Understanding the effects of entrapped gases in liquid sensing lines?
Understanding the effects of entrapped gases in liquid sensing lines?
(OP)
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
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





RE: Understanding the effects of entrapped gases in liquid sensing lines?
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.
David Simpson, PE
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
RE: Understanding the effects of entrapped gases in liquid sensing lines?
RE: Understanding the effects of entrapped gases in liquid sensing lines?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Understanding the effects of entrapped gases in liquid sensing lines?
RE: Understanding the effects of entrapped gases in liquid sensing lines?
This is a simple sketch to illustrate the basic principle.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Understanding the effects of entrapped gases in liquid sensing lines?
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
RE: Understanding the effects of entrapped gases in liquid sensing lines?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Understanding the effects of entrapped gases in liquid sensing lines?
Bill
--------------------
"Why not the best?"
Jimmy Carter