The Temperature measurement Conundrum

There is a very interesting quote by Prof. Bruce Bugbee(founder of Apogee Instruments), in one of his many online videos available on the internet - saying something like this -

“If you asked me in high-school - how easily and accurately can we measure air temperature ? I would have said, that’s a trivial measurement - get a air temperature thermometer and measure it.

If you had asked me the same question while I pursued by Masters - I would be a bit less sure, knowing atleast a few challenges.

And if you ask me now, as a practicing scientist - I would be even less sure about it all, on how accurately can we measure it at all. “

Our experience at APV Parbhani was no different! We wanted to look at the differential air temperature below solar modules as compared to between rows of solar modules. And inside shadenet, as compared to outside shadenet.

Why is it that something that a humble thermometer can measure - so difficult to assess?

Air gets its temperature from solar radiation. If you switched off the sun - in a few days, all air would go below freezing. Anything the sun’s ray’s touch - gets heated up.

However, what happens when the temperature sensor itself is exposed to radiation, then the sensor itself will have a solar heat gain and measure a higher temperature than the ambient air.

Hence temperature sensors have to be shielded from all radiation in a dark enclosure.

But hey - this dark enclosure cannot have stale air inside, else this air will not be a clear sample of the air outside! So again you are measuring the wrong temperature.

So you put a fan or an aspirator inside - and you have aspirated shields. They work fine - unless there is a dew accumulated on the shield. The fan evaporates this mist, makes the air inside cooler by latent heat of evaporation - and gives you a major error in temperature again! This concept is how the dry bulb temperature differs from the wet bulb temperature.

If you are using a water spray to reduce temperature of a enclosure, you would need to ensure that no droplets come and sit on the temperature sensor - else you would be measuring something completely different. But the air cooled by the spray should easily access the temperature bulb.

Hence, as said previously - air temperature measurement to a good level of accuracy is a tricky task, to say the least. You need to consider all the above complexities and process and interpret your temperature fields.