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'[EE]: Liquid Level Sensing Methods'
2002\10\18@121114 by Tim McDonough

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I'm researching various methods used to sense the level of liquids in containers in the 50-125 gallon range. So far I have found various web sites and manufacturers that use:

- mechanical floats
- ultrasound
- radio frequency
- capacitance

Due to cost considerations I'd prefer to develop my own electronics since I could incorporate other pieces of the control system plus the level sensing into a single package. Continuous level sensing isn't a strict requirement. Being able to detect the presence or absence of the liquid at half a dozen tank positions would be acceptable.

The liquid can be of varying viscosity and cleanliness (turbidity?) so mechanical apparatus that must move to function is undesirable since float switches, etc. tend to get crudded up quickly. A separate company's application with ultrasound proved both unreliable and expensive.

Temperature of the material is normally around 80F-100F with short excursions to 300F.

The tank itself is metal and grounded. Explosion concerns are not an issue. They are closed tanks, i.e.--no open tops but there is access to insert sensors, etc.

My assumption at this point is that a capacitive type system uses sense elements as plates of a capacitor and the material changing levels displaces air as the dielectric causing the value of the capacitor to change. This would then shift the frequency of an oscillator and the shift is correlated to container level.

Any of you experienced sensor engineers have suggestions or comments?

Tim

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2002\10\18@124254 by Dave Tweed

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Tim McDonough <.....timKILLspamspam@spam@MCDONOUGH.NET> wrote:
> I'm researching various methods used to sense the level of
> liquids in containers in the 50-125 gallon range. So far I have
> found various web sites and manufacturers that use:
>
> - mechanical floats
> - ultrasound
> - radio frequency
> - capacitance

Have you looked at Metritape (http://www.consiliumus.com/Metritape.htm)?
It's basically a plastic tube that deforms slightly under the static
pressure of the fluid and provides a variable resistance as a result.
This looks like a good all-around solution for your requirements, with
perhaps a simpler interface than a capacitive probe. I don't know if it
can handle your peak temperature, though.

-- Dave Tweed

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2002\10\18@131352 by Roman Black

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Tim McDonough wrote:
>
> I'm researching various methods used to sense the level of liquids in containers in the 50-125 gallon range. So far I have found various web sites and manufacturers that use:
>
> - mechanical floats
> - ultrasound
> - radio frequency
> - capacitance
>
> Due to cost considerations I'd prefer to develop my own electronics...
> Being able to detect the presence or absence of the liquid at half a dozen tank positions would be acceptable.
>
> The liquid can be of varying viscosity and cleanliness


When I worked in an industrial instrument fitters
we had exactly the same problem, with sludge tanks,
acid tanks and even sewerage tanks. :o{

The best system we had (and zero maintenance!) was
to insert a tube down into the liquid, attach it to
the compressed air mains via a small bleeder and then
just measure the pressure after the bleeder. Some had
just a pressure gauge, some had remote monitoring.
All your pressure sensors and electricals are well away
from the messy tanks. The small but continuous flow from
the air bleeder is enough to provide some passage of
air out of the tube, so they never get blocked.
Obviously the pressure in the tube is dependant on
the liquid depth.
-Roman

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2002\10\18@143933 by Chris Loiacono

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To follow Roman's lead, yet without supplying compressed air, a simple
pressure sensor such as one from motorola's MPS line at around $5-10 US at
the top of such a tube would suffice. The sensor range should be selected
according to the tank depth - there are sensors that will detect even the
slightest change in pressure that will work with even shallow depths as the
level rises and drops..
You may want a differential type sensor so barometric pressure will be
compensated for.
If you prefer something other than motorola, try David Ezekiel at servoflo:
EraseMEservoflo.davidspam_OUTspamTakeThisOuTverizon.net he specializes in sensing devices.


{Quote hidden}

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2002\10\18@154343 by Russell McMahon

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/
I'm researching various methods used to sense the level of liquids in
containers in the 50-125 gallon range. So far I have found various web sites
and manufacturers that use:
/

See "water level in fire truck tank" discussion some months ago.

Consider (as discussed then) an air column with a pressure sensor at the
top. This method is used in many (most?) washing machines for water level
sensing.

Also more recently see "Fuel tank sensor / real world solutions"

       RM

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2002\10\18@160716 by Mike Mansheim

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> I'm researching various methods used to sense the level of liquids in
> containers in the 50-125 gallon range.

I remember a lot of discussion of this a while back, especially regarding
the column of air technique.  Might be informative to look back and see
what was discussed then.
I can't seem to find the beginning of the thread, but here's part of it:
http://www.infosite.com/~jkeyzer/piclist/2000/Jun/0103.html

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2002\10\18@172331 by Brent Brown

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You didn't list conductivity type probes. I have done a few of these
and I've posted to the PIClist on the topic before. Your application
could be well suited to this method, especially because you have a
grounded metal tank and non-explosive atmosphere.

You need AC bias on a probe to prevent electrolysis. Basically you
use an oscillator through a resistor then a capacitor to the probe
and then an input from the capacitor to your micro. Frequency can be
just about anything, suggests a few hundred hertz. Conductivity
through the liquid to ground attenuates the signal, your micro
detects the lost signal as liquid contact on that probe. That's it.

I love using CMOS schmitt trigger parts for interfacing these things.
Gives a more defined trip point for each probe. The 40106 is a hex
schmitt inverter which is cool for a 6 level probe or use one
inverter as the oscillator and you have 5 inputs. I can post
schematics if you're interested.

The same scheme can be used to make a capacitive probe. Just increase
the frequency and resistor value and make the probe an insulated
plate.

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16 English Street, Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
eMail:  spamBeGonebrent.brownspamBeGonespamclear.net.nz

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2002\10\21@121549 by Tim McDonough

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On Sat, 19 Oct 2002 10:17:20 +1300, Brent Brown wrote:
>You didn't list conductivity type probes. I have done a few of these
>and I've posted to the PIClist on the topic before. Your application
>could be well suited to this method, especially because you have a
>grounded metal tank and non-explosive atmosphere.

I'm not certain if a conductivity type probe would work in this application. The material in the tanks is comprised of a lot of oil which I believe is not conductive. In fact it's used as a coolant in big transformers, etc. Am I off base here?

Tim

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2002\10\21@123423 by Dave Mumert

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Hi

Here is a page that has an article about building a capacitive fuel gauge
for homebuilt aircraft.  It should work just as well for oil.

http://www.rst-engr.com/kitplanes/

Dave

----- Original Message ----- I'm not certain if a conductivity type probe
would work in this application. The material in the tanks is comprised of a
lot of oil which I believe is not conductive. In fact it's used as a coolant
in big transformers, etc. Am I off base here?

Tim

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2002\10\21@142413 by Peter L. Peres

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On Mon, 21 Oct 2002, Tim McDonough wrote:

*>On Sat, 19 Oct 2002 10:17:20 +1300, Brent Brown wrote:
*>>You didn't list conductivity type probes. I have done a few of these
*>>and I've posted to the PIClist on the topic before. Your application
*>>could be well suited to this method, especially because you have a
*>>grounded metal tank and non-explosive atmosphere.
*>
*>I'm not certain if a conductivity type probe would work in this
*>application. The material in the tanks is comprised of a lot of oil
*>which I believe is not conductive. In fact it's used as a coolant in big
*>transformers, etc. Am I off base here?

Go capacitive imho. Oil wants capacitive.

Peter

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2002\10\21@144527 by Tim McDonough

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On Mon, 21 Oct 2002 10:32:41 -0600, Dave Mumert wrote:

>Here is a page that has an article about building a capacitive fuel
>gauge for homebuilt aircraft.  It should work just as well for oil.
>
>http://www.rst-engr.com/kitplanes/

Dave --

Looks promising. Thanks.

Tim

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