# Planar interdigitated electrodes

1. May 9, 2015

what is the formula to calculate planar interdigitated electrodes for dielectric sensor ?

2. May 10, 2015

### Baluncore

Welcome to PF.
There is no simple formula. It will depend on what scale you are considering and what response you require.
How big will your sensor be? What will it be sensing?

Last edited by a moderator: May 7, 2017
3. May 10, 2015

thanks Baluncore !

actually i want to design a dielectric sensor based on planar interdigitated electrodes that will be attached to a bottle filled with cooking oil to detect the quality of cooking oil.
until now i still confuse about it.

4. May 12, 2015

### Baluncore

To measure the water contamination of cooking oil you can measure the dielectric constant of the emulsion through the wall of a plastic or glass container. There are several design choices that can make that possible. It might also be possible to measure the dielectric loss angle of the emulsion, but I think the water content will be quite detectable by capacitance alone since the oil and water have very different dielectric constants.

A problem you may have is that wet oil may settle to the bottom of the container and your sensor may be measuring the characteristics of the clear oil that rises above the wet oil. It is also possible that water will react with the oil to make a hydrocarbon hydroxide. That sludge may have a similar dielectric constant to the good oil. It might be better to use density rather than dielectric constant in that case.

Planar interdigitated electrodes are good for sensing material that is very close to the electrodes. The electric field geometry between the electrodes will be of a similar form, independent of the scale.

Since the sensor must “look” through the wall of the container, it should use a large sensor with wide separation between electrodes and a deep field. That will sample a greater volume of the oil. A very small sensor with many fingers will measure the characteristics of the container wall material only, with very little influence from the oil. The less sensor fingers you use, the bigger can be the sensor gaps and the deeper the sensor field can sample into the oil. There may be a small irregular gap between the wall and the sensor. The effect of that gap will be less for a large sensor than for a small sensor. You might consider a grease between the sensor and the bottle to eliminate the airgap.

Another problem is screening the sensor from behind. If a metallic screen is used and if that is placed close to the back of the sensor, then it will increase the fixed capacitance and so reduce the influence of the fluid. I would consider placing a slab of “closed cell foam” behind the sensor. Outside that foam I would attach a sheet metal screen for the sensor assembly. The thickness of a low dielectric constant foam will reduce the fixed capacitance. The screen will keep out external electrical interference. The sensor electronics can also be placed inside that foam cavity.

It might be good to use two differential electrodes inside a grounded case. The advantage of that configuration would be better screening and isolation from the external environment.

So I am suggesting you use a large differential pair of electrodes rather than a planar interdigitated electrode array.

5. May 12, 2015