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Moisture content sensor, LDH

Monitoring moisture content in hydraulic and lubricating oils is critical to prevent mechanical damage to rotating components. Excessive moisture in lubricating systems rapidly degrade bearing surfaces leading to failure and secondary damage. In hydraulic systems, moisture damages high pressure surfaces on rotors, pistons and poppets. Corrosive effects on surfaces such as hydraulic cylinders and precision assemblies can be avoided.

ifm’s LDH sensor measures the relative moisture content present in machine oils in real time. Since allowable moisture content changes with temperature, the LDH uses both conductivity and temperature to calculate the allowed absolute moisture at any operating temperature. Both relative moisture and temperature signals are available from the sensor.


Hydraulic power units

The LDH continuously checks water content in the oil.  It provides an analog signal of relative humidity corresponding to 0…100 %.


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. As a general rule of thumb, moisture content above 60% requires some sort of corrective measures.


Q.  How does air bubbles in the oil affect the measurement?

A.  It is assumed that the air humidity is the same as the oil humidity at a given temperature, so the LDH can still provide a stable, reliable reading.

Q.  Does oil conductivity affect the measurement?

A.  Yes.  The permissible conductivity range is suitable for all common oils. Exceeding the maximum conductivity of 3 mS/m will cause inaccurate measurements.

Q.  How do particles in the oil affect the measurement?

A.  The sensing element is not affected by dirt or particles in the oil.  It cannot become clogged. However, the response time can be affected.

Q.  Can the sensor be used to detect oil in water as opposed to water in oil?

A.  No.  Operation in pure water short-circuits the electronics. The sensor is not damaged, but the measured values are meaningless.

Q.  Can the sensor be used to detect moisture content in air?

A.  Yes so long as the air pressure is within the rated pressure of the LDH. In production, the LDH sensors are calibrated with air.

Q.  Is there an IO-Link version of the LDH?

A.  Not at this time.

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