Author Topic: Anyone knows this weird RH meter?  (Read 1111 times)

Offline Eric in Thailand

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Anyone knows this weird RH meter?
« on: December 20, 2010, 09:43:48 AM »
I've been having some trouble getting reliable Relative Humidity readings using analogue as well as digital meters. Last week I came across this weird - at least for me - metering method. It consists of 2 thermometers. One is dry and one is wet (the bulb being inside a little water-filled container).
You read the temperature on the dry side (e.g. 26) and subtract the reading on the wet side (e.g. 22.5 Centigrades). You then take the difference, e.g. 3.5 degrees, and look up the intersecting value in a table where the dry value horizontal line intersects with the difference value vertical line (e.g. 3.5). The value at the intersection point gives you the RH% value (73%).
Sounds complicated, hence I'm including a couple of pics.
Anyone has experience with this device, knows the physics of its workings, or has knowledge about its accuracy?
« Last Edit: December 20, 2010, 10:02:51 AM by Eric in Thailand »


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Offline linuxboy

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Re: Anyone knows this weird RH meter?
« Reply #1 on: December 20, 2010, 10:33:34 AM »
Yes, this is a classic psychrometer. Excellent analog way to measure RH, but prone to some error, especially at lower RH levels.
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Offline JayW

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Re: Anyone knows this weird RH meter?
« Reply #2 on: December 27, 2010, 06:49:48 PM »
This actually is a fairly good way to measure humidity. It simply compares the wet bulb temp to the dry bulb. The moister the air the lower the temp will be on wet bulb due to evaporation. The biggest improvement is to use a sling psychrometer or even a fan on the wet bulb side to increase air flow and maximize evaporation.

In my cave I use several "Abbeon" hygrometers (they are based on the hair hygrometer) but they are always calibrated to a sling psychrometer and I have a good deal of faith in them. Like right now as we go into the winter months, when my hygrometer takes a dive I know its not kidding and need to pump up the cave humidity ASAP if I do not want to see some cracks developing.

The digital style hygrometers are not very accurate from my experience. We did sell one at one time but  they never read very true. We no longer offer any hygrometers because it is so difficult to find one at reasonable$.

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Offline John (CH)

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Re: Anyone knows this weird RH meter?
« Reply #3 on: December 27, 2010, 07:01:30 PM »
Jim, big welcome!

Agree, I find my digital hygrometers not accurate at very high % RH.

Offline JayW

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Re: Anyone knows this weird RH meter?
« Reply #4 on: November 17, 2012, 11:20:20 AM »

The digital style hygrometers are not very accurate from my experience. We did sell one at one time but  they never read very true. We no longer offer any hygrometers because it is so difficult to find one at reasonable$.

Ran across this old thread and a comment I made back in 2010 ... thought I should update my response because since then I have found another hygrometer that is perfect and can be found on our website (cheesemaking.com).
This one is very accurate .. within 1-2 percentage points from my standard measure here. They are also accurate between random samples off the shelf here.  The temp is also quite accurate.
Still standing by my old rule that "Bad info is worse than no info at all"
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Offline Mike Richards

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Re: Anyone knows this weird RH meter?
« Reply #5 on: November 17, 2012, 04:50:15 PM »
Just in case anyone reads this looking for info on the psychrometer: wet bulb vs dry bulb--  Jim said,

Quote
The moister the air the lower the temp will be on wet bulb due to evaporation
,

but he meant the opposite--the dryer the air, the lower the temperature on the wet bulb.  As water evaporates off of the wet bulb, it pulls energy from the bulb, lowering the temperature.  At 100% RH, there is no difference in the wet bulb and the dry bulb measurement because no water will be evaporating from the bulb.  At 0% RH you'll see the greatest difference.

One thing about psychrometers is that most charts produced for using them are created for use at sea level and aren't as accurate at higher elevations.  I don't know which, if any, digital meters take into account (or are independent of) total pressure.
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