Another one of those words that sound like it would mean something else; so no, it doesn't measure how psychotic someone is. :P
: a hygrometer consisting essentially of two similar thermometers with the bulb of one being kept wet so that the cooling that results from evaporation makes it register a lower temperature than the dry one and with the difference between the readings constituting a measure of the dryness of the atmosphere.
...basically, a way to determine the degree of humidity in the air.
— psy·chro·met·ric adjective
— psy·chrom·e·try noun
From psychro-, derived from Ancient Greek psychros, cold, frozen+ meter, from Ancient Greek metron, to measure.
International Scientific Vocabulary
First Known Use: circa 1741
Here's an illustration: