SFS H2O Relative Humidity Sensor

Cloud Based Condition Monitoring

A Platform for Intelligent Condition Monitoring.


Water is not desired in diesel fuel, hydraulic fluids, lubricants. High concentration of water can cause severe disturbance in operation and damage.

The SFS H2O measures the relative humidity of the fluid (diesel & oil) and directly displays the saturation degree in the water:

› 0%: Absolutely dry oil.

› 100%: The oil is completely saturated with water. Additional water will not be dissolved anymore and will present itself as free water.

In contrast to the humidity analysis from laboratories, where the absolute water content is defined in ppm (parts per million), the saturation limit of the oil can be determined by relative humidity measurement. The advantage of the relative humidity over the absolute water content is, that it is not necessary to know the oil or its saturation limit in order to determine if there is free or dissolved water.


› Mineral oils (e.g. HLP) have a comparatively low water absorption capacity. 500 ppm may signify that the oil is over-saturated and that free water exists.

› Ester oils (e.g. HEES) have a relatively high water capacity. 500 ppm may show that the oil is just saturated by 15%.

Please also note the characteristics of the relative humidity with different temperatures: Warm oil can dissolve more water than cold oil. Therefore, the relative humidity of the oil increases in case of no further water supply. Hot, relatively dry oil, may suddenly keep free water if the ambient temperature cools down.

The SFS H2O points out the current saturation of the oil with water, independent from oil type and temperature and additionally assures operation of systems by direct warning.

Measurement Principles:

The sensor records the relative oil humidity and oil temperature. Through an oil specific calibration it is possible to calculate the absolute humidity up to the saturation limit. The measuring values are given by RS 232 and the analogue outputs.

Design Characteristics:

The sensor is provided with a G¾ thread and can be integrated in the tank or via adapter in lines. Communication with the sensor either takes place over a serial interface or over two analog outputs (4 … 20 mA).