Level Measurement Using DP Transmitter

[James08]

When you use a differential pressure transmitter to measure the height of the fluid inside the tank (where the high pressure side of the DP transmitter is connected to the fluid and the low pressure side is open to the atmosphere), do you have to account for air buoyancy to get the true measurement values? Could you provide a piece of literature to prove it or disprove it?

 

[MortenA]

Are you thinking of air dissolved in the liquid or air above the liquid level?

If its air above the liquid level this would not have any influence unless its under a very high pressure (thus get a density worth considering). Air will have a density about 800 times less than that of water at ambient conditions. So if you are measuring a water level of 1 meter with 10 meters of air above it the the error would be aprox. 1 %

Best regards

Morten

 

[Ashereng]

I am not sure what you mean by air buoyancy. Buoyancy is a result of differring densities. Air in the fluid will be taken care of by the density in the equation to calculate hydrostatic head/pressure: density x gravity x height.

I am assuming you have a enclosed tank, with either floating roof or VRU. In this case, the DP transmitter should still work since the vapour pressure is more or less stable (the same). DP to level is a correlation. As long as everything stays relatively close to the original calibration conditions, it should continue to give the same results.

 

[jmw]

I think you are referring to the effect of the density of the fluid. In which case there is a correction for the density of the fluid in air to the density of the fluid in vacuo.

http://www.measurementuncertainty.org/mu/guide/index.html?content_frame=/mu/guide/appendix_g.html

see density effects.

JMW

http://www.ViscoAnalyser.com

 

[jmw]

PS try this description:
"The terms 'density in vacuo' or 'density in air' are sometimes used on fuel delivery or bunker receipt notes. As density is the absolute relationship between mass and volume and not its weight to volume, by definition density is in vacuo. Although often used, the term 'density in air' is incorrect and should be referred to as a 'weight factor'. This is because a substance weighed in air is supported to a small extent by the buoyancy of air acting on it. Thus the weight of a liquid in air is slightly less than the weight in vacuo. There is no simple relationship between density and 'weight factor' but for bunker fuels the difference approximates to 1.1 kg/m 3 . To convert density at 15°C to the 'weight factor' at 15°C, 1.1 kg/m 3 should be deducted"
Source:

http://www.kittiwake.com/Default.aspx/Page/KB/KB/25

JMW

http://www.ViscoAnalyser.com

 

[Ashereng]

jmw,

I am still not sure how weight in vacuo versus weight in air will affect a DP transmitter. As long as the properties of the fluid is that same as that for the calibration, the DP to height relatinship should stay the same.

 

[hacksaw]

James08,

You are using an open tank installation, but describing a narrow segment of a closed vessel measurement topic. So the answer is yes and no and some times maybe.

Could you be more specific?

 

[jmw]

Ashereng,
I'm never sure how relevant this is nor, I suspect, is it ever properly understood or accounted for throughout the fuel industry, but the question posed by James08 was about bouyancy and this is all I could think James08 was referring to.
Hence your answer is probably pertinent to James08'squestion.

JMW

http://www.ViscoAnalyser.com