Easy to use, for the dew point, slide the known wet bulb temperature under the depression (i.e. the dry bulb minus the wet bulb) and read the dew point under the arrow at zero. For the relative humidity, slide the dry bulb reading over the known dew point and read the relative humidity against the arrow.
Supplied in a Protective Pouch.
Scales used are traceable to the UK Meteorological Office.