Re: Measuring liquid level with bubbles using an ultrasonic transducer

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On 20 Feb 2007 03:38:25 -0800, overgeo@xxxxxxxxx wrote in message
<1171971504.954131.161090@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:

>Hello, i got a tank that needs a control of liquid level, i was
>thinking about using an ultrasonic transducer, but my problem is that,
>when the tank is being filling produces many many bubbles maybe if the
>tank has 4 meter, 1 meter are bubbles, my question is if the
>ultrasonic transducer will be able to measure the real level of the
>liquid?.

>In addition the tank is in almost vacuum, the transducer can has
>problems with this?.

What does "almost vacuum " mean?

Conventionally this is done with a differential "wet-wet" differential
pressure transducer that is vented to the airspace inside of the tank and
measures the pressure at the bottom of the tank. Did you try this and it
didn't work well?

This arrangement will measure the static pressure head in the tank and
account for changes in the pressure in the head space ("almost vacuum" or
whatever). The height of the fluid above the transducer inlet, h = pressure
head/(specific gravity * g)  where g = acceleration due to gravity.

Or simply calculate the static head (height of fluid above the pressure
transducer  inlet) as if it were water and multiply by the specific gravity
of the fluid.

A "stilling well" as suggested by Les is a good idea because it will help to
dampen the variation in output caused by filling but is not needed to deal
with the bubble problem. If the liquid is _extremely_ viscous and the bubbles
are substantially supported by the sides of the vessel, the measured amount
will be too small regardless of whether or not the transducer is in a
stilling well.

You would need to use a "wet-wet" transducer in which the materials used on
both the measurement and vent sides of the transducer are unharmed by the
fluid being used. Typically the material used is a stainless steel.

There are many, many sources of transducers and associated electronics. If
you provide more information on the application, budget and accuracy
required, we could be more specific about transducer recommendation. How and
when will you use the information? Only while filling?


... Marc
Marc_F_Hult
www.ECOntrol.org