How a pressure decay device make a minus value? 1

[amirfarzad]

Hi.
Is it possible for an air pressure decay device make a minus value in leak test?
I was testing some plastic fuel rails with 50 cm hose assembled on them. just one of them had a 10 cm hose but the volume, on the device, was set for 50s. the leakage was showd the same magnitude but minus for the part with smal hose.
Is there anybody can explain how?
I also cuted another hose and the minus result was repeated for this new one. Also performed the test by clamping device output pipe to be sure about internal leakage of the device. but this time it rises a plus valu! It seems to be ok.

 

[TStaples]

amirfarzad,

When doing differential pressure leak testing, the lengths of the hose must remain constant, as they are part of the overall volume of the assembly being tested. If you have a given leak rate, and a volume of x being the volume of the fuel rail assembly and the connecting hose, then reducing x by shortening the hose means that a given leak rate will produce a LARGER pressure drop in the test volume. The leak test machine assumes that YOU haven't changed the outlet volume, and reads it as a larger leakage rate than it actually is.

Also, with pressure decay testing, be careful to maintain constant thermal conditions for the assemblies under test. I once managed a fuel injector manufacturing line, and I had a customer in France who started rejecting our injectors due to tip leakage. At their request, I paid them a visit so I see first hand how "bad" my injectors were on their new, state of the art, leak test stand! These were injectors we were 100% testing on helium, so I knew they were good. Upon arrival I find the customer was measuring 4 injectors on a fuel rail assembly using a Alcatel pressure decay system integrated into a test stand by one of their mfg. engineers. The customer had located the test stand next to a window. In reviewing the data with the engine plant's quality manager, I noticed that they rarely rejected any assemblies on 2nd or 3rd shift. I was there in the morning. I observed the operation and noticed that when the sun would peek through the clouds and shine through the east facing window, it would illuminate both the machine, and the assemblies queued up for testing. While illuminated, they passed every assembly. Once the sun went back behind the clouds, they would start rejecting almost every assembly. I concluded that they had a random number generator based on heat-up or cool-down delta of the test assemblies, and then proved it to the disbelieving customer via a quick experiment. Moving the machine solved the problem.

Be very careful with pressure decay measurement systems.



-Tony Staples

http://www.tscombustion.com