuring the temp in the MT to control temp in the HLT creates feedback problems because the HLT temp is likely to overshoot while the MT is coming up to temp. In other words, if you're shooting for 153 in the MT, the HLT is going to keep right on heating up until the MT reaches that temp. This may mean that you have to stop recirculating through the HEX to avoid having the MT go above 153. That, of course, would defeat the purpose of the HERMS (I've seen systems where there's a cutoff valve so you can recirculate without going through the HEX, but apparently you don't need this if you have the TC in the HLT). Anyway, these are my impressions. Others may have other, better, smarter explanations!

My HERMS uses an RTD (I've used TCs and RTDs and prefer RTDs) in the HLT. My HERMS continually recirculates. There are a lot of people doing HERMS and only recirculate to maintain temps, then stop when mash temp is where they want it. I like the continuous recirculation, mainly because that's how I've always done it, and it's very hands-off on brew day. You could have a PID maintain HLT temp well above mash temp (168F maybe?) and have another PID or love controller control the pump that moves wort through the HERMS coil. You'd just want to make sure it has a long cycle time so that you're not powering the pump on and off excessively.

I keep my HLT about 4F over target mash temperature to maintain, and that works well for me. The challenge is ramping quickly. If you're willing to take the HLT to 175 or 180 and have decent flow through the grain bed you could ramp pretty quick, but then you need to add cold water to the HLT to bring it back down to sparge temps when you're ready to sparge. You'd also be raising portions of the wort to those temps, and I don't know enough about the science of brewing to know what effect that has. Denaturing enzymes is beyond my current knowledge . Then again, I also don't really understand consequences from a slow ramp. I'm just glad to be brewing again after a 3 month hiatus, and my first 20 gallons are fermenting nicely.

Good luck bubblehead brother!

Corgi, that very well may be the case but I would think that could be adjusted out. If not I'll just do it the way everyone else does and be happy with that. Just some of the questions bouncing around my head as I ponder this build.

I hear ya! One of the best things about planning/building these systems is that there's no end of possibilities, and no absolutely right or wrong way to do things. Hey, it's an adventure!