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The question What is the second thermometer in the image from the Esperanza Antarctic temperature record? shows what might be a wet/dry bulb setup.

I've given one to a friend to measure humidity for fun (it's me who thinks it's fun). I'd learned about them in Earth Science class a zillion years ago, so when I happened to see one recently I grabbed it.

I understand the very basics of how it can indicate humidity; the drier the air, the faster water will evaporate into it and so the colder the wet bulb will be with respect to the dry bulb.

To get a numerical value for humidity with this set up you use the coarse table on the front (there's a finer table on the back of the box) and look up the dry bulb temperature and the dry minus wet difference to find an approximate humidity. For example If I look up 19 and 2 °C for those respectively (roughly what's shown in the image) I get about 81% relative humidity.

But suppose I hooked up a camera to a Raspberry Pi and imaged the two thermometers, processed the image and determined the two temperatures and wanted to calculate the relative humidity without interpolating the table.

Question: How is relative humidity determined from a wet and dry bulb readings? What are the steps and where's a good source for the equations and parameters involved?

Click for full size

wet/dry bulb setup to measure relative humidity

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I'm going to go old school with this one.

The following pictures are the relevant pages from: Environmental Engineering in South African Mines, The Mine Ventilation Society of South Africa, 1989, pp 451-455. They give the psychometric equations that are required to calculate relative humidity from wet & dry bulb temperatures.

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For simplicity, I use the less accurate equations on p455, section 9.2.

As a slight diversion, the other way to measure wet & dry bulb temperatures is to use a whirling hygrometer. The wet bulb temperature is always read first because it will start to warm as soon as the hygrometer stops whirling.

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  • $\begingroup$ While screenshots of text are usually discouraged, this is the best possible answer for me at the moment, and I appreciate the speedy response! I'll take a look today and if nobody else does I'll add a summary answer based on what I've implemented, so that those who are visually impaired can have an answer as well. Thanks! $\endgroup$ – uhoh Feb 8 at 4:40

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