So far I have calibrated my hygrometer always with the salt-method. This works quite well with normal (small) hygrometers. But it is not really practical for my permanently installed humidification-system in the humidor cabinet. Therefore I have now built an electronic psychrometer. It consists of two electronic thermometers and an Arduino. So I can quite easily calibrate the hygrometer in the cabinet. Maybe this is interesting for some in the forum. In my blog I described the construction in more detail.