Ultrasound Level Transmitters vs. Pressure Level Transmitters

K

Thread Starter

khairil

To whom it may concern.

I am currently working on a project developing a monitoring system for water level in a tank. I have been given 2 proposals to use either:
- ultrasound level transmitters, or
- pressure level transmitter (if there are such transmitters)
to monitor the water level in the tank. Either transmitters will be installed at the surface of the water level.

I was informed by the contractor that the ultrasound level transmitter is better than the latter. What I would like know are these:
- what is the difference between the two transmitters?
- which would suit better for my application? i.e. to monitor water level in the tank

Thank you in advance to those who reply.

Best regards.
 
i suggest to use ultrasound transmitter rather than differential pressure transmitters because ultrasound transmitters are more accurate & less maintenance, mounting also easy.

thanks.
 
S

Scott Spencer

Ultrasonic measures the distance from a sensor to the top surface of the liquid. It can be affected by steam, dust, foam or turbulence of the surface for accuracy and repeatability. It does not rely on the physical properties of the liquid for measurement.

The pressure transmitter measures the hydrostatic head of a liquid column above the port of the transmitter. It can be affected by changes in specific gravity (temperature, compositon of solution) and would be suspended by cabel to the tank bottom. Vented cable systems are common and are prone to wear and tear on the cable, corrosion of the cell or failure from improper maintennce.

We supply both, application dependent, and can provide additional information if needed.

Good luck,
Scott Spencer
Engineering Performance
[email protected]
 
V

Verhappen, Ian

Either method will work though you will need to mount the DIFFERENTIAL pressure transmitter with one leg at the bottom of the tank and the other near the top. Level is then calculated using the formula deltaP = density x gravity x height. Since density and gravity should not change you then get height being proportional to the change in pressure. You need the upper tap to always be in the vapour space so you get "automatic" correction for changes in pressure in the vessel. If you, suspect the fluid density will change then you should use 2 dP transmitters with one set ALWAYS below the Low Liquid Level so you can use it to calculate liquid density.

Ultrasonic in some cases is affected by "steam" though otherwise it has the advantage of requiring only 1 nozzle and can be loop powered as well.

Hope this helps.

Ian Verhappen
 
B

Bruce Axtell

"Better" depends on the application.

Just a couple considerations--

It depends on what you are reading. If there is foam, agitators, turbulence, temperature extremes, etc., ultrasonic may give you troubles.

Pressure is measured at the bottom of the tank, not the top, thus requiring plumbing which can be subject to fouling.

Contacting the applications department of level control manufacturers may help you the most. They can steer you in the right direction.

Bruce
 
N

Nuno Moreira

Ultrasonic level transmitters are installed on the top of the tank and emits an ultrasonic wave that is reflected in the water surface. Based on the time of flight (actually half of the time of flight) of the wave, the distance between the top of the tank the water surface is computed.

A differential pressure transmitter (or gauge pressure transmitter) can be used to measure liquid level based on the liquid density. The diff. pressure trasmitter would sense the pressure difference between the two pressure intakes (one sensing the pressure on the bottom of the tank and the other one on the top of the tank). If the density of the liquid is known (1000 Kg/m3 for pure water) one can compute the liquid level, e.g. 10 linear meters of pure water would produce around 1 bar of pressure difference between the two sensing legs.

If the tank is at atmospheric pressure then, a gauge pressure transmitter can be used since the top pressure intake will always be 0 barg and diff. pressure will always be equal to the pressure sensed at the bottom leg.
Using the same measuring principle the submersible level transmitter can be used with the advantage of easier installation.

The main advantage of using the differential pressure transmitter is the price and simplicity of the instrument itself. In terms of installation price in the case of atmospheric tanks the price is similar on both options, since only one process connection is required (bottom of tank for pressure transmitter and top connection for ultrasonic level transmitter). Complexity of transmitter configuration is quite similar.

The main drawback of the diff. pressure transmitter is the errors caused by the dependency on the density of the liquid (either by temperature changes and by different composition... water is never pure water).

In terms of price:
-Submersible level transmitter ~$350 to $600
-Differential Pressure trasmitter ~$200 to $500
-Ultrasonic Level tranmitter ~$500 to ~$1000 depending on the measuring range

Hope this helps.
 
Have you considered a guided-wave radar? It installs on the top of the vessel and is independant of surface turbulence, density changes, temperature or liquid composition. Magnetrol makes an excellent product. A little more expensive than ultrasonic, but well worth it.
 
We have ultrasonics and hydrostatic pressure. I always recommend Noncontact Ultrasonics assuming the application is within specs.

The Pulsar Vanguard (http://pulsar-us.com/docs/LowLit-Vanguard.pdf) and dB transducers (http://pulsar-us.com/docs/LowLit-dB-Transducers.pdf) are extremely easy to set up and reliable. The programming is menu-driven, so a manual is not needed.

The Pulsar Oyster 700 (http://pulsar-us.com/docs/LowLit-Oyster.pdf) is a submersible pressure transducer that comes precalibrated to the specified pressure or water column of the tank height. It comes with a cable and is just lowered to the tank bottom from the top. Obviously we can't have agitators.

Please visit our website for futher information http://www.pulsar-us.com.

Good luck,
Robert
 
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