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2 PID-s controlling 1 SSR

 
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draj




Joined: 31 Jan 2021
Posts: 3



PostLink    Posted: Sun Jan 31, 2021 11:03 pm    Post subject: 2 PID-s controlling 1 SSR Reply with quote


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Hello,

I am designing an electric HERMS system with 3 PID controllers showing the temperatures of 1. HLT, 2. MT inlet from herms coil and 3. BK. I think it would make sense to make it possible to be able to switch control of the HLT heating element between the 1. HLT and 2. MT inlet PID controllers depending on the current brewing step.
My question, is it possible to use a contactor to disconnect the +12V DC linput ine of the SSR from one PID controller and connect it to another. Would it be okay for the PID controller, or it would detect it as an error or sg?
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kal
Forum Administrator



Joined: 12 Dec 2010
Posts: 11116
Location: Ottawa, Canada

Drinking: Pub Ale, Electric Creamsicle, Mild, Pliny the Younger, Belgian Dark Strong, Weizen, Russian Imperial Stout, Black Butte Porter


PostLink    Posted: Sun Jan 31, 2021 11:48 pm    Post subject: Reply with quote

Hi and welcome to the forum!

Don't measure the HT inlet from the HERMS coil. On a correctly designed setup this HERMS coil output temp will measure exactly the same temp as the HLT. It's redundant. I did this myself at first to confirm things. I recommend measuring the MLT output as that tells you when the entire grain bed has reached the target temp you set by the HLT. This is the design that is featured on our website build: https://shop.theelectricbrewery.com/pages/building-your-brewery

>> I think it would make sense to make it possible to be able to switch control of the HLT heating element between the 1. HLT and 2. MT inlet PID controllers depending on the current brewing step.

How and what do you mean exactly? The MT inlet doesn't have a heating element. So what are you controlling exactly?

Either way, for what it's worth, the heating system uses 3 parts: Temp probe to measure, heating element to heat, and PID to read the temp probe and tell the heating element when to fire. Every temp probe has slightly different offsets so you don't want to switch between 2 temp probes for 1 PID as it won't be accurate. You also don't want to switch between different heating elements/kettles as that'll confuse the PID. It learns the behaviour of the setup and if you switch between vessels i'll make it less accurate as every heating setup reacts differently.

Cheers!

Kal

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draj




Joined: 31 Jan 2021
Posts: 3



PostLink    Posted: Mon Feb 01, 2021 10:22 am    Post subject: Reply with quote

kal wrote:
Don't measure the HT inlet from the HERMS coil. On a correctly designed setup this HERMS coil output temp will measure exactly the same temp as the HLT. It's redundant.

Thanks this is a good point!

The idea would be to switch the control input of the SSR between 2 PID-s. In this case the PID-s would have their own temp probes with their own PID parameters. I think the benefit would be that one could just switch the control and forget about to take the temperature offset in calculation between the HLT and MLT.
For e.g. before mashing in, to heat the strike water HLT PID would be used to control the SSR. After the mash in the MLT PID controller would be used to keep precise temperature for recirculation.

Please see the addition of the red switch and lines on MLT pin7 and pin8:



PIDs-mod.jpg
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kal
Forum Administrator



Joined: 12 Dec 2010
Posts: 11116
Location: Ottawa, Canada

Drinking: Pub Ale, Electric Creamsicle, Mild, Pliny the Younger, Belgian Dark Strong, Weizen, Russian Imperial Stout, Black Butte Porter


PostLink    Posted: Mon Feb 01, 2021 1:13 pm    Post subject: Reply with quote

draj wrote:
The idea would be to switch the control input of the SSR between 2 PID-s. In this case the PID-s would have their own temp probes with their own PID parameters. I think the benefit would be that one could just switch the control and forget about to take the temperature offset in calculation between the HLT and MLT.
For e.g. before mashing in, to heat the strike water HLT PID would be used to control the SSR. After the mash in the MLT PID controller would be used to keep precise temperature for recirculation.

Ah! I get it now. This is a question that does comes up from time to time...

Using the Mash PID and temperature probe in the mash to control the HLT element which indirectly controls the mash temperature would be harder to do reliably as the design documented on this website as you make the system more decoupled, the heating and feedback are more indirect. There would likely be more lag/inconsistency. There are greater risks as well.

For example, you could never turn off the mash pump as the mash PID would then start running the HLT element hard because of the temperature drop it would sense, but it would never see the resulting heating since the mash pump is off, so the HLT water temperature would just keep climbing and climbing. So you'd have to make sure to always turn off the HLT heating element too whenever you turn off the mash pump to say, mash in. If you forgot the HLT water temperature would go through the roof and as soon as you turn the mash pump back on the mash temp would jump up past your target as the HLT water would have been overheated. A simply oversight like this could easily cook your batch into a tannic mess. The same would happen if you had flow issues or ran the pump slower when first mashing in. The PID would drive the element harder to compensate which may result in temperature overshoots in the HLT. A stuck mash that you don't notice would eventually result in boiling water in the HLT (a dangerous situation). None of these things are an issue with the system as designed today.

You may also have inconsistency in heating between batches due to the different amount of grain which could result in over/undershooting temperature. The different amount of grain and flow rate means the the system 'dampening factor' becomes different in every batch. With the system as designed today the only thing that varies batch to batch is how LONG it takes for the mash to reach the set temperature. The system cannot overshoot temperature as the closed loop (heating element/temp probe/PID) are closely coupled. When using the system as designed, I recommend brewers start with the same amount of water every time (twice the finished beer amount) so while the amount of strike/sparge water will vary, the heating will be more consistent from batch to batch as the tuning between HLT PID, temp probe, and heating element works well. This is because the PID always sees the same thing and the HLT pump is never turned off and the flow is very consistent while we're in heating/mashing mode.

Long story short: It's always more reliable to not decouple the heat source from the temperature probe on any closed loop system more than you have to. That said, this is a complex subject and gets into engineering / process control - feel free to experiment and see how it works doing things differently of course!

The HLT to MLT temperature lag is also a very common issue on some setups. While it won't affect anything (just set the HLT higher to compensate), some people want to get rid of the difference.

Using parts that are different from the ones I recommend in our build instructions can cause this. To minimize the difference, make sure to:

- Calibrate both the MLT and HLT temp probes per the instructions here: http://www.theelectricbrewery.com/control-panel-setup
Unless both are calibrated with a known accurate thermometer at the mash temp, you don’t know if what you are seeing is accurate.
- Make sure to set up the PIDs per the instructions here: http://www.theelectricbrewery.com/control-panel-setup
- Use the thicker walled hoses I recommend here: http://www.theelectricbrewery.com/hoses
Thicker hoses will lose less heat. Try to minimize distances as well.
- Set your mill gap crush per the recommendations here (0.045 to 0.050"): http://www.theelectricbrewery.com/grain-mill
You want to mill looser for good flow, not tighter.
- Use the pumps I recommend here to ensure adequate flow: http://www.theelectricbrewery.com/pumps
- Run both pumps at 100% open when mashing. To get good flow without any channeling use the kettles I recommend here: http://www.theelectricbrewery.com/kettles-overview

For example, users have reported on our forum and by email how they were seeing a temperature differential between the MLT and HLT and were able to make it go away by swapped out their hoses and pumps for what's recommended in the build instructions. A quote from one of them:

"I had originally tested the HERMS with the thinner 1/2 ID silicone and had my pumps setup with the inline heads (long story). I was landing about 1-2 degrees cooler in the mash kettle. I swapped out the pump heads for the front inlet and swapped out the silicone with what you recommend and I now hit the same temp!" - Todd W.

Other things that are sometimes done differently that can cause a differential:

- Make sure the heating elements are wired correctly and being fed 240V and not 120V. Running heating elements at 120V will result in 1/4 the power you expect. You can check this by looking at the amp meter. A 4500W element will draw 18.8 amps while a 5500W element will draw 22.9 amps.
- Brewing in a very cold location may cause the MLT temp to drop slightly by the time it reaches the MLT temp probe.

This question gets asked a lot on the forum too. These threads may provide further hints:

http://www.theelectricbrewery.com/forum/viewtopic.php?t=25474
http://www.theelectricbrewery.com/forum/viewtopic.php?t=26408
http://www.theelectricbrewery.com/forum/viewtopic.php?t=28283
http://www.theelectricbrewery.com/forum/viewtopic.php?t=25474
http://www.theelectricbrewery.com/forum/viewtopic.php?t=30387
http://www.theelectricbrewery.com/forum/viewtopic.php?t=26722
http://www.theelectricbrewery.com/forum/viewtopic.php?t=30603

Experimenting is definitely an important way to learn and ensures you design something perfectly suited for how you like to brew. Good luck and have fun building!

Kal

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draj




Joined: 31 Jan 2021
Posts: 3



PostLink    Posted: Tue Feb 02, 2021 7:22 pm    Post subject: Reply with quote

Thank you Kal for the detailed information.
I stand for simplicity, and in this case I will just aim for minimizing the temp differences between the HLT and MLT.
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kal
Forum Administrator



Joined: 12 Dec 2010
Posts: 11116
Location: Ottawa, Canada

Drinking: Pub Ale, Electric Creamsicle, Mild, Pliny the Younger, Belgian Dark Strong, Weizen, Russian Imperial Stout, Black Butte Porter


PostLink    Posted: Tue Feb 02, 2021 7:26 pm    Post subject: Reply with quote

By all means experiment and try things out - that's part of the fun and how we learn things as every setup is slightly different and people use things differently too. In my first few batches I had my MLT probe on my HLT HERMS coil output but then moved it to the MLT output for the reasons I stated above.... but it was a good test to ensure that my HLT HERMS coil output temp matched my HLT temp, so I confirmed it was impossible to transfer more heat from the HLT water to the mash MLT water going through the coil. So all good!

Kal

_________________
Our new shop with over 150 new products: shop.TheElectricBrewery.com
We ship worldwide and support our products and customers for life.
Purchasing through our affiliate links helps support our site at no extra cost to you. We thank you!
My basement/bar/brewery build 2.0
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