Liquid Level Detection

[Louis Bybee]

I recently finished the startup on a Waste Water Reclaim System. Most of the
system came up rather uneventfully with the exception of the gray water
storage tank. It had a tendency to form froth on the liquid surface.
Accurate liquid level detection in this tank is important for stable system
performance.

An ultrasonic level detector was factory installed.

It became apparent rather quickly that the sensor was detecting the froth,
and the resulting false reported liquid level in the tank was responsible
for frequent cycling, and system shut down.

I solved the issue in the short term by installing a large cylinder in the
tank, centered under the sensor, with holes in the lowest portion. That
allowed system startup, and tuning, to be completed.

My question is the availability of a non-contact sensor that will detect
true liquid level through foam, or froth?

Thank you.

Louis--
*********************************************
Remove the two fish in address to respond

 

[Cameron Dorrough]

I recently finished the startup on a Waste Water Reclaim System. Most of the
system came up rather uneventfully with the exception of the gray water
storage tank. It had a tendency to form froth on the liquid surface.
Accurate liquid level detection in this tank is important for stable system
performance.

An ultrasonic level detector was factory installed.

It became apparent rather quickly that the sensor was detecting the froth,
and the resulting false reported liquid level in the tank was responsible
for frequent cycling, and system shut down.

I solved the issue in the short term by installing a large cylinder in the
tank, centered under the sensor, with holes in the lowest portion. That
allowed system startup, and tuning, to be completed.

My question is the availability of a non-contact sensor that will detect
true liquid level through foam, or froth?

<link to sci.engr.control added>

If the ultrasonic sensor is any good, you should be able to adjust it to
work through foam. If it's a cheapie preset one, then you need to buy a
real one (like www.hawklevel.com or similar)

Why does it have to be non-contact? Displacer-type transmitters will likely
be much more reliable in a foam situation.

Cameron

 

[Walter Driedger]

Radar cuts through foam. On the whole radar seems to be a superior
solution.

Walter.

 

[**THE-RFI-EMI-GUY**]

How about a separate standpipe piped in from the bottom of the tank to a
location alongside. The standpipe would only have water displaced from
the bottom, no foam.

 

[Louis Bybee]

<link to sci.engr.control added>

If the ultrasonic sensor is any good, you should be able to adjust it to
work through foam. If it's a cheapie preset one, then you need to buy a
real one (like www.hawklevel.com or similar)

Why does it have to be non-contact? Displacer-type transmitters will likely
be much more reliable in a foam situation.

Cameron

Non-contact was a customer spec. As I understand it the owner remembered
having problems with material collection on previous contact type sensors.

I agree with you, and suggested a pressure transmitter with an air bleed,
but the owner wasn't receptive to anything except an ultrasonic (they were
in use in other areas of the plant (in non-foaming environments) without
trouble).

Thank you.

Louis

 

[ripper]

If the water is relatively clean you could use submersible pressure
transmitters, alternatively a DP cell.
If the water has suspended solids and you have compressed air to hand, you
could use the good old bubble tube and DP cell.
Also Milltronics make the radar type reportedly good for foam, but I haven't
tried them so cant comment.

 

[Louis Bybee]

That is essentially what I created. I used a large piece of PVC pipe
installed vertically in the tank with holes drilled in the very bottom, and
the top of the pipe was above the liquid/foam level.

Even with the water level never dropping below the holes in the pipe there
was enough turbulence in the water that some foam accumulated inside the
pipe, and had to be cleaned out once a month.

The owner insisted on that particular sensor, and a solution be found that
included the use of it.

Now knowing the limitations of an ultrasonic sensor in a foam environment, I
was looking for answers for "the next time".

Thank you.

Louis

 

[Dick Alvarez]

contact was a customer spec. As I understand it the owner remembered
having problems with material collection on previous contact type
sensors. ... The owner insisted on that particular sensor, and a
solution be found that included the use of it.>>

Unless a customer really knows what he is doing, some times it is
necessary, and quite ethical, to educate the customer. That
generally is one of the functions of an engineer. A good customer
appreciates it, and it builds good-will for you.

<<I was looking for answers for "the next time".>>

This is not my field of electrical engineering, but I got involved
in liquid level controls in our Boy Scout camps. I used mostly
Warrick Controls electromechanical and electronic conductivity
sensors and controls for water wells and tanks and for sewage
pumping. Probably you can choose the sensitivity to detect the
liquid but not to the foam. Warrick makes a variety of controls,
electrodes, electrode fittings, etc. Unless the foam is really
exotic, it should be possible to select electrodes that will be
resistant to the foam's effects. For example, you might want to use
standard rod-type electrodes that are coated with Teflon or
equivalent almost to the lower end. You probably would not want to
use simple wire-suspended shielded electrodes, as they could become
clogged with foam.

Warrick has a Web page. You may need to study it a bit to see
your choices on sensitivity and electrodes etc. Then contact their
representative. I used Calpacific Equipment Company, in Berkeley,
California. A Google search shows many representatives.

Other manufacturers make similar equipment, but I am less familiar
with them than with Warrick.

More elaborate systems, like ultrasonic, radar, etc., have their
places. But where a simple system will do the job, generally it
reduces cost and increases reliability. Beyond simple
float-switches, conductivity sensors are pretty simple.

It was a long time ago that I worked on such things in Scout
camps, so I an not current on such equipment. (It was a long time
ago, because the equipment still is there and functioning.)

Dick Alvarez
alvarez at alumni dot caltech dot edu

 

[ripper]

There is another product you can use, it is called Metritape, but it is
expensive.
Basically it is a resistor encaplsulated in a tube. The head of liquid
compresses the tube thus varying the resistance.
Do a search for Metritape.

 

[Louis Bybee]

I recently finished the startup on a Waste Water Reclaim System. Most of the
system came up rather uneventfully with the exception of the gray water
storage tank. It had a tendency to form froth on the liquid surface.
Accurate liquid level detection in this tank is important for stable system
performance.

An ultrasonic level detector was factory installed.

It became apparent rather quickly that the sensor was detecting the froth,
and the resulting false reported liquid level in the tank was responsible
for frequent cycling, and system shut down.

I solved the issue in the short term by installing a large cylinder in the
tank, centered under the sensor, with holes in the lowest portion. That
allowed system startup, and tuning, to be completed.

My question is the availability of a non-contact sensor that will detect
true liquid level through foam, or froth?

Thank you.

Louis--

Thanks to all who responded.

I have received some excellent suggestions, and sources, that will allow me
to offer reasonable solutions the next time this comes up.

The building owner refused to even consider other alternatives. Even with
multiple people advising that the sensor he requested was considerably less
than desirable for the application. Even the company that built the recovery
system advised in writing that they wouldn't be responsible for the results.
With our litigious society today I'm surprised they even consented to build
it.

The system has been running successfully (with monthly cleaning of the well
pipe) now for about six months. I can't believe how bull-headed the owner
was! He had his mind made up, and wasn't about to be confused with any
facts, or undesirable results! :-]

Thank you.

Louis

 

[Hugh Neary]

I recently finished the startup on a Waste Water Reclaim System. Most of the
system came up rather uneventfully with the exception of the gray water
storage tank. It had a tendency to form froth on the liquid surface.
Accurate liquid level detection in this tank is important for stable system
performance.

An ultrasonic level detector was factory installed.

It became apparent rather quickly that the sensor was detecting the froth,
and the resulting false reported liquid level in the tank was responsible
for frequent cycling, and system shut down.

I solved the issue in the short term by installing a large cylinder in the
tank, centered under the sensor, with holes in the lowest portion. That
allowed system startup, and tuning, to be completed.

My question is the availability of a non-contact sensor that will detect
true liquid level through foam, or froth?

Thank you.

Louis--
*********************************************
Remove the two fish in address to respond

Is the foam a uniform blanket, or is it isolated patches that float on
the surface. What is the chemical producing the foam?

If the foam is not too heavy or of uniform thickness and density it
should be possible to tweak the ultrasonic. Failing this a chemical
de-foaming agent or modification to your process would have to be
considered.

HN

 

[daestrom]

I recently finished the startup on a Waste Water Reclaim System. Most of the
system came up rather uneventfully with the exception of the gray water
storage tank. It had a tendency to form froth on the liquid surface.
Accurate liquid level detection in this tank is important for stable system
performance.

An ultrasonic level detector was factory installed.

It became apparent rather quickly that the sensor was detecting the froth,
and the resulting false reported liquid level in the tank was responsible
for frequent cycling, and system shut down.

I solved the issue in the short term by installing a large cylinder in the
tank, centered under the sensor, with holes in the lowest portion. That
allowed system startup, and tuning, to be completed.

My question is the availability of a non-contact sensor that will detect
true liquid level through foam, or froth?

Do you have access to some low-pressure air? One system that is pretty
immune to dirty water and contamination is to bubble air down through a tube
to near the tank bottom. Use a fine needle-valve to control the flow rate
to a small, steady amount. The pressure in the tube downstream of the
needle valve will be proportional to the liquid level. Then a pressure
switch or transducer can give you the level.

The inside of the tube is rarely wetted (only on a loss of air), so it
doesn't clog.

daestrom

 

[Louis Bybee]

Is the foam a uniform blanket, or is it isolated patches that float on
the surface. What is the chemical producing the foam?

If the foam is not too heavy or of uniform thickness and density it
should be possible to tweak the ultrasonic. Failing this a chemical
de-foaming agent or modification to your process would have to be
considered.

HN

The foam is non-uniform in level, and consistency. It is formed primarily
from soaps, and other related compounds. This is an effluent tank with a
rather high throughput so chemical treatment specifically for defoaming is
undesirable from many levels.

Others have suggested solutions that appear to be very workable.

Thank you.

Louis--
*********************************************
Remove the two fish in address to respond

 

[Louis Bybee]

Do you have access to some low-pressure air? One system that is pretty
immune to dirty water and contamination is to bubble air down through a tube
to near the tank bottom. Use a fine needle-valve to control the flow rate
to a small, steady amount. The pressure in the tube downstream of the
needle valve will be proportional to the liquid level. Then a pressure
switch or transducer can give you the level.

The inside of the tube is rarely wetted (only on a loss of air), so it
doesn't clog.

daestrom

One of the sensor types I suggested was a pressure transducer with purge air
(along the lines of a bubble tube). I have used these in the past where
corrosive chemical level was measured.

The customer didn't want any external level measuring devices such as, sight
glasses - still wells - etc., on the tank. He insisted on specifying
sensors, and left others to make the misapplied ones work.

This owner was a real pain. His requests were rarely based on rational data.
I can only imagine the pleasure of working for him on a regular basis.

Thank you.

Louis--
*********************************************
Remove the two fish in address to respond

 

[Rowbotth]

If you can live with a range where the detector operates - for instance,
the tank may be 50 feet deep, but if you can live with a detection
system that could be installed in the top 10 to 20 feet - you might be
able to do something with a radioactive device. These can be programmed
to detect the level of the substance you are looking to control, and
they would definitely ignore foam.

I know Ronan Engineering used to make these - as far as I know they are
still in business. Seems like an anwer.

HR.

 

[daestrom]

One of the sensor types I suggested was a pressure transducer with purge air
(along the lines of a bubble tube). I have used these in the past where
corrosive chemical level was measured.

The customer didn't want any external level measuring devices such as, sight
glasses - still wells - etc., on the tank. He insisted on specifying
sensors, and left others to make the misapplied ones work.

This owner was a real pain. His requests were rarely based on rational data.
I can only imagine the pleasure of working for him on a regular basis.

Yeah, I know just the type. Had similar customers myself. That's when you
look at your quarterly income statement and decide just how badly you need
the work ;-) Hate it when they hire you for your expertise, then ignore
your advice. The worst ones are those that then criticize the results and
conveniently 'forget' that half the design was their idea.

Oh well, you can only do so much....

daestrom

 

[B J Conner]

Suggest a gamma ray ray radiation system. Some of the better I deals you
have had may seem more practical..

 

[rob]

Gamma rays?

Suggest a gamma ray ray radiation system. Some of the better I deals you
have had may seem more practical..