I'm not sure if this would work with tubs, not sure if the magnets are strong enough to hold the humidity gauge in place but might be worth a shot.
http://bit.ly/1156jRX
http://www.amazon.com/gp/aw/d/B0007VWEQA
The hydrometer can be calibrated using a salt test. http://www.youtube.com/watch?v=d7Ap96SlGZA
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