Extech rh25 heat index psychrometer extech rh25 p the extech instruments rh25 is a heat index psychrometer which will measure temperature and relative humidity using a precision capacitance rh sensor the rh25 also calculates and displays heat index dew point wet bulb and wet bulb globe temperatures p p the high temperature alarm feature alerts the user when heat index or wet bulb globe levels exceed user preset limits p p data hold and min max avg memory utilities make it easier