Dual Loop PID

[atoughram]

Mostly a lurker here but have been around for a while. I've also had a RIMS brew stand for over twenty years in one iteration or another. I was commenting on a RIMS thread on Reddit (same username btw) and one of the commenters brought up a dual loop PID, which got me thinking. The commenter mentioned two temperature sensors on a dual PID controller, and I looked the controller up and they do exist but are prohibitively expensive. My current system has a typical RIMS setup where the RIMS tube has an element at the incoming end and a PT100 at the outlet end, all connected to a PID controller. This has always bothered me, because I was really controlling the outlet temperature of the RIMS tube, not the Mash Tun temperature, but the delta is only a few degrees and I've put up with it.



What I was thinking about doing is adding another PID controller, with the PT100 in the mash tun, replacing the thermometer in the picture. The original PID controller has an SCR output, and the new controller has a relay output. I'd wire the original controller SCR positive output wire through the output contacts of the second PID controller in series and then to the SSR. I could set the original PID controller to control the output temp of the RIMS tube to some higher temperature that doesnt denature the enzymes, and set the new PID controller to whatever I wanted the mash tun temp to be. I'm thinking that this would closley control the mash tun temperature, but avoid scorching on the elements if the mash compacted. I've got all the parts but dont want to waste my time in rewiring the thing a few times. Has anyone done this in the past? I cant forsee any pitfalls, but I'd like to hear your thoughts.

 

[doug293cz]

If you know what the temp offset (RIMS out vs. bulk mash) is, and can just add the offset to the RIMS controller setpoint value, what is the benefit of complicating things?

Brew on

 

[atoughram]

True - I do know the delta - it's three degrees. And my only complaint is the flow slows down after the grain bed gets compacted.

 

[doug293cz]

True - I do know the delta - it's three degrees. And my only complaint is the flow slows down after the grain bed gets compacted.

The solution to bed compaction due to recirculation is to split the return flow as described by @Bobby_M in this post.

Brew on

 

[atoughram]

I think I'll try beta glucanase first, thanks for the read!

 

[Bobby_M]

The brewzilla controllers implement a similar function when you add the blu-tooth probe thermometer to the mash. You tell the controller to use the mash probe as the control target but you can limit how far above that target you'll allow the bottom heated area to go (the heat delta or offset, however you want to state it).

Mash Controller (MC) with the probe in the mash tun drain flow or stabbed approximately in the middle could have low current dry contacts.
RIMS Controller (RC) with the probe at the exit of the RIMS tube can have an SSR control output that runs through the above dry contacts before getting to the SSR.

MC setpoint at 150F, set as on/off controller with hysteresis set to 1F.
RC setpoint at 155F, set as a PID with short cycle times.

When MC probe reads a mash temp of 149, it will allow RC to actively heat and will work to heat the RIMS tube output to 155F.
When MC probe matches the setpoint, it will disallow RC to heat.

I'm not sure it's worth it, but that's my stance on 3 vessel systems in general so YMMV.

 

[atoughram]

Thanks Bobby - I may try it and see how it works. It makes sense to me and may be helpful in a step mash configuration. I'm trying Beta Glucanase too, I think that will help flow on the grain bed.

 

[HighVoltageMan!]

I think I'll try beta glucanase first, thanks for the read!

I've mentioned in the past that the key to well working RIM system is a high flow rate.

I designed and built my own RIM system @ 15 years ago around a Sabco mash tun and a Watlow PID. I did not follow convention wisdom at the time and placed a 6" RTD probe into the grain bed. All other systems at the time had the RTD on the outlet of the tun prior to the external heater. I was told placing the probe in the grain bed would cause all kinds of problems.

I have had a few hiccups over the years, but the decision to place the RTD probe into the grain bed turned out very well. The temperature displayed on the Watlow is the the same as the grain bed. Over the years I have check to see if the temperature was uniform and the temperature differential was .5F or less.

Beta glunase is really effective when it comes to preventing a compacted grain bed. The other thing that helps a lot is wet milling of the grain, which prevents the hulls from breaking up during the milling.

I have a March pump that circulates the liquor in the mash and it runs wide open. Because of this, the enzymes are not denatured and there is no scorching in the heater because the high flow keeps the rise of the temperature in the heater to a minimum. The flow is high enough that the grain bed maintains a uniform temperature throughout. No second PID, no offsets needed to have extremely tight control of the mash.

Keep it simple.

 

[wickedbeernut]

Mostly a lurker here but have been around for a while. I've also had a RIMS brew stand for over twenty years in one iteration or another. I was commenting on a RIMS thread on Reddit (same username btw) and one of the commenters brought up a dual loop PID, which got me thinking. The commenter mentioned two temperature sensors on a dual PID controller, and I looked the controller up and they do exist but are prohibitively expensive. My current system has a typical RIMS setup where the RIMS tube has an element at the incoming end and a PT100 at the outlet end, all connected to a PID controller. This has always bothered me, because I was really controlling the outlet temperature of the RIMS tube, not the Mash Tun temperature, but the delta is only a few degrees and I've put up with it.



What I was thinking about doing is adding another PID controller, with the PT100 in the mash tun, replacing the thermometer in the picture. The original PID controller has an SCR output, and the new controller has a relay output. I'd wire the original controller SCR positive output wire through the output contacts of the second PID controller in series and then to the SSR. I could set the original PID controller to control the output temp of the RIMS tube to some higher temperature that doesnt denature the enzymes, and set the new PID controller to whatever I wanted the mash tun temp to be. I'm thinking that this would closley control the mash tun temperature, but avoid scorching on the elements if the mash compacted. I've got all the parts but dont want to waste my time in rewiring the thing a few times. Has anyone done this in the past? I cant forsee any pitfalls, but I'd like to hear your thoughts.

I have somewhat of a "dual PID" controller. The first PID controls the power as a fixed percentage delivered to the RIMS element, similar to a PID-controlled boil kettle. The second PID monitors / controls the temperature based on a probe at the egress of the RIMS tube (in order to avoid scorching). The first PID simply monitors the temperature in the mash tun. I manually adjust the set temperature of the second PID to compensate for temperature loss.

I agree. Ideally, the temperature would be controlled based on a probe in the mash tun with a "scorch detection override" (some delta temperature above the mash tun set temperature taking into account reasonable temperature loss) based on a probe at the egress of the RIMS tube.

You're on the right track.

 

[Thorrak]

The brewzilla controllers implement a similar function when you add the blu-tooth probe thermometer to the mash. You tell the controller to use the mash probe as the control target but you can limit how far above that target you'll allow the bottom heated area to go (the heat delta or offset, however you want to state it).

Mash Controller (MC) with the probe in the mash tun drain flow or stabbed approximately in the middle could have low current dry contacts.
RIMS Controller (RC) with the probe at the exit of the RIMS tube can have an SSR control output that runs through the above dry contacts before getting to the SSR.

MC setpoint at 150F, set as on/off controller with hysteresis set to 1F.
RC setpoint at 155F, set as a PID with short cycle times.

When MC probe reads a mash temp of 149, it will allow RC to actively heat and will work to heat the RIMS tube output to 155F.
When MC probe matches the setpoint, it will disallow RC to heat.

I'm not sure it's worth it, but that's my stance on 3 vessel systems in general so YMMV.

What's your recommendation these days -- 2 vessel or BIAB?

 

[Bobby_M]

What's your recommendation these days -- 2 vessel or BIAB?

Firmly one vessel. I can't justify the extra overhead of a second vessel in terms of upfront cost, footprint, cleaning, and fiddling on brew day.

Being convinced that starting the boil with the clearest of wort is a SLIGHT benefit, one MAY be able to justify a second vessel. I If I were to implement a second vessel, it would only be a temporary wort holding tank and probably mount it up high and install a bottom drain to be able to gravity drain out of it. At the end of the mash, I'd divert the recirculating mash liquor to that holding tank until the liquor started running cloudy. Pull the bag out and give the mashing/boiling kettle a quick rinse out. Then gravity drain the wort back into the boil kettle. The holding tank would have a CIP ball installed permanently so that a quick hot tap water rinse out would be enough after most brew days.

This solution negates having to run complicated second heating elements via RIMS or dedicated HERMS vessels, etc. You still make most of the mess in a single kettle, One element, one probe, very precise temp control.