Air Pressure Test

Hello all;
I am trying to design a pressure tester with RSLogix 500 to detect a air pressure leak in metal tubing. The leak rate cannot be more than 100th of a psi per 4 seconds at a pressured chamber of 60, 40 and 100psi. The metal tubing will be manually sealed at both ends of tube using some type of destako clamping system. The metal tubing is 1/4, 5/16, 3/8 and up to 1 1/2 inches in OD at varied wall thicknesses. Tubing has copper brazed fittings on both ends and different brackets brazed on also. I realize fill timing would be different for diameter size and stabilization time would be criticle as well. Some of the tubing would be used for fuel lines so it is esential not to have any leakage. The system we have tried using has a SLC 500 using a pressure transducer but when you get down to trying to detect 100th of a psi droppage it is not consistent. Can this be done without using transducers? It has to be idiot proof.
Thanks ya'all;
Willie

Trying to make it idiot proof would be the tough part!

Most commonly available industrial pressure transducers have errors in the range of 0.25% to 0.1%, so trying to measure 1/100 PSI over a range of 100PSI will be difficult.

One method to look at is to use a "differential pressure transmitter". This instrument has two piping connections (ports) and measures the difference in pressure between them. If one port is connected to a steady 100PSI reference pressure, and the other port connected to the pipe being leak tested, a small differential pressure of even 1/100 PSI can be detected.

For example, if a 0-1 PSI differential pressure transmitter (DPT) is used, with exactly 100 PSI on both ports, it will output a zero signal. If the pressure on the test pipe drops by 0.5 PSI, the DPT will output 50%.

This type of arrangement is commonly used in the instrumentation world to measure small differential pressures at up to several thousand PSI.

Any major instrumentation vendor will have these, such as Honeywell, Rosemount or ABB. The DPTs usually output 4-20 mA which is easily interfaced to any PLC. A quality DPT will cost about $500-$1000 US.

You will also need a balancing manifold valve which the vendor can supply.

"differential pressure transmitter".

I have never considered a DPT. Actually I have never heard of one. A DPT from your post looks like it would be the ticket. I will check into this. One of the greatest plagues of our plant has been "leakers." It is scarry to shut down a major industry because one of your parts failed "the test."
Thanks again;
Willie

Click to expand...

FYI: www.uson.com - they probably have what you want.

Hope that works for you Willie. I did a quick Google search and came up with a spec sheet for an ABB instrument:

http://www.instrumentation2000.com/catalog/pressure/06/020655b.html

You will probably need one calibrated in inches of water column, rather than PSI because of the small differential you want to measure. Make sure the instrument you select can handle the full 100 PSI pressure too.

Thanks for your reply edlu.

FYI: www.uson.com - they probably have what you want.
We have these all over the house but we have had leakers come back from them also. I am trying to come up with something better, and they may can produce something better but I haven't seen it yet.
Thanks;
Willie

I work In a cnc machine shop that makes oil pans and thermostat housing for Honda Autos. We currently use Cosmo leaktesting equip for all critical process that require leak test. We have 7 of these units and dont have a lot of problems except with big temp changes, then we may have to readjust a little.
Here is a link to thier site.
http://www.cosmoinstruments.com/index-products.htm

Bob

Per bob1371 reply

dont have a lot of problems except with big temp changes.

When you say temperature changes Bob just how much of a temperature change are you talking about? Is your plant air conditioned? What kind of temperatures does the units opperate at the best? We notice that with the Uson PT's also so we do a comp test every hour or at the start of each shift.
Thanks;
Willie

Click to expand...

The tempurature problems a just a couple times a day. Usually midway thru day shift and again in the evening as it cools down a bit. Our shop isnt air conditioned and we have Die Casting shops on both sides of us. On top of this most of our leak testers are located just after a Washer (hot water) which the instruction manual clearly says dont do.

I have two methods in use now.
We pressurize very complex compressors with lots of tubing on it.
We just simply put them under water and pressurize them to find leaks.
We just look at them to see bubbles but it is possible to automate this by putting a hood over the bassin and when air comes out of the hood the ****er leaks.
We also use a big bunker and pressures up to 30 bars.
You can use a analog pressure sensor and lots of software and plc
you also can use a switch used inside a washing machine to detect waterlevel. it is very cheap and it reacts on 0.001 bar.
However it is not possible to pressure it. And the old way to use a watertube is still possible off course just have a valve over the two tubes and close it to check. If level rises its leaking.

leak test

Might check out the following link....

These are currently used as "GO"/"NO-GO" for AGA testing of gas ranges.

Output you can link with SLC/PLC as well as data aquistion if you need logging.



http://www.iti-qualitek.com/products/apdleak.htm

Regards,
Bobby

Thanks bbpon for info.

bbpon;
We are currently checking out all these possibilities that you and others have mentioned. 98% of our products have to be pressure tested. The leak ratio of course varies. I did notice that these PT's have the differential pressure transducers that Glen Horne mentioned earlier in one of the replies to this thread. Uson has been one of our major suppliers for PT's so we are looking at them a little harder to see if this is or can be part of their Pressure Testers.
Thanks again;
Willie

We also use Uson meters. Recently we have also purchased some Furness meters. Check them out at: http://www.furness-controls.com/

Steve

FUKUDA Air Leak Tester

I recently using a FUKUDA Air Leak Tester (FL-600)10,(FL-2700)10 for both of our plant that makes parts for Toyota and i tell you it works great and very user friendly and can replace Cosmo machine just plug and play and very good pricing too.

There are sometimes better ways to detect a leak than measuring pressure drop. One method that is commonly used is a Helium spectrometer. Its a bit more money, but its very reliable and will find even the smallest leaks.