Sensors

The AS 2000 features the following over the AS 1000 series: a LED display/keypad, two analogue outputs, four easily adjustable switch outputs and a serial RS485 interface, which can be easily calibrated. This translates into a higher price tag than its predecessor.

The latest version is also more reliable than before as a result of its compact and rugged design.

Water movement in and out of the sensor is driven by water vapour pressure. The water vapour pressure of the water in the oil is confronted by the water vapour pressure of the water in the sensor. Water is moved into and out of the sensor until a pressure equilibrium is reached. If the equilibrium is disturbed by adding water (increasing the water vapour pressure of the oil) or drying the oil (decreasing the water vapour pressure), water again starts to move into or out of the sensor.  

The addition of water to dry oil causes the water vapour pressure to increase until the oil is saturated with water. The point where oil cannot dissolve any more water is the saturation point and the corresponding pressure the saturated water vapour pressure. 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 vapour pressure has the same value for all oil types at a given temperature. By definition, at this point the saturation level is 100%.

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

Differences in fluid properties, including oil age, fluid type and additive levels make it challenging to determine harmful levels of water through ppm readings. For example, 300ppm in a synthetic ester would be ideal, but the same water content would have disasters consequences.

Furthermore, this gives only a quantitative measure and does not answer the question as to whether or not the water content is still acceptable. By contrast, the saturation level provides a clear indication of the fluid’s condition as it is directly related to the oil’s saturation concentration (saturation point).  

Water levels should remain well below the saturation point – as a guideline HYDAC recommends maintaining saturation levels below 45 per cent in all equipment. This is because the effects of free and emulsified water are more harmful than those of dissolved water. However, even water in solution can cause damage and therefore every reasonable effort should be made to keep saturation levels as low as possible. This is because there is no such thing as too little water.

A measured saturation level is equivalent, at a given temperature, to a certain water content in ppm. If the saturation curve of the fluid is known, the saturation level can be converted into a ppm value (contact HYDAC for further information). Such curves are rarely provided by oil suppliers because determination is laborious and there is no standardised procedure available. Furthermore, the saturation curve changes with the ageing of the oil and would only be valid for new oil. 

All mineral-based oils, HFD, HETG, HEES and Skydrol (phosphate ester version) have been tested successfully (for other fluids contact HYDAC for more information). The viscosity range is specified in the datasheet, with the general rule of thumb being that the AquaSensor does not have any viscosity limit from a measuring point of view. However, low viscosity oils provide for a higher response speed. This is because there is a quicker exchange of oil close to the sensor. Furthermore, high flow rates and high viscosity would cause too much mechanical pressure on the AquaSensor.

The datasheet specifies a certain temperature range and maximum flow rate. 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 they would display 100 per cent saturation at the time. Furthermore, the AquaSensor must not be used in water-based fluids such as oil in water emulsions (HFA), water in oil emulsions (HFB) or water glycol (HFC).

One effect is that the amount of water an oil can dissolve is slightly pressure dependent. Another effect is a changing water absorbing capacity of the sensor itself. The total effect is however marginal at about 0,02 per cent FS/bar.

This depends on the application such as the sensor being placed downstream of a potential water source if the intent is to monitor a potential water leak 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 the hydraulic tank.