The HP21 is a highly accurate hand-held indicator that displays RH, temperature and the dew or frost point.
The HP21 is best suitable for using as a master for calibration and humidity validation 
The HP21 operates with a regular 9V alkaline battery.
 
Measuring range 0…100 %RH
Measurement accuracy at 23 °C
Repeatability ±1.0 %RH 0.3 %RH
Long term stability Sensor time constant Temperature measurement < 1 %RH / year
Typical 10 sec, 63% of a 35 to 80 %RH step change (1m/sec air flow at sensor
Display Probe material LC, 1 or 2 decimals resolution, backlight, trend, alarm and low battery indication Polycarbonate