Multiple Liquid Level Sensing


Thread Starter

CP Pang

I am working on a palm oil mill project in Indonesia. We use the clarifier tank to separate the crude palm oil from the sludge. The efficiency for this separation process depends on the operator's observation on the liquid oil that is skimmed out via a skimmer valve. If the sludge oil is seen skimmed out from the skimmer than, the skimmer valve height is increased. The operator's activity is subjective, and the yield of separation varies from each shifts. We plan to automate this operation. The skimmer valve can be adjusted easily using servomotor or pneumatic diagphram actuating valves. But my problem is the level sensing of multiple liquid composition. i.e., the oil, sludge and sediments. How do you do that? My concept of the automation does not only skim out the first layer of the oil but also involve draining of the sludge oil for secondary recovery process and at the same time the capability of draining the sediments for bigger head space - to avoid upstream process interuption via level control and higher retention time in the tank for heating, thus better separation and yield. I need your helps and suggestion. Thank you.

Jeff Pinegar

CP Pang wrote:
I am working on a palm oil mill project in Indonesia. << clip ....>> my
problem is the level sensing of multiple liquid composition. i.e., the oil,
sludge and sediments. How do you do that? << clip >>

I once worked for a division of MTS Systems. They make a product for measuring the levels of multiple liquids in one tank.
I don't know the specifics, but I am sure application consideration include the difference in specific gravity of the liquids and the rate of level change.

Good luck
Jeff Pinegar

Adolfo Jimmy Saldivias


You can use a servo level and interfase detector from Enraf, model XTG.

This device works by the Archimedes principle.

It uses a displacer to measure.

The displacer is a body manufactured with a very exact conditions to yield a volume of 105 cubic centimeters and weigth of 223 grams.

Following Archimedes, if you have such a body in air, it will weight 223 grams, but if you submerge the displacer it will "lose" weight, due to the "push" due to the displaced oil.

You use a weight sensor that will sense this "apparent weight". When you just touch the oil surface, the system will detect a level, when the "apparent weight" of the displacer becomes 218 grams.

You can do this measurement with an accuracy of 1mm.

After this, you can continue submerging the displacer until it touches the interfase oil/sludge, because there will be an additional "push" coming from the different density of oil as compared to sludge.

You can sense this Interface with an accuracy of 2 mm.

Finally, you can continue submerging the displacer until it reaches the interface sludge/sediments.

Again this measurement can be done with a 2mm accuracy.

You can also measure the density of the oil.

Once you fully submerge the displacer into the oil you will have the weight of the oil, as well as the volumen. If you divide weight by volumen you will obtain density.

This measurement can be done, in 10 points along the height of the tank. The system will give you 10 point readings of density (profile) and also the average value for these measurements. So you will end up with 11 measured values.

You can use the same principle to measure density at 10 points around the Oil/sludge interfase. And also the average value for this 10 points.

You can check this at You will be provided with these explanation, but only with the pdf of the equipment. That's why I explained
it here.

I have written an article on these technology at our website. Sorry only in spanish. In any case if you have further questions, you may contact me at any time.

Hope it helps.
Jimmy Saldivias
Phone: 591-4-523438
Fax: 591-4-523413

Willem de Jong

Your application can be done with Rhosonics sensors. These sensors measure the velocity and absorption of sound and since there is a large difference between the products, it is well possible to detect differences. The best choice is the sensor that can be positioned at the side of the tank. Locate the sensors at the spots where you want the interfaces to be detected. The sensors tell you exactly if there is oil, sludge or even water at the point of measurement. An advantage is that the measurement is maintenance free, i.e. not sensitive to pollution of the sensor surface. Willem de Jong Rhosonics
Almeg Controls makes interface level switches by adjusting the of the floats on the probe. Another model floats on the surface like a spider. An adjustable probe senses the base liquid - if the spider rises lifts or floats above the set point - a relay is activated - simple. Is this what you need?

Willem de Jong

Our MP-130 probe measures the ultrasonic properties of fluid, and we use it for exactly the same application in oil/water and water/toluene decanters. The controller Model 8100 is converting the measurements into concentration of water and oil, thus enabling you to detect the emulsion or sludge continously. Together with a simple PLC, it is possible to control the outlet valves so that you will have a constant quality of oil. Look at our website ( for more information about the probe and the 8100. Best regards, Willem de Jong Rhosonics NL Phone: +31 35 543 1998