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Capacitive & Psychrometric
Humidity Sensors
The ALMEMO® humidity/temperature sensors are equiped with a meltable capacitive thin film
sensor for measuring humidity and with a highly precise NTC- or Pt100 W measuring element
for temperature measurement.
Capacitive measurement of air humidity
With capacitive sensors, a moisture-sensitive polymer layer is applied on a glass
substrate between two metal layers. As a result of water absorption, corresponding to the
relative air humidity, the dielectric constant and thus the capacitance of the thin-film
sensor alters. The measured signal is directly proportional to the relative humidity and
is independent of the ambient pressure.
Callable humidity values (table 1):
Temperature
Relative humidity
Dew point
Mixture ratio
Partial steam pressure
Enthalpy |
°C
% rH
°C
g/kg
mbar
kJ/kg |
For all ALMEMO® instruments these values are called measuring channels.
Depending on how many measuring channels are existing in the instrument, it is possible to
call up a maximum of 4 humidity values as indicated in table 1. If another order should be
desired, this can be modified in the ALMEMO® connector. All calculation formulas are in
the microprocessor of the instrument used.
Psychrometric measurement of air humidity:
Psychrometers are high-precision instruments with a dry and a wet temperature sensor. The
evaporation will cool down the wet sensor. To enable this, an air velocity of at least 2
m/s for cooling down is ensured by means of a ventilator within the psychrometer. From the
temperature difference (psychrometric difference) the humidity values are calculated. The
calculation formulas for the ALMEMO®-instruments are equal to those of the german
meteorological office related to nominal atmospheric pressure of 1013 mbar. For
high-precision measurements, differing atmospheric pressure can be corrected.
| Advantage: |
No ageing of the sensor, with the exception of contamination of the wick.
High degree of accuracy, of considerable value for the measurement. |
| Disadvantage: |
Long-term measurement limited by the supply of water necessary and
maintenance of the wick.
Can only be operated with difficulty at sub-zero temperatures. |
| Specifications |
| Humidity measuring
circuit |
| Measuring range |
5 to 98% R.H. (0 to 100% R.H. total range) |
| Sensor |
Capacitive thin film sensor |
| Accuracy |
± 2% R.H. at nominal temperature |
| Repeatability |
< 1% R.H. at nominal temperature |
| Nominal temperature |
25°C ± 3°C |
| Operating pressure |
0.04 to 20 bar |
| Response time |
T90: approx. 8 s (without filter) |
| Temperature measuring
circuit |
| Sensor |
Pt100 or NTC type N |
| Accuracy |
Pt100: IEC 751, class B
NTC type N: -20 to 0°C: ± 0.4°C
0 to 70°C: ± 0.1°C
70 to 80°C: ± 0.6°C |
| Repeatability |
0.1°C |
{ALMEMO® MAIN PAGE}{PRODUCTS} |
KAYTECK 
Capacitive humidity sensors for climate