MIMF

This may be a stupid question but in the great discussion on dew point calibration, Mario recommended a cheap mechanical hygrometer and said that they are easily calibrated. I was just looking at such a device at the local Woodcraft and for the life of me I saw no way to calibrate it. There were no adjustment screw or even an obvious way to rotate the dial. There was just a simple coil spring mechanism and the instructions said to take it onto a high humidity area such as a shower once a year to "charge" the unit and set the calibration. Am I missing something?

Those can be calibrated if done carefully. Inside the back, you can see where the spring is stuck to the inside of the face of the unit. You can gently move the point of contact, until the hand is in the right place, as the sticky stuff is not hard, or it wasn't on the ones I've bought. I have two that stay pretty accurate now. I check them and my Abbeon to a wet/dry thermometer set up once a week or so.

If you spend a couple of dollars more you can find ones that have a hole in the center of the back with a little screw inside.