# Simplified level probe

Discussion in 'The Projects Forum' started by beaster, Jun 9, 2010.

1. ### beaster Thread Starter New Member

Jun 7, 2010
3
0
OK my first post went nowhere so I'll simplify the problem and then try to expand on it later.

Essentially I'm looking for a way/probe that can indicate when I reach the interface of a binary liquid system (ie oil and water).
The probe will be lowered through the top liquid/layer and would indicate when the probe enters the lower liquid. Of note the top liquid has a much lower resistance than the lower liquid.

Any help would be greatly appreciated !

2. ### Bychon Member

Mar 12, 2010
469
41
Are you saying water floats on your oil or you have electrically conductive oil, or something else?

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
I'd approach it not from the electrical conductivity angle, but the dielectric constant.

Here's a page with lots of dielectric constants:
http://clippercontrols.com/pages/dielectric-values#O

The dielectric constant of a perfect vacuum is 1.
The dielectric constant of air at sea level isn't much higher; something like 1.0000001123 or thereabouts.
The dielectric constant of oils is in the 2 to 3-something range.
The dielectric constant of water at room temperature is 80.

So, if you had a couple of parallel metal conductive plates forming a capacitor, and they were used as part of the timing circuit in a free-running oscillator, you would see a sudden drop in frequency of the oscillator when the plates had water in between them instead of oil or air.

The plates should be electrically insulated from the liquid. A thin coat of lacquer, conformal coat or epoxy paint should be sufficient.

4. ### beaster Thread Starter New Member

Jun 7, 2010
3
0
I solved my problem

Unfortunately capacitance approach was not an option due to the extreme temperature and corrosive nature of one of the liquids. I simply used a voltage divider with an LED in parallel as an indicator. When I'm in the higher resistance liquid the LED is on and when I move to the lower resistance liquid the LED is off.
Extremely simple and it works !