Aerocon Systems Aqua Environment
Cheap enough for them to ship. That device accepts the humidity cards from here:
Humidity Indicator Plugs, Cards & Desiccators
You want the 0.95" 3-section 10/20/30% cards.
And no, checking once in a while is NOT good enough. You need continuous monitoring - when the 20% indicator just
starts to turn your dessicant (molecular sieve) is shot and within minutes you will have oil vapor in the product gas.