Reusing PID for multiple temperature probes

[t_met]

I'm just starting some research on bringing electric/HERMS into my brewery.

One of the ideas I was having was sharing a PID for MT & BK. I would have a disconnect at the temperature probe. The actual temperature probes would stay attached to the MT and BK (one temp probe for each vessel) So when mashing/sparging was done, I would disconnect the temperature probe wire and connect it to the BK's temperature probe.

First of all is this possible? I'm assuming that PID/temperature probes need to be calibrated, but I'm not too concerned on the accuracy of the BK since I'm only using it to tell me when I'm getting close to a boil. I'd like it to alarm when I'm near so I can prevent a boil over.

Any other pitfalls?

 

[shortyjacobs]

The RTD temp probes are usually pretty dead on. At most, it's a linear deflection, (RTD reads 155, real temp is 157), that can either be accomodated by settings in the PID or by you remembering the deflection and accounting for it.

The biggest problem is tuning. PIDs are "smart", and have parameters in them to allow them to hold temp precisely. Most people just "autotune" the PID, which means you let it figure out what the best settings are for what it's controlling.

The PID parameters will be DIFFERENT between MT and BK. You don't really care about holding a precise temp in a BK though, since the only temp you care about in the BK is "boiling"....so just autotune the PID on the MT, and you'd be just fine.

Short answer - No, I don't see any pitfalls.

 

[t_met]

Thanks for the information.
I'm just using the MT & BK as monitoring. I'm using a gas burner BK and a cooler mashtun. I'll be using a different PID for controlling element in the HERMS vessel.

 

[shortyjacobs]

Also, if you don't want to deal with disconnecting/reconnecting probe wires all the time, you can get a three pole, dual throw switch that lets you flip between probes.

http://www.amazon.com/dp/B006LU4HIY/?tag=skimlinks_replacement-20

If you're just using it for monitoring, then no worries, you don't even care about autotuning.