Free water in oil sensing

Hi Juha:

We have not done any investigations with water detection in oil. Depending on the system and what it is doing, it may or may not be feasible. Is the oil and water completed separated or are they mixed? If they are mixed, then there is no way to distinguish between the two. Are you trying to determine how much free water is in the oil tank? Is this a liquid level sensing application?

If you can elaborate on the application and system itself, I can answer your questions with more confidence.

Thanks
-David Wang
Capacitive Sensing Application

Hello David

thank you for your answer. Application is  water in the lubrication oil alarm. Other possible information like oil level are benefits if available. Goal is to find out if there are any water at the bottom of the oil tank so called free water.   Dielectric constant of water depends quite a lot about temperature being between 50..about 90. About lubrication oils I do not have exact data but they are somewhere between 3...5. Difference is so large that it should be possible to sense. It is also possible to arrange fresh clean oil for the reference cap.

Juha

So if all the water is on the bottom, it may be possible to put a liquid level setup without the reference near the bottom. So you would have a parallel plate configuration. If the sensor is completed covered with water, the capacitance should be fairly high. If the sensor is half covered with water and half with oil, the capacitance should be decreased. This setup will give you a WATER/NO WATER presence type of detection.

A prototype will need to be completed to determine how much of a change you will see at different water/oil height ratios covering the sensor.

Thanks
-David Wang
Capacitive Sensing Applications

Thank you again. Evaluation board is ordered. When I get it I start practical tests.
Parallel plate cap connected to one channel for WATER/NO WATER presence detection is good starting point. Do you have any opinion how rest of the channels should be used if I try also oil level indication with the same sensor system.
Juha

You can implement the Out of Phase liquid level sensing technique to measure the level of the oil. Since it has a very low dielectric constant, the sensors will probably have to be wider than the dimensions in the OOP app note that was tested using water. You will have your LEVEL and REF sensors which will occupy CIN1 and CIN2. CIN4 will be used to put the system into OOP (floating channel). So CIN3 can be used for the WATER/NO WATER detection near the bottom.

The issue that I see since there are two different dielectric constants is that if the height of the oil is changed (emptied or filled), then the capacitance versus liquid height will not be linear. Water will be near the bottom so it will affect your REF sensor. If OIL/WATER is filled or emptied, then your measurements will not change as noted in the app notes due to different dielectric constants and ratios. This will also need to be prototyped.

Thanks
-David Wang
Capacitive Sensing Applications