Air bubble on the surface of a pressure sensor in a liquid

[TeejT]

Conceptually speaking, say there is a pressure sensor immersed in water and that sensor has an air bubble trapped at the pressure sensing surface of the sensor (assume the bubble does not rise). Would the pressure the sensor "sees" be equal to the pressure of the liquid above the air bubble?
Say the sensor is lowered in water until the hydrostatic pressure reaches 1000 psi and the bubble was trapped at the pressure sensing surface of the sensor as it was initially immersed in the water at the surface (so the air in the bubble is initially at 1 atm).
Thank you!

 

[BigInch]

The pressure around the bubble is essentially equal, differing by only the diameter of the bubble x density of the fluid in the vertical direction.

As you increase the pressure of a liquid, water for example at 15 psia to 1000 psia, theoretically a trapped air bubble of 1 mm3 at 15 psia would decrease in size to 15/1000 th of its original volume, or 0.015 mm3 ... pretty small.

If the bubble was composed of water vapor, not air, when you reached its vapor pressure, somewhere around 0.45 psia at normal temperatures, the bubble would disappear, collapsing to become part of the liquid water surroundings.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

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[BigInch]

assuming capillary forces, surface tensions, material attraction forces, etc. are all so insignificant as to be neglected.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[TeejT]

Okay, so let's say the bubble is very large (so even factoring the decrease in volume it is still significant, say it covers the entire pressure sensing surface of the sensor), and you are below the vapor pressure of water, so it does not collapse to become part of the liquid water.
At this point does the sensor sense a pressure different than the pressure of the water above the bubble?
And secondly, what if instead of a vertical column of water we are instead considering the same large bubble covering the sensor in the piping of a hydraulic fluid system where the pressure in the system is increased by a piston?
Thanks again!

 

[jmw]

The impact on the actual density is one thing.
The second aspect to consider is if an air bubble affects the sensors ability to measure correctly.
I don't know of any pressure sensors that are affected, but other sensors can be such as vibrating element sensors.


JMW

http://www.ViscoAnalyser.com

 

[Apakrat]

Hydrostatic yield stress test on 11 miles of 20" OD pipe had a very minor difference in volume per stroke to make pressure rise each time a new test was done after repairing a piping defect found by testing..

Don't remember the difference and am too lazy to dig out 40 year old log books.

We had 11 failures instigated by hydrostatic testing on this natural gas line. No provision was attempted to eliminate trapped air after each repair.

Company Specs interpretation of DOT regulations required 24 hours of NO leaks AFTER pressuring by hydrostatic yield stress test.



At 74th year working on IR-One2 - - UHK PhD - - -

 

[BigInch]

I imagine that 11 miles was not tested in one section, although it could have been), it was a high pressure test and they probably had vented most of the residual air out. Lower pressure tests can be much more significantly affected than high pressure tests, as the relative volumetric change is significantly higher at lower pressures.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Apakrat]

Yes, BigInch, it was in one section, but there was not any source of removing air except to run a pig between launcher and trap.

If You have access, write up of this was in 3 of the Oil and Gas Journal publications in the fall months of 1969 about a Transwestern Pipeline Company emergency re-hab project.

Testing was done in the 1500psi range on line that had been in service about 5 years. The entire connecting nat. gas field was shut down waiting for clearance by hydrostatic testing.



At 74th year working on IR-One2 - - UHK PhD - - -

 

[BigInch]

That's the best way to remove air. Theoretically there would only remain some air dissolved in the water, which if temperatures were constant, would pretty much stay where it was. An 11 km test section is entirely possible, if elevation change is limited.

And practically this confirms that high pressure reduces any remaining bubbles to essentially nothing, right. All air compressibility would have to have been eliminated, air volumes apx = 0, to obtain low stroke counts for high pressure increase/stroke.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Apakrat]

The last line failure occurred after volume input was causing the steel to yield as pressure/volume graph was being plotted.

Occurrence of this failure built my animosity against using gaseous substances for high pressure testing for safety reasoning. Scary length of noise and shaking during failure. Extremely scary sight of two 40 ft joints of unraveled torn steel pipe.

The final pressuring had a beginning yield point higher than the previous starting yield curve plotting.

It took about 3 days for pressure to hold at no loss for a continuous 24 hour period. The first two days had losses equaling about 3 gallons per day.

This project caused future worries about entrapped air in pipelines to be about safety but of little concern about structural testing.

At 74th year working on IR-One2 - - UHK PhD - - -