[time-nuts] Temp/Humidity control systems?

Neville Michie namichie at gmail.com
Thu Oct 27 18:39:04 EDT 2016

You are correct to question commercial humidity sensors.
It seems to have come about because no-one can make a dollar by selling humidity.
Manufacturers do not tell the truth, they think ours is as good as theirs so we 
should claim the same accuracy. People buy these sensors, believe them, and buy more.

It is not hard to measure humidity/temperature. (they should be measured together).
Cover the bulb of an ASTM32C thermometer with cotton gauze. Insert it through the wall
 of a short length (18”) of 4” metal tube, insert another ASTM32C thermometer through the side 
of the tube 4” upstream. Put a computer fan on the outlet of the tube sucking air over 
the thermometer bulbs at about 4m/s. Wet the thermometer bulb, but NEVER touch it with your fingers.
In about 3 minutes you can take two temperature readings. There are a number of tables and calculation methods,
some much worse that others that will convert these values to air temperature, Relative humidity,
Dew Point temperature etc. You get accuracy of 1% from temperatures measured to 0.1C.
If anyone is interested I have basic routines for XCEL spreadsheet use to do the hard work.
This is based on the WMO Reference Psychrometer developed by Russel Wylie of NML Australia.

> On 28 Oct 2016, at 12:43 AM, Ron Bean <time at rbean.users.panix.com> wrote:
>> * You cannot "feel" absolute humidity, always measure it.
> And since this is time-nuts: Measuring humidity accurately is tricky. 
> According to people who have tested them, commercial electronic humidity 
> sensors, when tested in a lab, have never come anywhere close to the 
> accuracy claimed in the data sheet. The best you can hope for is 
> consistent readings, not absolute accuracy.
> The exception is the "cold mirror" type of sensor, which measures the 
> dewpoint by cooling a mirror and bouncing a light off it to sense the 
> temperature where dew condenses on it. Those are expensive, and they 
> require maintenance to keep the mirror clean.
> BTW some of us are more sensitive to humidity than others. I can't tell 
> you the RH of a room, but I can tell you when it's too dry for comfort. 
> I want it as close to 50% as I can get it without growing mold on the 
> walls. Some "experts" claim that 30% is good enough for anyone, but 
> they're wrong.
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