The humidity sensor is a condenser the dielectric of which is a hygroscopic polymer. As the dielectric constant for water is around 80, a strong variation of capacity is obtained with the variation of the humidity content of this polymer.
The particular advantages of this type of sensor are good linearity, insensitivity to temperature variations, brief response time and long life.
The temperature is measured by a Pt100 platinum thermoresistance (the temperature working range is limited by the working range of the humidity sensor).
The sensor is flat, with a high surface/volume ratio; this allows a good response time.
The HD 8901 is easy to use, and measures:
- Relative humidity
- Absolute humidity
- Dew point temperature
- Temperature in °C and °F
- Relative values
- Maximum and minimum values
The heart of the instrument is a microprocessor, with the functions of the instrument including:
- AUTO-POWER-OFF: the instrument switches of automatically after about 8 minutes.
- Relative measurements.
- REC: stores the maximum and minimum value of the two inputs in all functions.
- HOLD: blocks the reading on the display, while internally the instrument continues to update the values.
A beep sounds to indicate that a key has been activated; a beep every 30 seconds indicates that the battery is running low; also, the H symbol appears on the display.