In general, I like the concept of testing the compressed air by taking a small sample into a lower pressure or ambient pressure container. I do this to take air samples, to check for carbon monoxide when I travel.
But I am under the impression that this humidity meter doesn’t read low enough to provide useful information. Please correct me if I am wrong.
The humidity level that I like to see out of my compressed air should be in a dew point of about -40F when it is brought back to ambient pressure. If I am assuming an ambient temperature of about 65F, then the relative humidity of the air coming out of my cylinder should be about 0.9%.
You can use this website to calculate from dew point to relative humidity (at ambient pressure):
In other words, you need a meter that reads ultra-low humidity accurately, with readings of dew point lower than -20F and or relative humidity lower than 2% accurately.
I found this humidity sensor that reads dew point down to -22F. I think that is barely good enough to get you in the ballpark, but I would like to find something that measure even lower. This meter is $36, which is not bad.
https://www.amazon.com/dp/B01CY29RCI/?c ... _lig_dp_it
I would recommend at least using this meter with your method. I may try that.
Notice that this meter and many others will specify a relative humidity measuring range from 0% to 100%, but then they have a accuracy of + or – 2% RH. I don’t think that is accurately enough if you are trying to detect a relative humidity under 1%.
A relative humidity of 10% at a temperature of 65F produces a dew point of 7F. That seems very dry, but it is not dry enough for compressed air.
The reason those disc that read at 10% are somewhat useful in the sight-glass is because they are testing high pressure air. The relative humidity changes with air pressure, but I have not found good data to correlate RH (or dew point) as a function of pressure.
The dew point goes up with pressure and that is why you need extremely dry air to keep from getting condensation inside your steel cylinders and avoiding rust, etc.
An interesting piece of trivia: Fire fighters in the Navy are particularly concern about moisture in the compressed breathing air, when fighting fires in cold temperatures. They are now running SCBA with cylinders at much higher pressures than our SCUBA cylinders, and much higher risk of moisture freezing in the regulator first-stage.