cooler thermostat help

Karl sez:

". . . So, can a thermistor probe cable be lengthened?. . ."

I'm fairly certain thermistor probe cables can be lengthened; not exactly my baliwick, though. Do some reading in Wikipedia and etc.

Bob Swinney

So, I'm looking for a better thermostat. First I looked at an electronic thermostat - item E on the same Mcmaster page 603. I could move it about four foot down the wall as it has a longer bulb length. That would put it

Then I noticed item 331K21 on page 544. If I could lengthen the cables from

Karl

Thermistor temp probes can definitely be lengthened within reason and just need a little recalibration perhaps.

If you want to do things the correct industrial way, you use a real industrial temperature controller like 38615K13 on page 544 with a thermocouple type sensor (pick a type J on page 538) you can readily locate anywhere (type J extension on page 542)

Yea, you can roll your own, and McMaster has TC wire as well. I expect the OP just want's to get the job done, and not practice TIG welding his own thermocouples however.

Pete C.

"Karl Townsend" wrote: (clip) My old tired eyes almost can't see the dial anymore, trying to look through bifocals to the ceiling while holding a flashlight (clip) ^^^^^^^^^^^^^^^^^

"Remote sensor thermostat". I got one last fall for the apartments - the sensor is in one apartment on the 2nd floor and the thermostat is in the basement. Just ran ordinary wire between them. Don't remember the model, but it is Honeywell & I got it on eBay. IIRC, it was a model that could be either internal or remote sensor, switch or option selected.

Bob

Help me out with thermocouple vs. thermister.

My understanding, thermocouples are just a joint of two dissimilar metals. can measure temperature over a large range, but can't control accurately to a small range of control. I'm after +/- 1 degree F or better

Thermistors are a temperature sensitive resistor with less range and more accuracy, and can do +/- 1 degree control.

The only thermistor controller in McmasterCarr is the one I'm asking about.

Karl

Yeah, but Karl's son is one of those TIG welder guys that practices on beer cans. For what he wants to do, welding his own seems like the best choice.

I didn't find the TC wire in McMaster, used the Omega site since it had a good writeup.

I've always used 'K' wire, you menti> RoyJ wrote:

Certainly an option. Save a few $ in TC connectors.

McMaster page 541, bottom.

No particular reason, just first in the list of supported TC types for the controller.

Exact opposite, thermocouples are much more accurate than thermisters. Thermisters are cheaper than thermocouples, and good enough for consumer stuff like on PC cooling fans.

Additionally the autotuning PID controller referenced will learn the response times of your refrigeration system and tune itself for very tight control.

J t/c's come on a lot of stuff because they're cheaper. They work fine, but don't use one around moisture or corrosive environments. The iron (J = iron/constantan) will corrode and fail. Actually, when I need as much accuracy as I can get, I use RTD's. Most temp controllers support them, and the signal is much more robust than the t/c millivolt signal. I have them assembled with Brad Harrison Nanochange connectors by Sandelius in Houston. That makes it easy to swap in a spare. The normal four prong connector made from two t/c connectors of copper is too big and clunky.

I still use J's when they come installed by the OEM. But when I use them, I calibrate the entire loop with a dry block tester at least once per year. I have found that J's will drift significantly over a period of years in a temperature cycling application, like an oven or an extruder that doesn't run continuously.

Pete Keillor

There are standard thermostats with a capillary tube and a remote sensing bulb. Generally, they con'd come with VERY long tubes, but 6 feet is no problem. You should be able to put the part with the dial in a more convenient place. You should be able to find these at a refrigeration supply outfit.

Jon

Make sure you're dealing with a thermistor, not a thermocouple. Thermistors are "thermal resistors" and can use regular copper wire for hook up. Thermocouples use two different types of materials and require the same pairing for hookup wire. One causes a varying voltage drop to an applied voltage, the other generates a (small) varying voltage. Temperature controls can be designed to use one or the other, but they don't substitute types without some redesign. With a thermistor, you may have to adjust for the voltage drop of the additional wire, the temperature control may think it's cooler(or warmer) than it really is because of the additional resistance, depends on the coefficient of the thermistor, positive or negative.

Stan

I used the Penn A419 (1760K77) on a Sauna where nothing else worked, too cool for an oven stat, too hot for a room... It's fine except when one of the residents tries messing with anything other than the setpoint - it's either totally locked or totally unlocked, no 'setpoint change only' option... Then I have to go and reset it.

They have factory longer lead thermistor sensors available. Put it in conduit for damage protection. And the Penn has nifty features - all the setpoints and timing is programmable...

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BINGO! I'm orderring two on Monday. McMaster doesn't list the long lead but says they are available. Can you suggest a vendor to just order from?

Karl

You can use the thermistor that comes with it and splice on some more wire - ISTR that there's a gauge restriction, but not onerous - needs 18 Ga or 16 Ga copper thermostat wire.

Might call for twisted, but that's used as speaker wire for home theatre pre-installs. Available at The Borg.

If McMaster gives you a long lead time, try a local refrigeration house - Johnstone Supply is national, and has locations everywhere.

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