Understanding the effects of entrapped gases in liquid sensing lines?

[CiaoMongo]

Hi Folks,
Wondering if anyone can point me in the right direction or provide some useful insights on a phenomena I've frequently observed on sensing lines where entrapped air has been an issue.
What I'm specifically curious about is, how, under certain circumstances, can measured pressure be artificially increased by the presence of air entrapped in a liquid sensing line?

In my current workplace, we have a very simplistic rig where we measure pressure from a static volume of oil (open to atmospheric pressure) via a ~2m length of 12mm hose/sensing line to a remote dP sensor (compensating for atmospheric pressure) that is mounted lower than the volume of oil. During the process of filling/refilling the volume or connecting/disconnecting the sensing line we're able to VERY reliably create a scenario where the presence of entrapped air in the line creates a higher than normal pressure reading for any given volume of oil (or column of oil if you will).
When we prime/purge the line with oil, the pressure ALWAYS DECREASES to a value consistent with the column of oil we're measuring.

This honestly baffles me because I've always considered pressure is pressure is pressure in any fluid until you start dealing with dynamic fluid issues where resonance and flow may start confounding things.
In our case, the system is relatively static when we make these observations and as far as my logic is concerned, air in the line should cause a lower than normal pressure reading due to the mass in the column being reduced and ultimately exerting less pressure on the sensor.

Is there something obvious I'm missing here?
I'd really love for someone to challenge my thinking here or help me understand how this can occur.

Cheers,
Ciao

 

[zdas04]

Could you provide a drawing of the relative heights of the transmitter, tank take-off, and fluid level? I was unable to picture these from your description.

Pressure is pressure, but in smaller lines (12 mm is almost too big to see this) we often see trapped pockets of liquid that tends to create a seal between gas pockets. The cohesive forces between the liquid and the small pipe result in slightly different pressure from one pocket to the next. The cumulative effect of a number of these pockets can be measurable. I haven't noticed a bias in one direction or the other, but I wasn't looking for a bias, I was just looking for differences from the source to the sensor and found that.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[CiaoMongo]

I should add that I do have some loose theories/ponderings that the phenomena relates somehow to film strength of the oil around the entrapped air or air acting somewhat like a restrictor/blockage but these are very incomplete theories and I'm not able to find much support for them.

 

[itsmoked]

It may be buoyancy. The bubble is trying to rise based on densities not pressure. This would force the oil above into more compression - higher pressure.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

Bubbles in sensing lines will definitely affect the readings on differential pressure sensors. This may be up or down depending on which of the two lines has bubbles. This is simply due to the fact liquids have greater density than gas. If you lower the sensor the pressure on both taps will increase but the differential pressure will not change. If an air bubble gets into one line, the weight of fluid in that line will decrease, and change the differential pressure at the sensor.

 

[waross]

Hills and valleys in the impulse line will do that.

This is a simple sketch to illustrate the basic principle.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeHalloran]

For salt water in lumens smaller than ~2mm diameter, a bubble or a drop can actually stop the flow against a modest head, effectively acting as a shutoff valve. My friend Dicky first pointed it out in the context of blood cell counters, but we couldn't find a way to do it intentionally, which would have been useful.

Some friends of mine made a beautiful test panel for monitoring exhaust gas pressures in boats, where a pressure above a couple of feet of water will void the engine warranty. It had four Magnehelic gages and four inputs for monitoring a pair of V-type engines.
They would show up at a boat with a huge coil of polypropylene tubing, cut it to length, and shove it into push-connect fittings, so they could monitor pressures while sitting on an upper deck, not in the engine room.

Unfortunately, they used 1/8" OD tubing, and a single drop of water anywhere in its length would screw up the readings, and not in a way that you could back-calibrate and recover useful data. I found that 1/4" OD tubing was better, but still subject to occasional water problems, and refrigeration hose was better still, but much more expensive and heavier than the small polypro stuff.





Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

The problem is legend in chlorination systems for well systems. Chlorine solutions often off-gas creating a tiny bubble which can block all further flow in 1/4" lines. Always a bummer to check a system and discover "Hey no chlorine has been used this week!" Oh oh.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

As I understand the issue it is vapour causing an INCREASE in indicated pressure, not blocking the flow or causing reduced indicated pressure.

Bill
--------------------
"Why not the best?"
Jimmy Carter