PDA

View Full Version : OT- Taking Temperature of a Pipe



rws
04-27-2018, 05:17 PM
Checking the charge on a heat pump using sub cooling, you have to read the temp of the high pressure line at the condenser. Of course contractors use contact digital multi meters, clamp on devices, etc. My question is would I get a good enough reading using a pocket probe type thermometer, by laying the probe right on the tubing and wrapping the probe and tube with insulation and taping it closed? Between the contact and the environment of the probe should let the probe do it's job, right? Thoughts?

Machine
04-27-2018, 05:23 PM
I'm not an HVAC tech, but I have worked with temperature probes on industrial facilities before (fuel distro systems). As long as the probe and the instrument the probe is connected to are properly calibrated, and you give the sensor enough time to equilibriate before taking a reading, I think what you described sounds fine.

flylo
04-27-2018, 05:23 PM
Would a laser indicator work?

boslab
04-27-2018, 05:28 PM
Dab of heat transfer grease, tape probe. It does work,
Mark

deltap
04-27-2018, 05:57 PM
Non contact laser. I have seen them for as little as $20. Spray flat black paint on area to be tested. No need for insulation. Use gage pressure converted to temperature or shoot return bend about 1/3 of the way down from top of condenser and on liquid line where it exits unit. Subtract for subcooling.

mixdenny
04-27-2018, 11:57 PM
The non contacting guns work great for things like this. One quibble, a laser has nothing to do with reading temperatures with any of them. Just indicates where you are pointing and the real spot is much larger then that. You need to read the specs to see how large the spot is. On a typical 12 to 1, if you are 12" away you are measuring a one inch circle and averaging the reading.

Dennis

J Tiers
04-28-2018, 12:06 AM
If the "probe" is a thermocouple, lay it along the pipe so the wire does not heatsink it as badly. Then make sure the junction is contacting the pipe, and tape will insulate it somewhat so it does not lose heat to the air

rws
04-28-2018, 08:34 AM
Dab of heat transfer grease, tape probe. It does work,
Mark

What is heat transfer grease?

Machine
04-28-2018, 09:45 AM
Not sure what level of accuracy you need, but the cheap laser detectors typically do not have the accuracy and precision a dedicated thermocouple is capable of.

lakeside53
04-28-2018, 11:33 AM
They are actually hopeless unless you calibrate for the emissivity of different materials/surface conditions. An earlier poster had the right idea - put a matt black patch on the pipe. Even that though requires calibration, but at least the readings will consistently relative.

Evenly heat a cast iron plate that has one part "shiny" and the other "matt'. Sample each and note the difference.

I inserted a cylinder sleeve into a big compressor block. The block had to be heated evenly to 450F. IR measured differences between machined and rough surfaces was over 70F! My Fluke with thermocouple showed consistent readings.

lakeside53
04-28-2018, 11:34 AM
What is heat transfer grease?

Typically a silicone product that removes air gaps and conducts well. Used extensively in electronics and commonly available.

boslab
04-28-2018, 12:08 PM
Thank you lakeside, I was just going to answer, white heat transfer grease is used (as I just did) under the thermistors on combination boilers over here, also heat sinks on pcs thermocouples embedded in heated platten presses, I don’t know what’s in it but it does remind me of lithium grease ( calms down plumbers)
If your stuck anti seize copper grease works but not as well I’m told though I don’t see a lot of difference
Mark

rws
04-29-2018, 04:15 AM
I like this grease idea, I'll search for some. Makes sense!

J Tiers
04-29-2018, 11:14 AM
If you want accuracy, the grease is a good idea , but also lay the wire along the pipe for 50 mm or so and tie it down. That tends to put heat into the wire, and keep it from acting as a heatsink to cool the junction. Grease is very helpful, but not perfect.

If the counterexample is swaged in a hole, or the like, then the wire is less of a problem, because the contact is much better.