I finally got around to calibrating the hygrometer in my Omega. Hygrometers are typically calibrated once a year, but it has been suggested that the analog ones (like the Omega’s Barigo model .1748 unit) should be calibrated every time they are moved to a new location. In any case, I had reason to suspect that the reading was off.
The basic technique is to place the hygrometer into a chamber in which the RH is known to be at 100%. The indicator needle is then adjusted so that it point to the 100% mark.
My humidification chamber comprised a deep bowl lined on the bottom with a wet hand towel, covered by a wet (but not dripping) hand towel. The hygrometer was placed face-down on the towel in the bowl, and left for about 30 minutes. When I returned and took the hygrometer out of the bowl the indicator needle was reading past 100%.
To adjust the position of the indicator, a broad-ish screwdriver was inserted into the slot visible through the centre hole (of eight) on the rear of the case, and turned. The mechanism was pretty stiff and it took quite a bit of force to move it. I also tapped the case of the hygrometer after each tweak in order to get the indicator needle to settle in the new position. The hygrometer reacts to changes in RH within 15 to 20 seconds of any change, which makes fine adjustment of the indicator pretty difficult. It took me a couple of hours of repeated humidify/adjust cycles before I was satisfied with the result.
I then left the hygrometer on the shelf with my two other digital hygrometers for a couple of hours, to check if all were in agreement (they were, within about 1% RH) and to allow moisture condensation inside the case of the hygrometer to evaporate.