The HD 8901 is easy to use. 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. 
The functions of the instrument are: 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 battery symbol appears on the display.

The HD 8901 is a portable instrument R670D with interchangeable probes for measuring absolute and relative humidity, dew point and temperature.

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 thermo resistance (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.

TECHNICAL DATA HD 8901

No. of inputs: 2. Input A for humidity/temperature probes HD 8501S, HD 8501SAT/500, HD 8501 SS/500.

Input B for only temperature probes of the type TP 870, TP 870/C, TP 870/A, TP 870/P.

• Measuring ranges: Probe HD 8501S5 ... 98% R.H. -10...+70°C (up to +90°C for a short time)

• *Precision: Relative humidity 5 ... 90% R.H.: ±2.5% R.H. points 90 ... 98% R.H.: ±4/-2.5% R.H points

• Temperature: ±0.2 °C + linearization error (0.04°C/°C) ±l digit - Resolution: Relative humidity ±l % R. H. Temperature: ±0. 1 °C

• Response time: Relative humidity: 45% R.H. -> 90% R.H: **T63 = 2 sec. T90 = 10 sec. Temperature: T90 = 55 sec. (air moving at a speed of 1 m/sec.)

• Probe HD 8501SAT/500: 5 ... 98% R.H. in the temperature range -30 ...+130°C.

*Precision, including the probe: 
5 to 90% R.H. 2% R. H. 90 to 100% R. H. = ±4/-2.5% R. H.

Temperature measuring range with the TP 870 series of probes:
 -200°C (-328°F) ...+800°C (+1472°F) in two scales with automatic change of sca1e.

Precision, including the probe: 
-50°C (-58°F) ...+200°C (392°F) ± 0.2% of reading ± 0.2°C (±0.4°F) -200°C (-328°F)...-50°C (-58°F) and +200°C (+392°F) ...+800°C (+1472°F): ±0.4% of reading ±l °C (2°F).

Functions: automatic switch-off, automatic change of temperature scale, HOLD, storage of MAX, MIN and relative measurements. Indications on the LCD display: height of figures 12.5 mm, symbols HOLD, °C, °F, % R.H., RCD, MAX, MIN, REL, g/M3, Err for no probe, broken probe or inverted connector.

Instrument range: -10°C ...+50°C, 0 ... 90% R.H. Power supply: 9V battery, IEC 6LF22.

Battery Iife: 200 hours with an alkaline battery.

Probe connection: Input A, DIN 5-poIe connector, humidity and temperature probe Input B, DIN 8-poIe connector, temperature probe.

*This precision refers to measurements Iasting Iess than 15 minutes. If the hygrometric sensor is exposed to humidity IeveIs close to saturation for periods of over 30 minutes an excess deviation of up to +6% R.H. is measured. This deviation is memorized in the sensor when normal humidity IeveIs return ( Iess than 90% R.H.). 
The sensor resumes its original characteristics if it is exposed to humidity IeveIs of Iess than 80% R.H. for about 24 hours.

**This response time refers to a measurement in air at a constant temperature. 
For a correct measurement the temperature of the humidity sensor must be the same as that of the air of which you wish to measure the humidity. Differences in temperature between the sensor and the air cause errors in measurement because the temperature influences the relative humidity of the air. The higher (or Iower) temperature of the case radiates heat to the sensor.