IFM LDH100 - Relative and Absolute Humidity; Signal Processing

5
UK
4.1.3 Relative and absolute humidity
The relative humidity (φ) is the ratio of the amount of dissolved water actually
contained in the oil (ρ
w
) and the maximum possible amount of the saturation
limit (ρ
w, max
):
φ =
ρ
w
ρ
w, max
x 100 %
ρ
w
Absolute humidity or the amount of dissolved water actually contained in the
oil. Is typically indicated in ppm (parts per million).
ρ
w, max
Absolute humidity at the saturation limit (in ppm).
In contrast, the absolute water content is not a physically measured value� It is
determined on the basis of the relative humidity and the saturation limit using the
formula below�
ρ
w
=
φ x ρ
w, max
100 %
The saturation limit ρ
w, max
depends first of all on the type of oil� For further
information about the type of oil please contact the manufacturer of the oil�
Secondly the saturation limit is very dependant on the temperature� Therefore
the relative humidity changes with the temperature� The same applies when
the absolute humidity remains constant� Usually oils absorb more water with
increasing temperature�
4.2 Processing of the measured signals
The unit detects the measured values continuously and outputs them as analogue
signals� It has two 4���20 mA current outputs with the following fixed measurement
limits:
Measured value Output signal
OUT 1 -20...120 °C (oil temperature) 4���20 mA linear
OUT 2 0���100 % rel� humidity 4���20 mA linear