Remote pressure gauge?

'Should be fine'..is my vote...

Adding a bleed would be worse as there would be pressure drop associated with the flow.

It depends slightly on how dynamic this 'dynamic operating pressure' is..but I still vote for 'it's fine'.

Del

Damn Del, you're always the first to respond anytime I post. thanks by the way.

As far as "how dynamic" is concerned, according to manufacturing, it is critical. We have a part that that can be run in the field at low pressures so we test at low pressures to make sure the vaccum is effective in the low ranges. I'm not sure where they get the spec of 20psi dynamic, but that is what I have to design a test fixture for manufacturing around.

I'm just learning fluid mechanics and it seems to me that 20psi dynamic in 3/8" tube versus 20psi dynamic in 1/2" tube would produce 2 different flowrates. I think this would affect the vacuum. Our parts are spec'd in flowrate. Shouldn't we be adjusting flowrate and not pressure?

It's one of those theory vs practice things...

BTW are we talking gas or liquid?

Theoretically any vessels connected to eachother will equalise to the same pressure...the question is how quick...after all the Earth's atmosphere is one big open system and so should be at the same pressure throughout....however the air cant rush from high to low pressure quick enough to equalise it, it also gets diverted by Coriollis force into the swirling weather systems we are familiar with.

Back to your question... Is your length of pipe significant?
It depends on the rate of change of the pressure at the far end!

Put it this way...long long ago ..far far away..I designed a commercially successful respiratory instrument, which sensed pressure down a 3mm tube about 40 mm long, this was connected to the cavity where the spring rested...the spring was attached to a piston in a tube through which the patient blew. The pressure sensor reacted to a Forced Expiration Manouvre quickly enough sampling every 5millisecond as I recall.

My vote is still...'it's fine'..but hey that's 'cos a) I don't know the full set up. b) I'm an optimist.. c) Cat's don't care about this stuff anyhow

If the tubing is filled with water, for every 27" of vertical offset between the pressure measurement point and the gauge there will be 1 PSI indicated error. If the gauge is at the same elevation as the pressure measurement point you will not introduce this error.

If the tubing is air filled the elevation differential will be insignificant.

The difference in elevation is 6". Both the regulator and gauge are mounted on the same panel so the manufacturing personnel can easily read and adjust if necessary. It was not possible to connect the 2 directly in the configuration, so the gauge is mounted just above the regulator. The is a 1/4" 90 elbow conncted to a 1/4" tee coming off the regulator. One port on the tee has a 1/4"npt x 1/4" barb fitting facing "up" towards the gauge, and the other port is plugged for later use by calibration. The gauge has a 1/4"fnpt x 1/4" barb ell fitting facing "down" towards the regulator. There is a 4" peice of 1/4" braided vinyl tubing connecting the 2.

Will there be large inaacuracy due to the fittings?

No.... you will have good accuracy with those fittings, assuming the flow rate through the Tee piece is not too great.

John.

The pressure read by the guage will be accurate,. The air in the tube may only introduce a miniscule delay in the gauge responding as the air will compress as the tube pressurises. However at 20psi this will be insignificant. (fluid does not compress & thus would give instantaneous response). As the tube is capped (by the gauge & plug), no flow will occur. In a static system (ie no flow) pressure will always be equal at all points in the system. The point about pressure difference due to height is also correct, but in your cast is also not an issue.

So, Del is right... "Should be Fine"!

Tony

Who's old enough to remember fitting an oil pressure gauge to their first car to make it look sportier, this is no different, should work just fine.

Thanks for the detailed answered everyone supplied. This gives me more confidence in the test fixture's overall design. Again, thanks.

I always put an isolation and a bleed off valve before a gauge but I do it so we can change the gauge on the run. It is one of those unwritten rules in the paper mill. There is nothing worse than having to shut a system or (several for that matter) down because of a bad gauge around here. On higher pressure steam pipe and boilers we use a double block and bleed and of course a pig tail to keep the steam from directly hitting the gauge without the water buffer, but again this is for safety reasons and to guard the equipment.

pipewelder

It's always a good practice to plan for preventative maintenance or even reactive maintenance when designing a system. This is only a test fixture for production and a very simple one at that. Everything has been designed "modular", so replacement of any faulty component or "module" is very simple. It takes less than 30 minutes to completely disassemble and reassemble the entire thing.

If I can throw my two cents worth in, I vote for "no problem." If, as one respondent pondered, there is a concern with the elevation difference in the system, simply shut the system down and read the static pressure on the gage. This will be the weight of the water in the tube (in psi). Deduct this from the test reading and that would yield the system results. My guess is that, at only 20psi, the static reading will not even be able to be read unless you are using some type of altitude gage that is very sensitive to small fluctuations in pressure.