Measuring Relative Humidity (Another Method)

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Darryl Young
Posts: 1668
Joined: Fri Jul 30, 2010 6:44 pm
Location: Arkansas

Measuring Relative Humidity (Another Method)

Post by Darryl Young »

Mario Proulx posted an accurate way to measure relative humidity over on the MIMF. This is simpler and more straight forward than the the drybulb/wet bulb method and all you need is a decently acurate thermometer. I can't describe the procedure any better than Mario so here is a link to his post on the MIMF:

http://www.mimf.com/phpbb/viewtopic.php ... 583#p15583

I made up a spreadsheet where you enter the 2 Fairenheit temps and it calculates the realtive humidity so if anyone wants it, here it is:
RHFromDewPoint.xlsx
Also, if you want a decently accurate, competitively priced digital thermometer, here are a couple of suggestions. Both have 0.5C (0.9F) accuracy at room temps. Also digital thermometers are notorius for taking forever to settle on the real temperature but these will go from ambient temp to freezing in 5 or 6 seconds. And bonus, these are great food thermometers! And they are only $19 (even cheaper if you buy 2 or more). These would make great bending thermometers except they only go up to 302F (bummer). I own the first one below.

http://thermoworks.com/products/low_cost/rt600c.html

http://thermoworks.com/products/low_cost/rt301wa.html
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Darryl Young
Posts: 1668
Joined: Fri Jul 30, 2010 6:44 pm
Location: Arkansas

Re: Measuring Relative Humidity (Another Method)

Post by Darryl Young »

I gave this RH measument method a test tonight.

My thermometer (linked above) showed the air temp in the kitchen was 71.6F. Put water in a glass with about 5 ice cubes from the fridge and started slowly stirring. In about 2 minutes moisture finally started forming on the outside of the glass and the temp of the water inside the glass was 33.8F. Plugged these numbers into the spreadsheet and it calculated a RH of 25%.

If you guys remember, a year or two ago Ken Cierp recommended a cheap humidity sensor made by Aprilaire and I bought two of them (maybe off ebay, can't recall). Well, I went and picked it up and it showed a slightly different temp (less than 1F difference) and it showed exactly 25% RH. I guess that is a decent, digital RH measurement instrument.
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