Temperature difference: Air vs. Thermocouple

I have a NIST-traceable Fluke 289 multimeter and thermocouple. The other day, I was measuring some unrelated temperatures and decided to check out the temperature environment near my AIR. Relative to the thermocouple, the AIR was reading 5-6 degrees high. The thermocouple tip was placed within inches of the AIR, and the entire thermocoupld/multimeter ensemble was in the same shaded environment.

Understandably, some may think the calibration on the thermocouple is off, but it agrees closely with other known temperature sources and thermometers.

Anyone have any thoughts as to why such a big difference?

Check temp readings early in the morning right before sun comes up.
If temps are close then, your air is being affected by solar radiation from
somewhere even if its in shade. Maybe a nearby fence, wall or the ground.

But if the probe was inches away from the AIR, I would think that the same radiation effects would be in play, or at least in play enough to not be measuring such a significant difference.

Yes, the probe will pick up radiated heat while standing in the shade.
The air unit has a lot of mass so if the temp was rising or falling it could
be lagging behind. You need to verify both read the same in a steady state
environment. I have a digital instant read temp hum sensor that reads exact
as my air in a steady environment, and use it to spot check my air readings.

why not put a normal thermometer for an hour next to the air as a third opinion

Hi Sunny, its an obvious suggestion…and assuming the calibration of the thermometer is satisfactory, it’s a good suggestion. I just don’t happen to own one. :slight_smile:

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if you trust the Fluke, stick it outside to catch up with the location and any radiant heat for a day, then turn it on and check it. You need to let the Fluke catch up to ambient as the Air sees it.