Hygrometer Calibration

[Nature]

I recently had to replace my Hydroset II, the calibratable rectangular one.

I picked up this Acurite 00309B at Lowe’s for like $12. Amazon has several similar as low as $8. Pretty sure most of these have the same guts/ sensors.
I like that it tracks high/low for a 24 hour period so I can see if there are fluctuations. Not that I would be concerned if there were unless it was happening all of the time. Also, the large numerals are pleasing for my aging eyes.



I verified accuracy with the salt test in a gasket sealed container. It was pretty much dead on at 75% RH. Ziplock type bags aren’t as air/moisture tight as many believe.
This is my salt test set-up. A glass dish (plastic is fine too) With about 1/4 inch salt in the bottom and enough water to moisten all of the salt. There is just barely a skim of water on the top surface.

You don’t need this much salt, but it is more stable that way and reaches equilibrium faster. The increased surface area of the dish helps too.


For the adjustable ones, you don’t have to adjust while still in the bag. Just note the difference and turn the dial however many clicks / marks as indicated. It is only adjusting the numeral displayed and not establishing a set-point. IIR, you have to press and hold the calibration button again after turning? Refer to your instructions to be sure.

I think too much stock is placed on the accuracy/ precision of these digital hygrometers. They all are +/- 2% or worse (Not 2% of the reading, but a flat 2%. This is a 4% absolute variance and almost 6% of the reading at 70% RH. We use NIST calibrated hygrometers in the lab at work costing almost $100 or more that are still stated as +/- 2% for humidity. I think most of the price tag is for the NIST sticker and certificate.

Not too much technology in these to vary too much, just the care taken to adjust the trimpot and the consistency of components.



Sent from my iPhone using Tapatalk