Diferential pressure measurment
Diferential pressure measurment
(OP)
This is out of my field.
As usual, I don't know as much as I thought.
I installed a differential pressure gauge across the heat exchanger in my home geotherm system.
Object is to monitor flow, as psi drop per gpm is a listed spec.
Seemed simple enough.![[blush] blush](https://www.tipmaster.com/images/blush.gif)
The "high" side is 18" lower than the sensing point for the low side.
It is a closed system pressurized at about 20 psi. Water/ glycol mix.
Gauge is mounted about 5' above sensing points. Plumbed with clear poly 1/4" tubing.
Gauge does not read zero after slowly pressurizing both sides. Had a few leaks to start with but I still can't seem to come up with the proper way to set this up. ??????
My electrical analogy is lacking in that I assumed pressure in a closed static system will be equal. (ie. voltage with no current flow)
Couple questions:
If I leave it reading + or - 1, will this be the starting point of my dif pressure reading?
Do I have to level fluid level in tubes? (not easy)
Is it necessary to bleed air from the tubing to gauge? (hope to leave air cushion)
Am I correct in assuming the weight of the water columns (feed tubing) is cause for my error?
As usual, I don't know as much as I thought.
I installed a differential pressure gauge across the heat exchanger in my home geotherm system.
Object is to monitor flow, as psi drop per gpm is a listed spec.
Seemed simple enough.
![[blush] blush](https://www.tipmaster.com/images/blush.gif)
The "high" side is 18" lower than the sensing point for the low side.
It is a closed system pressurized at about 20 psi. Water/ glycol mix.
Gauge is mounted about 5' above sensing points. Plumbed with clear poly 1/4" tubing.
Gauge does not read zero after slowly pressurizing both sides. Had a few leaks to start with but I still can't seem to come up with the proper way to set this up. ??????
My electrical analogy is lacking in that I assumed pressure in a closed static system will be equal. (ie. voltage with no current flow)
Couple questions:
If I leave it reading + or - 1, will this be the starting point of my dif pressure reading?
Do I have to level fluid level in tubes? (not easy)
Is it necessary to bleed air from the tubing to gauge? (hope to leave air cushion)
Am I correct in assuming the weight of the water columns (feed tubing) is cause for my error?





RE: Diferential pressure measurment
I'd vent the air, or you'd have to add the air column weight to the liquid column weight, or neglect the air weight and only measure the liquid column height... in all cases, it complictates the readings.
http://virtualpipeline.spaces.msn.com
RE: Diferential pressure measurment
Why leave an "air cushion" anyway?
Brian
RE: Diferential pressure measurment
RE: Diferential pressure measurment
My thinking was to cushion the pumping pulsations that I noticed on my pressure gauge. Not needed at all. Just a dumb idea.
It seems to be not as simple as using the initial reading as a starting point, as the fluid levels will change with flow. I see that now!
I'll just bleed the air out. A little air between the last fitting and the gauge should not be significant?
The proper way to do this for both gauges is an orifice ?
I wish I had more head room. It looks like a closed vertical loop from one side to the other would self level and show foh visually in the tube. I'm looking at 15 foh dp.
Now, to highjack my own thread with another question that will once again show my ignorance of fluid flow.
At one time I would winterize my marine v8 motor by disconnecting the water line feeding the exhause and block.
Insert a funnel.
Hold it a foot or so higher than any part of the (open to atmosphere) water jackets.
Pour in antifreeze until it displaced any standing water and ran out the normal way.
This of course did NOT work without adding conciderably more pressure to force fluid thru the maze.
I honestly do not understand what type of chamber internally would prevent the higher level of fluid from flowing thru and seeking the lower level.
Air lock is a term I've heard, but thought it was only related to pumping.
RE: Diferential pressure measurment
Air or vapor lock can happen in any "gooseneck" configuration. Even with a higher gravity head driving the fluid, it can take awhile to clear in small diameter lines, especially if the internals rise up to almost as high as the fluid entry point, (which is why most liquid cooled engines come equipped with water pumps), as theoretically if the levels could be arranged nicely, an engine could be (at least partially) cooled by heat driven circulation only.
http://virtualpipeline.spaces.msn.com
RE: Diferential pressure measurment
Actually it IS a dif pressure gauge. Although one side has a tee going to a gauge to measure the static pressure.
I just bled the lines and all is fine now. I was just totally suprised that air in the lines would change things.
So, you are saying that in the motor situation, it will eventually fill under any "open" condition, as long as the fill level is higher than any internal dam? That was my gut feeling, but it just didn't seem to work that way.
I can't visualize any type of trap that would not fill and pass fluid.
In this case the fill line and funnel would just remain full.
RE: Diferential pressure measurment
The diagram illustrates that with atmospheric pressure at the end of the goosenecks, how each water column adds pressure (0.4333 psi/ft) as it moves higher. (I said the weight of this (special) air in each column is zero and this special air is also not compressible.
As you can see, the additional pressure to lift the liquid columns trapped between vapor-locked goosenecks can get relatively high. And we still haven't cleared the invert elevations of the goosnecks.
http://virtualpipeline.spaces.msn.com
RE: Diferential pressure measurment
http://virtualpipeline.spaces.msn.com
RE: Diferential pressure measurment
It might also be worth mentioning that if the liquid was gasoline and the pressure at any one of the tops of the goosneck was less than about 8.5 psia at 60ºF, or probably 20 psia or more if its temperature was high, a vapor pocket would appear. At that point in time, the vapor would weigh approximately zero and you would be looking at something almost like the start-up case above where all locked heads must be added together, except you would have some flow going already, but in any case an immediate increase in inlet pressure would be needed to keep things moving along.
http://virtualpipeline.spaces.msn.com