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What is the operating principle of the AquaSensor?

A special capacitance sensor is used in the Aquasensor. Water molecules are moved into and out of the sensor by changes in the saturation level of the oil. The capacity value of the sensor changes with the population of the water molecules in the sensor and is directly related to the saturation level. The change in capacity value is electronically converted and displayed as the saturation level in percent.

How does the water move into and out of the sensor?

The driving force behind this is the water vapor pressure. The water vapor pressure of the water in the oil is confronted by the water vapor pressure of the water in the sensor. Water is moved into and out of the sensor until the pressure equilibrium is reached. If the equilibrium is disturbed by adding water (increasing the water vapor pressure of the oil) or drying the oil (decreasing the water vapor pressure), water again starts to move into or out of the sensor.

How does the sensor know the saturation point of a given oil, since different oils can dissolve different amounts of water?

When adding water to dry oil, the water vapor pressure increases until the oil is saturated with water. This point is called the saturation point and the corresponding pressure is called the saturated water vapor pressure. The oil cannot dissolve any more water. When this point is exceeded, free or emulsified water is the result.
Although different oils can dissolve different amounts of water (in ppm) at the saturation point, the saturated water vapor pressure has the same value for all oil types at a given temperature. By definition, at this point the saturation level is 100%.

Temperature 20°C 20°C
Saturation Level 100% 100%
Saturated water vapor Pressure 2.3 kPa 2.3 kPa
Water concentration at saturation 78 PPM 358 PPM

What relation between water vapor pressure and saturation level?

Technically speaking, the AquaSensor measures the saturation level of oil by looking at the water vapor pressure. The simple relation between water vapor pressure and saturation level is:

measured water vapor pressure
Saturation level [%]= ——————————————– X 100
saturated water vapor pressure

What is the advantage of measuring the saturation level rather than PPM?

Using readings in PPM to ascertain a harmful level of water is difficult due to differences in fluid properties, including oil age, fluid type, and additive levels. Furthermore, this gives only a quantitative measure and does not answer the question whether or not the water content is still acceptable. By contrast, the saturation level provides a clear indication of the fluid’s condition.

What is the desired saturation level in hydraulic and lubrication systems?

Since the effects of free and emulsified water are more harmful than those of dissolved water, water levels should remain well below the saturation point. However, even water in solution can cause damage and therefore every reasonable effort should be made to keep saturation levels as low as possible. There is no such thing as too little water. As a guideline, we recommend maintaining saturation levels below 45% in all equipment.

What is the best location for installing the AquaSensor in a hydraulic/lubrication system?

This depends on the application. If the intent is to monitor a potential weak source, the sensor should be placed downstream of the potential water source. Generally speaking, the fluid should circulate freely around the sensor as increased flow increases the response speed. Therefore, it is advisable to install the sensor in the return line rather than in the hydraulic tank.

What fluids can be used with the AquaSensor?

​All mineral-based oils, HFD, HETG, HEES, and Skydrol have been tested successfully. For other fluids, please contact us.

What viscosity limits are there?

The AquaSensor does not have any viscosity limitations. However, low viscosity oils provide for a higher response speed. This is because there is a quicker exchange of oil close to the sensor.

Are there any other limitations to using the sensor?

​There is a certain temperature range and a maximum flow rate specified in the data sheet. Apart from that, the only systems in which the AquaSensor is of no use are those with a free water condition at all times (without any intention to change this condition) as it would display 100% saturation at the time.

How often does the AquaSensor need to be calibrated?

The AquaSensor is calibrated at the factory but can also be recalculated at the customer’s site. When calibrating the AquaSensor, it has to be brought into contact with a known level of saturation (saturation level standard) and the displayed value compared to the known one. We recommend calibrating the AquaSensor at least once a year.
The available HYDAC AS Calibration and Adjustment Set (stock no. 3122629) is supplied with two saturation levels standards (11.3% and 75.5%).


Dissolved / In Solution
These terms refer to a homogenous mixture of two fluids – in this case oil and water – implying that the individual water molecules are discrete and mixed with the oil molecules. The water is in solution. The sample cannot be separated by allowing the solution to stand at a given temperature. The fluid is clear.

Free Water
This describes the condition in which a fluid is saturated and is past the point where water is the solution. If more water is added to the oil, the water sinks to the bottom and the oil rises to the top. The visible horizontal line at the boundary between the two elements is called the interface.

Another example of free water is emulsions. They form when enough mechanical agitation acts on the fluid so that the free water forms a cloudy mixture of water and hydrocarbons. The mechanical shearing action creates very small water droplets which have too much surface tension to join and form an interface. This is still free water as it is not in solution, but it does not create an interface boundary, causing a visible cloud or haze instead.

Saturation / Saturation Point
At this point the fluid carries as much water in the dissolved state as it possibly can at a given temperature. At this point the saturation level is 100%. If any more water were to be added, a free water condition would result and that would be the beginning of an emulsion or interface. When the saturation point is given, a corresponding temperature is also given because saturation varies according to temperature.

Saturation Level / Percent Saturation
This is the degree of saturation which indicates what percent of maximum possible water in a dissolved state is in the oil. A reading of 0% would indicate oil free of water, while a reading of 100% would indicate oil that is saturated with water.

Water Vapor Pressure
This is the pressure exerted by water vapor. Water gives off vapor, consisting of molecules that have evaporated and are in a gaseous state. The presence of water in oil results in a water vapor pressure on the surface of the oil. This water vapor pressure depends on the water content, the type of oil (including additives and particles), and temperature. If the ambient water vapor pressure is higher than that of the oil, water moves into the oil. By contrast, if the ambient water vapor is lower, water evaporates out of the oil.

Saturated Water Vapor Pressure
When adding water to oil, the water vapor pressure increases until a maximum value. The vapor is then said to be saturated vapor and the pressure it exerts saturated water vapor pressure. In oil this is the case when a maximum amount of water is dissolved.