two pids one rtd?

I have a few watlow SD series controllers. One thermowell in the fermenter. I thought this would be fine... but the temp swing in my shed enough to require both heating and cooling in the same day. I want to share the 3-wire pt100 rtds between both pids. Yeh or neh? Would think it's ok, as they are just measuring the I-V relationship, but I'm not sure how the third wire is handled. I use 3-wire because I made my own cables with a pretty long run of doorbell wire....

Thanks...!

I suppose it wouldn't be too hard to test. I could just bridge the connections between the pid inputs....

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I don't know if you can do this. The controller supplies current to the RTD. If you hook up two PIDs to one RTD, they are both going to be trying to provide current.

In theory, it *could* work, but it does mean that you are connecting a "hot" line on two PIDs together. If they are the exact same model of controller, you might be OK.

Might work with an On-Off-On switch to switch the white wire on the RTD from one to the other or off.

StMarcos said:

I have a few watlow SD series controllers. One thermowell in the fermenter. I thought this would be fine... but the temp swing in my shed enough to require both heating and cooling in the same day. I want to share the 3-wire pt100 rtds between both pids. Yeh or neh? Would think it's ok, as they are just measuring the I-V relationship, but I'm not sure how the third wire is handled. I use 3-wire because I made my own cables with a pretty long run of doorbell wire....

Thanks...!

Click to expand...

Don't quote me but I thought the 3rd wire was used to correct for errors introduced from long wire runs.

mattd2 said:

Don't quote me but I thought the 3rd wire was used to correct for errors introduced from long wire runs.

Click to expand...

correct. there are two legs coming from the actual RTD sensor head. from there two connection wires are attached to one of the sensor legs, a third wire to the other. the two wires that are attached together form a loop. measure the resistance of that loop, and you can calculate how much resistance is added to the RTD sensor due to wire legnth (which would otherwise be impossible to measure, as the RTD resistance is variable, obviously).

on 4-wire probes- two are connected, one to each side of the RTD sensor, the other 2 are separate and looped as to isolate the wire legnth compensation measurement from the RTD element completely. this can improve accuracy where you need very fine measurements.


to the orig question- most PID controllers have an 'alarm output', in addition to the SSR trigger. the alarm can be set to go off when the temperature drops too much (when in heating mode, or raises too much when in cooling mode). you can use the alarm to trigger a "low" SSR, and the main output triggers the "high" SSR. that way you only need one PID to do both.
i would not try to short the ports on two PIDs together unless you built in some backflow protection circuitry like a diode. you could end up frying your controllers.

Get a PID that has both heating and cooling

http://www.auberins.com/index.php?main_page=index&cPath=1&zenid=0214924c08c480570aca3ddecba4a38c

Nah, I prefer three MCs and one DJ...

As far as I can tell, the pids I have can only be used as a control or alarm output, but not both at the same time. They're Watlow SD31-HCAA-AAOR. I should have paid the extra 25 bucks to get an extra channel. This is still probably the best bet. I could even sell the two I have now to help fund the transition.

I toyed with the idea of using a combination of nc and no relays to switch between both the pids and the rtds. Just got messy. I can see how my original idea would not work. The pids output 390uA and check the voltage needed to maintain this current, compute the resistance, and thus the T. Having 2 would definitely screw with this ability.

Watlow did get back to me, and it's no surprise that they recommended I get another controller.

It is odd that there are no separate output and alarm connections. I can't imagine why someone would use a fully functional pid just as an alarm, but then again I've just been using the on-off function.

Any room to stick another RTD in the thermowell. Or just attached to the outside of the fermenter, not the best way but you could get it to work with an offset to take into account the diference between the inside and shell temp of the fermenter.

No room in the thermowell. Both the rtd and the thermowell are threaded. I think I will test the 'tape on the outside' tek. If I can get the rtd on the outside to match the one in the thermowell, I'll only need to get another ssr. A lot cheaper than another controller! Not nearly as elegant, unfortunately.

My next biggest hurdle is dealing with all the condensation dripping onto the fermenter valves and all over the floor. It's not insulated or isolated from the humidity in my shed.

I taped one to the outside and covered with some pipe insulation. It actually responds to the chill cycle faster than the thermowell, by about 0.1-0.2F. That's actually about how far apart the two of them read in comparison with one another. I need to pack some more Cu mesh in the thermowell... as it is now, the rtd is just floating in there.

I ordered some flexible heating mat from Omega. 2.5w/in^2 and got about 100in^2. Also snagged another ssr.

http://www.omega.com/ppt/pptsc.asp?ref=SRMU_HEATER&Nav=heam01

the alarm and control relay should work in tandem. otherwise whats the point of having an alarm if it wouldnt work to tell you when the control relay wasnt functioning?

ive never seen a controller that was either relay, or alarm, not both.

do you have a link to your specific controller?

http://www.instrumart.com/products/18977/watlow-sd31-pid-temperature-controller

There's a .pdf manual on the lower right of that page.

It specifically an SD31-HCAA-AAOR

There is only one output, that you select as heat, cool, process alarm, or deviation alarm. There are spare terminals on the back (3 and 4) for the 2nd output, but I'm guessing that they aren't doing anything because I didn't get the model with 2 outputs. Terminal 1,2 are for input power, and 5,6 are for the switched DC output that I have going to an ssr.

Cheers....

Got an extra ssr and one of those heat mats. It's 11x12'' and 330w. The taped on rtd has been reading very close to the thermowell one, although they seem about 2.5F out of whack right now. I'm thinking that the 3wire function is getting screwed up somehow on the thermowell one, as it has about 15' of lead wire, is reading 2.5degF higher, and it's right after I arfed around with the control panel. Maybe jostled something.

Gotta say the heat pad is nice. About 3w/in^2. Got the sticky back and install was a breeze. 60 bux or so.

Awesome. They are 0.1degF apart now. Taping onto stainless does work quite well. Not sure I'd vouch for carboys though.

according to the data sheet, the alarm and mechanical relay are active together. the mechanical relay is on the screw ports, number 5+6. the alarm channel would be on the smaller ports, 5,6 and 7 depending on the alarm (if im looking at the correct model).

look at page 11. "output 1- process" would be where you wire the alarm, "output 2 mechanical relay" or "output 2 solid state relay (page 12)" would be where you place your relay.

Not quite. There is no output 2. With just one output, you have to specify (at ordering - they are different models) mechanical relay, ssr, switched dc, or 5V process. Once you have one (I have the switched DC - it controls an external ssr), you can use the software interface to select heat, cool, or alarm. It's a shame but appears that's the way it is....