- LDH oil humidity sensor
- Technology
LDH Oil humidity sensor: Technology
ifm’s oil humidity sensors measure the relative moisture in oil by means of a capacitive measuring element. Water is generally not wanted in applications using hydraulic liquids and lubricants. High concentrations of water can seriously affect operation or even cause damage. ifm IO-Link enabled oil humidity sensors ensure accurate and reliable humidity and temperature data.
Principle of operation
The LDH is only suitable for measuring the amount of water molecules trapped between the oil molecules (moisture). Free water molecules (1) penetrate a water permeable diaphragm electrode (2) and affects the counter electrode (3). The capacitance measurement between the two electrodes is proportional to the percentage of moisture saturation (4).
0% represents completely dry oil (no water content) and 100 % represents completely saturated oil. Fully saturated oil has no free water content. The LDH sensor will read 100% before there is any visible free water. As a general rule of thumb, moisture content above 60% requires some sort of corrective measures.
Relative and absolute humidity measurements
ρw: Absolute humidity or the amount of dissolved water actually contained in the oil. Is typically indicated in ppm (parts per million).
ρw, max: Absolute humidity at the saturation limit (in ppm).
The relative humidity (φ) is the ratio of the amount of dissolved water actually contained in the oil (ρw) and the maximum possible amount of the saturation limit (ρw, max):
φ = (ρw / ρw, max) x 100%
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.
ρw = (φ x ρw, max) / 100 %
The saturation limit ρw, 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 dependent 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 (increased water absorbing capacity).
Temperature monitoring
The unit measures the oil temperature by means of a platinum measuring resistor (PT1000).