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Q:
What is the operating principle of the
AquaSensor?
A: 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.
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Q:
How does the water move into and out of
the sensor?
A: 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.
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Q:
How does the sensor know the saturation
point of a given oil, since different oils can dissolve different
amounts of water?
A: 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%.
| EXAMPLE: |
Oil
A |
Oil
B |
| 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 |
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Q:
What relation between water vapor pressure
and saturation level?
A: 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:
| Saturation
level [%] = |
measured
water vapor pressure |
x
100 |
|
| saturated
water vapor pressure |
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Q:
What is the advantage of measuring the
saturation level rather than PPM?
A: 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.
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Q:
What is the desired saturation level in
hydraulic and lubrication systems?
A: 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.
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Q:
What is the best location for installing
the AquaSensor in a hydraulic/lubrication system?
A: 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.
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Q:
What fluids can be used with the AquaSensor?
A: All mineral-based oils, HFD, HETG, HEES, and Skydrol have
been tested successfully. For other fluids, please contact us.
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Q:
What viscosity limits are there?
A: 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.
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Q:
Are there any other limitations to using
the sensor?
A: 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.
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Q:
How often does the AquaSensor need to
be calibrated?
A: 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%).
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Glossary
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.
Emulsions
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.
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