detecting moisture in sOIL

strantor · Mar 1, 2016

Do you think that one of these Vegetronix soil moisture sensors would work for detecting moisture in OIL?

From their website:

our probe measures the dielectric constant of the soil using transmission line techniques

I was reading some propaganda about handheld "On-line Impedance-type Moisture Sensors" (for petroleum/oil) and it said:

A probe senses water at a representative point of the system. The devices change capacitance as water concentration increases and decreases.

So it seems like it's the same concept but what is your prediction for "would it work"?

kubeek · Mar 1, 2016

I can´t imagine that an electrode covered in oil would be able to detect conductivity of water, which implies it would need to detect subtle changes in impedance or more exactly in capacitance.
What are the ratios of water to oil that you are interested in? I would imagine detecting 1 part in 100 would be tricky, and in any use of oil I can imagine such number would be an awful lot and render the oil useles.

JimB · Mar 1, 2016

I have done some basic experiments and tests on capacitive oil/water probes.
From memory oil has a dielectric constant of about 2, and water has a dielectric constant of about 60.

Details of my initial experiments are in this thread, towards the end.
https://www.electro-tech-online.com...ill-run-to-an-led-display.107910/#post-883915

As for moisture in soil, I guess that would also change the capacitance.

JimB

cowboybob · Mar 1, 2016

Off the top of my head, I'd say no. But...

Since they explain their system's operation as:
"7... All we can say is that it's - magic. ... ", there's no way to tell. They do say it's "non-conductive". It's obviously self-powered.

The capacitive water/oil sensors use the properties of the oil/water fluid mix to alter the frequency of an oscillator. This is a fairly mature sensing scheme. And obviously designed for very harsh environments, i.e., not potting soil.

You may have already done this, but I did a google on "capacitance as water concentration in oil" and got a good number of relevant hits.

cowboybob · Mar 1, 2016

Stupid site double posted on me...

At least, I think that's what happened... .

audioguru · Mar 1, 2016

A soil moisture detector measures the resistance of the salty water in soil. When the soil is dry then there is no salty water so the resistance is much higher.
Condensation of water in the air that is in oil probably is pure water with no salts so its resistance would be near infinity, an insulator. Therefore a soil moisture detector will not detect it.

gophert · Mar 1, 2016

audioguru said:
A soil moisture detector measures the resistance of the salty water in soil. When the soil is dry then there is no salty water so the resistance is much higher.
Condensation of water in the air that is in oil probably is pure water with no salts so its resistance would be near infinity, an insulator. Therefore a soil moisture detector will not detect it.

audioguru / strantor
The dielectric constant of oil changes when water is added (very slightly solvable but oil/water emulsion will also change the dielectric constant as well. Here us one example of how it can be detected with a pair of plates - the changing properties of the fluid changes the capacitance of the plates.

https://www.google.com/patents/US5642098

atferrari · Mar 2, 2016

Strantor,

In tankers, to detect and measure water in oil (not as an emulsion but - we hoped - consistently settled in the bottom), we used a device that looked like a common sounding tape with a somewhat bulky "bob" at the end.

Submerged in the tank, it sounded a beeper when reached the water interface. Never had the time to look into the details. Maybe it was in line with Jim's design. With the vessel in calm water, repeatability was very good.

strantor · Mar 2, 2016

kubeek said:
What are the ratios of water to oil that you are interested in?

I don't need awesome accuracy or resolution. Ultimately I don't need to measure the content of water, just detect if water is present above a threshold value. Arbitrarily I pick 10%; if my volume of oil consists of 10% or more water, then output a digital high for an idiot light.

strantor · Mar 2, 2016

JimB said:
I have done some basic experiments and tests on capacitive oil/water probes.
From memory oil has a dielectric constant of about 2, and water has a dielectric constant of about 60.

Details of my initial experiments are in this thread, towards the end.
https://www.electro-tech-online.com...ill-run-to-an-led-display.107910/#post-883915

As for moisture in soil, I guess that would also change the capacitance.

JimB

I like this a lot. Thanks for the info. I think I'll try out your capacitive probe.

gary350 · Mar 2, 2016

Why don't you heat the oil to 300 degrees there wont be moisture in it then.

gophert · Mar 2, 2016

gary350 said:
Why don't you heat the oil to 300 degrees there wont be moisture in it then.

Because he will never notice the leak until it is large enough to overwhelm the heater that is heating the oil.

And, then he will need a big air conditioner to cool the room where the heater is located.

And, all the guys walking past the pipes full of hot oil might bet burned.

And, ...

JimB · Mar 2, 2016

strantor said:
I like this a lot. Thanks for the info. I think I'll try out your capacitive probe.

Thank you.

I did some more work on the two wire probe and oscillator, it is summarised here in pictures:
The oscillator:

A calibration graph of oscillator Frequency vs Capacitance (Ctest)

Graphed results of probe in air and in water.

I hope this is helpful for you.

JimB

Les Jones · Mar 2, 2016

Here is a capacity (and inductance.) meter design with a built in display that work quite well (I have built one.) It would suit the range of capacitance values you are interested in. It is the same as the design that was in the EPE magazine March 2010

Les.

chemelec · Mar 2, 2016

This Circuit Might detect the Presence of that Water:

http://chemelec.com/Projects/Fluid-Sensor/Fluid-Sensor.htm

jpanhalt · Mar 2, 2016

10% water in oil seems like a fairly high threshold. At that point, I suspect those methods based on physical displacement of the oil would be preferred. You could also look at light scatter for physically suspended water. At lower concentrations, you might want to consider IR. In the near IR, water shows a strong absorption around 1500 nm:

Surprisingly, the publication citation isn't shown on the Google link. To get it, you have to join for "free." I wasn't up for that, but here is the search header from Google:
**broken link removed**

And to tease your appetite, here is another figure showing results:

That apparently is dissolved water. The same IR source could be used for measuring light scatter too.

John

strantor · Mar 2, 2016

I wonder if a plain old dishwasher turbidity sensor would do the trick.

jpanhalt · Mar 2, 2016

It might and is an easy test to do. While scatter and turbidity are similar in some ways, scatter offers some different dimensions. I assumed that water droplets would have different reflective properties than suspended solids, like dirt. Thus an off center detector, or even a forward detector might provide differentiation between dirt and water or other immiscible liquid.

Idea #2: can you add a small amount of water soluble dye?

John

gophert · Mar 2, 2016

strantor said:
I wonder if a plain old dishwasher turbidity sensor would do the trick.

What kind of oil is it & what is it used for? Refined vegetable oil, mineral oil, used motor oil, crude oil, ... Lubricating oils change quite a bit over time. So do heat-transfer oils depending on service conditions. The expected changes during the life of the oil could be an issue.

Also, is there reservoir tank for the oil. A 10% volume change when water is present may allow a simple float switch to be used.

strantor · Mar 2, 2016

gophert said:
What kind of oil is it & what is it used for? Refined vegetable oil, mineral oil, used motor oil, crude oil, ... Lubricating oils change quite a bit over time. So do heat-transfer oils depending on service conditions. The expected changes during the life of the oil could be an issue.

Also, is there reservoir tank for the oil. A 10% volume change when water is present may allow a simple float switch to be used.

http://www.royalpurpleindustrial.com/products/marine-hydraulic-oil/

It's an under water electric motor full of hydraulic oil for pressure compensation. No reservoir.

detecting moisture in sOIL

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