Can I run 2 different PIDs from one thermocouple?

[MonkeyWrench]

Looking at making myself a HLT-Less RIMS system like this one: https://www.homebrewtalk.com/f51/rims-build-129646/. I think I'll be going with a single LD 1500w element in one end of the manifold, and a HD 5500w element in the other end for sparging. My tap water is 48* right now and gets colder in the dead of winter so I am pretty sure I need this type of setup for my application.

Can I run just one thermocouple on the outlet to operate both PIDs, or will I need two different ones? I know I could always run a panel mount and swap the thermocouple from the 1500w PID to the 5500w PID, but I want to plan to be able to run both elements at the same time.

Thanks,
Steve

 

[PintOfBitter]

The parallel input impedances of the two PIDs would probably cause each PID to read the potential across the thermocouple as half its true value. (assuming the PID units are identical) Just a common sense guess based upon my limited knowledge of thermocouple circuits. You probably want two thermocouples.

EDIT:
Now I'm second guessing myself. The above would be true if the thermocouple behaved like a current source, but I believe it is closer to a hypothetically ideal voltage source. Try it and find out? Easy to add another thermocouple!

 

[MonkeyWrench]

Ya, I'm not sure how these work, but if they are like a NTC thermistor, they run off of resistance, so ya, if you run them in parallel, you'd only be reading half of what it actually is.

I just don't know enough about PIDs on whether you can make it learn the temperature at a given resistance, say freezing and boiling and it would know the rest.

I'm just looking at parts to build the manifold and wanted to price in an extra fitting for another probe or not. I suppose I'll just add one for simplicity sake.

Steve

 

[leboeuf]

Each pid will need it's own thermocouple placed in its area of control. If they're both wired to the same thermo there will be an instrument in the system that is out of each pid's control...

Basically it may work, but is severely unadvisable... If you have access to matlab/labview/etc you could simulate it and hit us back with the results

 

[OhioBrewingTechnologies]

I think you could do this IF the PIDs use isolated inputs. The thermocouple is a voltage source. Consider the following:

1. Use thermocouple wire, not copper wire, and keep the polarity correct when wiring between them. Thermocouples sense temperature DIFFERENCE, not absolute temperature. The PIDs will have absolute sensors where the thermocouple wire meets the terminals.

2. If the PIDs put a small periodic current through the thermocouple to help detect disconnection, that's a problem if one sources current while the other is measuring temperature. Don't know how they actually do it so this could be BS.

 

[Want2Brew]

Could you get away with using one PID and one thermocouple and just switch the out put of the PID between the two SCR's for the heating elements? Then you could have a 3 position switch set up like this, (5500/both/1500)

 

[MonkeyWrench]

Well, sounds like more work (if it will even work) than needed. I'll just get a cross fitting and install a sensor on each side and the liquid path will go straight through.

I was actually trying to simplify things, not complicate them

Thanks for the help. This will be my first adventure using PIDs, should be fun.