Sensor design - dielectric constant of water

[Klaus Kragelund]

Hi

I'm working on a water sensor (simply detect water or no water). The
dielectric constant of water is about 80. But what about the value of
the constant with different pureties of the water (salt, pH)?

The application is a pump which has a metallic surface connected to the
earth wire. The sensor consists of a metal plate housed in a plastic
enclousure mounted on the chassis of the pump. The capacitance measured
is the one from the metal plate to the chassis - growing in value when
the water is present.

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Regards

Klaus

 

[[email protected]]

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Remember to use AC, otherwise you'll electrolyze the water and the
measured impedance will go up.

 

[Dan Hollands]

Since normal water is not a very good insulator it will make a very leaky
capacitor

All sensors I have seen use the resistance to detect presence or absence of
water

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
[email protected]
www.QuickScoreRace.com

 

[petrus bitbyter]

Hi

I'm working on a water sensor (simply detect water or no water). The
dielectric constant of water is about 80. But what about the value of
the constant with different pureties of the water (salt, pH)?

The application is a pump which has a metallic surface connected to the
earth wire. The sensor consists of a metal plate housed in a plastic
enclousure mounted on the chassis of the pump. The capacitance measured
is the one from the metal plate to the chassis - growing in value when
the water is present.

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Regards

Klaus

Klaus,

As so often, it depends. Pure water is an insulator so the capacitive
approach may work pretty well. But pure water is rare and you need only very
little pollution to make a conductor. Measuring conductivity using an AC
current may work better but you have to account for a wide range of
conductivity values. (Unless the water has a more or less constant pollution
of course but I understand from your question that this is not the case.) To
use a capacitive sensor you can insulate the plate from the water. The
effect can be compared with old wet electrolityc capacitors. With fluid high
capacitance, without low capacitance.

petrus bitbyter

 

[Yukio YANO]

Hi

I'm working on a water sensor (simply detect water or no water). The
dielectric constant of water is about 80. But what about the value of
the constant with different pureties of the water (salt, pH)?

The application is a pump which has a metallic surface connected to the
earth wire. The sensor consists of a metal plate housed in a plastic
enclousure mounted on the chassis of the pump. The capacitance measured
is the one from the metal plate to the chassis - growing in value when
the water is present.

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Regards

Klaus

If you have to move enough water to require a pump why not a float
sensor ie a magnetic float anda reed switch !.I can agree with "Exotic
sensors for small volumes but for Gallons !

Yukio YANO

 

[Klaus Vestergaard Kragelund]

If you have to move enough water to require a pump why not a float
sensor ie a magnetic float anda reed switch !.I can agree with "Exotic
sensors for small volumes but for Gallons !

Yukio YANO

The current design is with a float sensor, but we are trying to optimize
the costs and design of the product

Regards

Klaus

 

[Klaus Vestergaard Kragelund]

Klaus,

As so often, it depends. Pure water is an insulator so the capacitive
approach may work pretty well. But pure water is rare and you need only very
little pollution to make a conductor. Measuring conductivity using an AC
current may work better but you have to account for a wide range of
conductivity values. (Unless the water has a more or less constant pollution
of course but I understand from your question that this is not the case.) To
use a capacitive sensor you can insulate the plate from the water. The
effect can be compared with old wet electrolityc capacitors. With fluid high
capacitance, without low capacitance.

petrus bitbyter

Thanks

Yes - the current sensor design is isolated from the water. So in effect
the path is a capacitor with first a plastic distance before reaching
the electrolytic - and then direct contact to the other electrode. We
are measuring 4pF with no water and >20pF with water (1inch^2 area). But
the capacitance measurement is I suspect heavily "poluted" by the
conductance of the water.

Thanks

Klaus

 

[Jim Thompson]

Hi

I'm working on a water sensor (simply detect water or no water).

This was done back in the early '70's with a simple metal collar
around the upper radiator hose and measuring capacitance from the
collar to ground.... no water means you've lost your lower capacitor
plate.

I've also used a copper pipe inside PVC, then a wire cage about 0.5"
outside of that, to measure (irrigation canal) water DEPTH.

The
dielectric constant of water is about 80. But what about the value of
the constant with different pureties of the water (salt, pH)?

The application is a pump which has a metallic surface connected to the
earth wire. The sensor consists of a metal plate housed in a plastic
enclousure mounted on the chassis of the pump. The capacitance measured
is the one from the metal plate to the chassis - growing in value when
the water is present.

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Regards

Klaus

...Jim Thompson

 

[Jim Thompson]

Thanks

Yes - the current sensor design is isolated from the water. So in effect
the path is a capacitor with first a plastic distance before reaching
the electrolytic - and then direct contact to the other electrode. We
are measuring 4pF with no water and >20pF with water (1inch^2 area). But
the capacitance measurement is I suspect heavily "poluted" by the
conductance of the water.

Thanks

Klaus

See my other post. Don't fret over the "polluted" water, consider it
a conductor... make IT the "other electrode".

...Jim Thompson

 

[Adrian Jansen]

Hi

I'm working on a water sensor (simply detect water or no water). The
dielectric constant of water is about 80. But what about the value of
the constant with different pureties of the water (salt, pH)?

The application is a pump which has a metallic surface connected to the
earth wire. The sensor consists of a metal plate housed in a plastic
enclousure mounted on the chassis of the pump. The capacitance measured
is the one from the metal plate to the chassis - growing in value when
the water is present.

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Regards

Klaus

Honeywell make some very nice liquid sensors based on total internal
reflection of a IR LED. Single unit, single hole mount, no contact
between the liquid and electronics.


--
Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

 

[Roger Lascelles]

Hi

I'm working on a water sensor (simply detect water or no water). The
dielectric constant of water is about 80. But what about the value of
the constant with different pureties of the water (salt, pH)?

The application is a pump which has a metallic surface connected to the
earth wire. The sensor consists of a metal plate housed in a plastic
enclousure mounted on the chassis of the pump. The capacitance measured
is the one from the metal plate to the chassis - growing in value when
the water is present.

Right now it looks to be a capacitive sensor design. But I also might
opt for a conductivity measurement of the water instead. Any comments
and experience in this field?

Regards

Klaus

You don't need to reinvent this - its all been done before for low cost
water level sensing.

Use a metal pipe standing vertically in the water.

Down the center of the pipe you run a teflon insulated wire which goes down
the bottom, round a rod and back to the top, so that only insulation is
under the water and you don't have to seal the ends. Between the wire and
the pipe you have a nice capacitor with zero DC leakage. Capacitance is
pretty well proportional to the water depth and is insensitive to water
salinity and wire positioning. The pipe protects the whole thing nicely.

The secret is that the water acts mostly as an AC short and the teflon is
the dielectric.

Don't use PVC wire - eventually the PVC leaks.

To measure the capacitance, you use a 7555 timer with the wet capacitor in
the timing circuit. The pipe is the earthy side of the capacitor. The 7555
is stable with temperature. The logic output of the 7555 runs to your
microprocessor.

Google will find you schematics, pictures of this sort of thing.

Roger Lascelles

 

[John Woodgate]

I read in sci.electronics.design that Klaus Vestergaard Kragelund

We are measuring 4pF with no water and >20pF with water (1inch^2
area). But the capacitance measurement is I suspect heavily "poluted"
by the conductance of the water.

It doesn't matter, or at least you can make it not matter. If you use a
bridge to measure the capacitance, you simply balance out the resistive
component. If you measure voltage and current, measure the current with
a phase detector to isolate the quadrature component. Use as high a
frequency as is convenient, so as to make the capacitive reactance as
close to the same order of magnitude as the resistance as you can.