Better thermocouple sample readings

Discussion of the physical aspects
Post Reply
ocluke
Posts: 10
Joined: Fri Mar 09, 2012 12:21 pm
Bot?: No

Better thermocouple sample readings

Post by ocluke »

I've attempted to find this answer out myself through the wiki, but am still having a hard time figuring it out. Any assistance would be helpful.

I measure the temperature inside my cellar fridge by having the thermocouple inside a 500ml water bottle full of Starsan sanitizer. For whatever reason, the temperature measurements seem to swing about 4-5 degrees up and down in any given 5-10 seconds. I can't imagine that the water flowing over the tip (which is roughly in the middle of the water, not on the edge) would actually fluctuate that much, but maybe that is the case. Or, maybe it's due to the 30 ft extension of 24 AWG CL2 shielded wire I added between the thermoprobe and the BCS.

What I'm looking for is a way to get an "average" reading over, say 15-20 seconds, rather than a constant reading. That way the fridge doesn't constantly kick on and off and waste electricity based on a split second low reading. Is this possible? I've read the Output PWM Control page, but am still unclear about the exact way to go about this. I've changed the hysteresis to 4 degrees to help account for some of the erratic swing, but I'd rather turn that down to 2 or 3 degrees and adjust other parameters to get a more "average" reading.

I would also like it to be that when the fridge does turn on, it will stay on for a minimum amount of time (say, 3 minutes), regardless of the temperature swing.
JonW
Site Admin
Posts: 1726
Joined: Sun Jul 18, 2010 7:51 am
Bot?: No
Location: Huntington Beach, CA
Contact:

Re: Better thermocouple sample readings

Post by JonW »

Is this the only temp probe experiencing this type of fluctuation or are other probes doing this as well?

If it is only one probe, then swap with another probe to see if the problem follows the probe.

Do you have a jumper run from the BCS ground output to earth ground? (Same as your AC ground).

Longer cable runs on the temp probes have shown to cause temperature fluctuations and grounding the AC & DC systems together has solved the issue.
ocluke
Posts: 10
Joined: Fri Mar 09, 2012 12:21 pm
Bot?: No

Re: Better thermocouple sample readings

Post by ocluke »

Is this the only temp probe experiencing this type of fluctuation or are other probes doing this as well?
I get fairly good swings on my brew rig as well, but it's the only fridge one with this sort of a swing. It's also the only fridge probe with a cable extension.
If it is only one probe, then swap with another probe to see if the problem follows the probe.
This is a hard to get to temp probe (had to be run up the wall of the fridge), so it's a bit of work to swap it out. I'll try this, but I'd prefer it to be a last resort.
Do you have a jumper run from the BCS ground output to earth ground? (Same as your AC ground).
Yes, there's a jumper from my BCS ground to the SSR ground.
JonW
Site Admin
Posts: 1726
Joined: Sun Jul 18, 2010 7:51 am
Bot?: No
Location: Huntington Beach, CA
Contact:

Re: Better thermocouple sample readings

Post by JonW »

ocluke wrote:
Is this the only temp probe experiencing this type of fluctuation or are other probes doing this as well?
I get fairly good swings on my brew rig as well, but it's the only fridge one with this sort of a swing. It's also the only fridge probe with a cable extension.
If it is only one probe, then swap with another probe to see if the problem follows the probe.
This is a hard to get to temp probe (had to be run up the wall of the fridge), so it's a bit of work to swap it out. I'll try this, but I'd prefer it to be a last resort.
Do you have a jumper run from the BCS ground output to earth ground? (Same as your AC ground).
Yes, there's a jumper from my BCS ground to the SSR ground.
It's not a jumper to the SSR - it is a jumper from the BCS DC ground to the AC ground. This is likely your problem.
You don't need an average reading over 15-20 seconds because the temperature doesn't swing like that over 15-20 seconds. If you fix the grounding issue, the temperature probe will stabilize the temp.

Also, using differential/hysteresis uses a temperature differential, not a time duration. Once you fix the temp flucuations, using a 1 degree differential for the hysteresis should be at least a couple of minutes. If you still think it is short cycling, change it to 2 degrees. If this doesn't meet your needs, then create your own temperature and time logic using multiple states - you can do it any way you like then.

Bottom line - put the ground jumper in place to fix the temp swings and your other issues go away.
Post Reply