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DP Transmitter for Level Measurement in Vacuum Conditions
Can we use a DP transmitter to measure the water level in a vacuum tank? If not, what is the solution for it?

Currently we are using DP to do the level measurement. But once the vacuum pump starts to vacuum the tank, the DP level keeps dropping and becomes negative. Can this application work by using a DP transmitter?

Do you have the LP connection connected to the top of the tank? The differential should not drop under vacuum if it is connected.


Certainly! Just connect the vacuum area to the low pressure side of the DP transmitter, and the liquid level to the high pressure side. This will work well unless the tank contents begins to boil under vacuum conditions - then the density changes and the level reading will be incorrect.

Dick Caro
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270

By Stefan Lynge Christensen on 3 October, 2008 - 4:20 pm

I agree with Dick.

The principle is used on power plants in condensator and low pressure feedwater preheaters.

We use to compensate the level calculation for the density changes that Dick mentioned, with the water-steam tables in DCS.

You have to use differential pressure arrangement where both measuring pipes are filled with water. Condensing pot is needed on the top measuring point (reference), which will fill the measuring pipe with water.

If your pressure in the tank corresponds to a boiling temperature lower than the temperature surrounding the tank, you will risk that the water boils in your pipe an the water level will be undefined. Then you will calculate a level which is higher than the reality in the tank (30 Celsius is about 0,04 bar absolute).

We use to add a little amount of water continuously to the reference pipe to avoid that problem.


I agree with all of the above and would add, that where the range of the dp measurement is very small, or where there is a small head of liquid in the impulse lines, a special DP transmitter should selected. This will ensure the silicon fill pressure and the type of silicon oil filling have been optimized for vacuum service.

By Stewart Emery on 5 October, 2008 - 4:49 pm

If you can not add fill liquid to the high static line, use an up incline and a vapor condensation cup to define the high point, so extra condensate will drain back to the vessel under vacuum. These are used both for boilers and condensers where there is a sizable vapor pressure to the liquid in the tank.


How about the direct type condenser, which the cooling water directly contact with steam from turbine, then create vacuum inside the condenser and there is some situation where the level control get fluctuation (hunting), but other situation it was normal. Is it due to the effect of vacuum which create the level get fluctuated?

By Robert Scott on 4 October, 2008 - 7:58 pm

If your DP reading drops when you apply a vacuum to the tank, then there are two possiblilities. One is that you have one side of the DP transducer connected to ambient air where it should be connected to the air in the top of the tank (and subject to vacuum). The other possibility is that the particular DP transducer you are using does not have very good common mode rejection. I have seen DP transducers that are affected by the ambient pressure even though both ports were connected to the same point.

If the problem is that you aren't connected to the air in the top of the tank, then that is easily solved. If that is not the problem, then tell me how much the DP reading changes when the vacuum reaches some known level. If the change in reading is about the same as the amount of vacuum applied, then you are almost certaining reading tank-bottom against ambient instead of tank-bottom against tank-top.

Robert Scott
Real-Time Specialties

What process did you try to measured?

Is that a Boiler Feed Water or similar with sugar refinery Vacuum Pan or Evaporators? If the Boiler Feed Water, the replies above are absolutely right, but if you try to measure the evaporators you should check the buffer of the tank. Is there's a leak? That can allow vacuum pressure to "drag" the liquids out of the process connection pipes. If it's used for the vacuum pan the DP should be measured in the vacuum state level conditions, and that means you need to recalibrate and readjust the range of that transmitter to vacuum range.

it is a Geothermal Power Plant, and we have 2 unit (2 condenser). One level control using DP transmitter and other use guided wave transmitter, but both almost has same phenomena which some condition it was fluctuating and other not. I don't it was due to the effect of vacuum inside or the condensation effect.

Is this something that just started to occur?
Are you the original poster?

Is the probe in a bridle or connected to the tank directly?


By Bart Schaminee on 31 October, 2009 - 10:49 am

More expensive, but a better measurement mechanism for measuring level fluid in vacuum environment is ultrasonic level measurement. Hope this helps.

Ultrasonic requires a physical gas medium (like air) for propagation; ultrasonic does not work in a vacuum.

Perhaps Mr. Schaminee means some other top mounted non-contact technology like radar or laser.

Try Using Displacer type level transmitter it works

> Currently we are using DP to do the level measurement. But once the
> vacuum pump starts to vacuum the tank, the DP level keeps dropping
> and becomes negative. Can this application work by using a DP transmitter?