Air flow test in capillary tubes

Hi guys,

We want to be able to test air flow in capillary tubes, roughly .1" in diameter, and 3' long for the tube. Air will be the medium for the test.

Anyone know of a flowmeter that will detect obstructions (due to the brazing process when we assemble them)?


Thanks!

I've used these to test for both tiny leaks and small air flows:

http://www.dwyer-inst.com/Product/Pressure/DifferentialPressure/Gages/Series2000

Differential pressure is not a bad idea.

A 'standard' tube should drop some pressure due to the small tube size. Any increase in DP would indicate an obstruction. Any decrease and you probably have a leak.

The challenge will be to have a constant upstream pressure. At a minimum a pressure regulator at an output pressure that the supply can maintain during a test. Regulators initially 'adjust' during no flow-to-flow conditions, so you need to let a regulator stabilize. If the upstream pressure varies, the DP will vary.

And there's a bunch of thermal dispersion mass flow meters the size of a pack of cigarettes, Brook, Aalborg, Omega, Sierra, Alicat, in no particlar order, usually with tube fittings for small diameter tubing.

Thanks guys!,

@Gas: I've used many a Magnehelic. I prefer to use them for dirty filter detection. And I've spent many a cold morning on the roof of the paint shop at Spring Hill trying to get that D$&! Dwyer to work. Not a fan.

@danw: We use Sierras here at work, the ones we use are for very low flow rates (for our leak detectors), and they do well enough for that purpose.

We tried making a obstruction tester in house (on the cheap, with no purchased parts). Using a pressure sensor on the 5mm coils I was only seeing a .2 PSI drop with one circuit blocked (50% blockage). Not too much there to work with...

For these assemblies we will have a higher pressure drop, and the flow will be lower, maybe our Sierras can work with that.

What I will probably do is to fixture the thing up, and invite flowmeter sales people in to show me how good their kit is.


Thanks again!

>Using a pressure sensor on the 5mm coils I was only seeing a .2 PSI drop with one circuit blocked (50% blockage).


What's the pressure drop for a good tube?


20% of 1 PSI is 5.5" water column, which is what industrial DP's measure all the time.


Can you increase the flow rate which will increase both the 'normal' DP and the blockage DP? Do you need to limit yourself to the 'normal' flow rate when testing for blockage?



But a Brooks style thermal dispersion is probably 1/4 the cost of an industrial DP transmitter.

Root boy,
We have used those sensors to test for leaks on automatic glass making machines and they have always been very reliable.
BUT...they were inside and everything was relatively clean.

gas said:

Root boy,
We have used those sensors to test for leaks on automatic glass making machines and they have always been very reliable.
BUT...they were inside and everything was relatively clean.

Click to expand...

This one was outside on a Thermal Oxidizer, and it would always fail during the dead of winter. So it would either be freezing, or just a gentle winter rain when we had to fix the thing.

The unit had been swapped out several times for new ones, but it would always shut down in winter.

danw said:

>Using a pressure sensor on the 5mm coils I was only seeing a .2 PSI drop with one circuit blocked (50% blockage).


What's the pressure drop for a good tube?

20% of 1 PSI is 5.5" water column, which is what industrial DP's measure all the time.

Click to expand...

I don't know as of yet. This was a project that belonged to one of our guys who moved on to greener pastures. He's now living in the UK.

danw said:

Can you increase the flow rate which will increase both the 'normal' DP and the blockage DP? Do you need to limit yourself to the 'normal' flow rate when testing for blockage?

Click to expand...

We like to do a leak test at 60PSI first. And I see no reason to use less pressure for the flow test. But if I'm stuck using the small Sierras, then it will probably have to be lowered. Or add a second flow meter for the obstruction test.

danw said:

But a Brooks style thermal dispersion is probably 1/4 the cost of an industrial DP transmitter.

Click to expand...

Thanks! I'll have to check them out.