If you have a decent digital multi meter, you can get UEI or FieldPiece attachments that will work with it to give humidity readings. I have a UEI meter that runs the Fieldpiece Humidity head for it, which was 40 bucks at my local Johnstone Supply (which are everywhere and an HVAC supply house). I did have to buy a set of new leads for my multi-meter that had removable lead tips which plug into the meter and the Humidity head. Not something that you would want to leave on in your safe, but very accurate and reasonably priced for occasional monitoring.