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goybar · 01-02-2013, 01:15 PM

Can you use multiple RTDs sensors on a PID?

Multiple inputs to help get more consistent temperature throughout the mash?

I do BIAB, and recirculate with a pump, I monitor the temperature from the bottom of the kettle (sight glass with Thermometer), and at the wort return (into mash).

I'm currently using propane, but will be going electric once the funds and plan is finalized.

I would like to insert a thermowell into the lid of the BK/MT to get a reading there as well as under the "false bottom" (wire rack today/possibly Bayou Classic basket in future).

Am I just being to overly concerned about consistent mash temps?

As long as I'm recirculating it really shouldn't be an issue?

Should I just RDWHAHB?

Chris

aquenne · 01-02-2013, 02:37 PM

A PID controller will perform best when controlling 1 enviroment. When you run the auto tune to set the P, I and D gains, the controller is measuring the way the system reacts to changes (the "inertia"). The various places were you want to have different sensors would change those auto tune terms.

For the price of a PID controller, I would run seperate ones for each control location.

warthog · 01-02-2013, 09:13 PM

most pid controllers have only one rtd input. if you were to attach multiple sensors, the result would likely be totally messed up. rtd's are resistance based, a bunch of resistors in parallel will not give you the average resistance. 1/r= 1/r1+1/r2, so your resistance is exactly 1/2 the average resistance (since you are thinking about using 2 probes - it would be 1/3 the average with 3 probes, and so on). the resulting temperature will be considerably lower than the actual temperature.

choosing the best location for a probe, is always a challenge (unless you are an alien abductor). in reality, you will have hot spots and cold spots, even if you recirculating. but they will be very small variations. i assume that you are actually recirculating the runnings, (as opposed to running a loop of hot water around your bag) in which case i would suggest that you monitor the temperature of the recirculating wort, as it reenters your BK/MT. you will likely have to apply some offset, to equate the temperature of your returning wort to the average temperature of your mash. we all play around a bit to figure out what the best settings are for our particular brewing rigs.

aquenne · 01-03-2013, 11:58 AM

oh I took what you wanted to do was switch inputs from various sensors.

If your thinking about hooking up multiple RTDs, you will need to create some kind of conditioning circuit to average the resistances. Its do-able, but your probably further ahead, to stick with one sensor.

goybar · 01-03-2013, 12:29 PM

Thanks for the replies.

Warthog, yes I recirculate by taking the wort from the bottom (ball valve) and pumping it back up through the lid. I currently play the game of monitoring both the output and input of the wort and trying to adjust the temperature manually. Going electric and using a PID will in all likely hood be better and more consistent than my curent method.

Aquenne, I was thinking/hoping that the PID might be able to read multiple rtd's at the same time and use its logic and auto tuning to compensate for the differences.

It was just a thought. I'll stick with one RTD...... and try to figure out the best placement...

Chris

**BadNewsBrewery** · 01-03-2013, 03:25 PM

I have my RTD in a tee off the outlet from the tank, before it goes into the pump. It's the same way Kal does it, and in my opinion the best way - you get a steady flow of liquid (no stagnation) and with the pump running your constantly getting a nice mix too.

**jCOSbrew** · 01-03-2013, 07:10 PM

I use a cheap waterproof rtd and stick it right in the middle of the mash. In theory the input or output temp should be just as accurate with good recirc and the correct offset.

If you want to check extra temps an oven or BBQ thermo might work. I have also seen some panel mount thermometers for about $5 on eBay and amazon.

**Shockerengr** · 01-03-2013, 08:25 PM

it's possible to wire 4 RTDs together in a way that averages their readings together. it would appear to the PID as if it was a single RTD, but you'd be getting an average reading.

That would only work if they are all reading the same enviroment however (say 4 different spots of the mash tun)

To wire it, you put two sets of RTDs in series, and then run those two sets in parallel.

This may not be what you're after however.

warthog · 01-03-2013, 08:36 PM

not a bad idea. it should work

goybar · 01-03-2013, 10:41 PM

Quote:

Originally Posted by Shockerengr

it's possible to wire 4 RTDs together in a way that averages their readings together. it would appear to the PID as if it was a single RTD, but you'd be getting an average reading.

That would only work if they are all reading the same enviroment however (say 4 different spots of the mash tun)

To wire it, you put two sets of RTDs in series, and then run those two sets in parallel.

This may not be what you're after however.

That is inline with what I had in mind.

So if it is possible, does it make sense to do it?

Will the results lead to a more efficient, consistent mash, that is repeatable?

Or will it led to a less consistent mash, or end up with a mash that is too high or low than intended?

Is it worth the expense of an additional 3 RTDs?

Anything will be more consistent than my current method, which will no doubt lead to better results.

But I would also like to make the system, with in reason, the best it can be.

Chris