Negative leak rate during testing and R & R issues

We are using Massflow unit (MF520) for leak testing the motor, it is the pressure differential unit. In few cases the leak value is showing in negative value (Ex. -0.68ml/min). During a customer audit, customer says negative leak value is not allowed - check your equipment. When we checking with equipment manufacturer he is telling negative leak is allowed - but he is not clearly explained with theory. Anybody can explain is it negative leak is allowed during leak testing and what the theory behind it ? Also we are not achieving the R & R what are the factors need to be focused to improve the situation in leak testing...

 

When we recieved a negative it was usually a calibration issue.

I know on our equipment we would calibrate it to a near "perfect" part. This was supposedly to account for any other minor leaks in the system. If that near perfect part had any leaks, then parts with less leak would show negative.

It may be able to be explained away if there is residual heat in the product when testing. Thus raising the heat of the air in the closed system and raising the pressure.

Ours never did do well on an R&R.

Our parts were machined then washed to reduce debri where we would be sealing. We would still wipe it down carefully before testing. At times we would use a small amount of grease so that it would create a tighter seal and yet an operator was able to take it on and off. Are you doing any impregnation? We would often have residual material after impregnation as well. Seals are obviously often a problem. And of course at times there are issues with the actual aluminum casting that we were able to prove through xray.

 

I don't know any specifics about the particular meter or your use but I've seen issues in the past on air leak gages where you get issues from cooling/heating of the air after it expands. We saw that you'd have the pressurized air expand into a volume and cool accordingly; after a bit of time the part, gage, and surrounding environment would warm the interior air back up to ambient and the pressure would rise (ideal gas law). It would give the appearance/indication that the part was not leaking or would be negative leaking (gaining air); we were also befuddled for a while. In our situation it was with a specialty built gage (2 chamber leak tester) but perhaps you might be seeing something similar.

You might also get some insight from the manufacturer but I have no experience with them.

David

 

The Calibration is done by the equipment manufacture at his facility and he sends the calibration report. Our frequency is once in a year. Actual testing condition is ambient temperature (28Deg to 40 Deg - Depends on climatic condition) in the shop floor. The Motor is hydraulic motor, motors which are used for skid steer loaders and construction machines...
Test parameters : Test pressure 2 bar +/-0.2 bar, Fill 40 Seconds, Stabilization 100 Seconds, Test 10 Seconds and Dump 5 Seconds. Test Volume & Reference volume 1000 Cc and Part value 1000cc (motor). Allowable leak value defined as 2.5ml/min. (they are considering + / - 2.5ml/min... Whether it is the correct way of measurement - Negative leak ? I don't know), Further how to differentiate whether it is test issue or product issue. We have contingency if the test is fail we can dip the motor in pool tank to check the air bubbles are coming... Most of the cases in pool tank test, no leak observation but the equipment value is varies from -1.5ml/min to +7.5ml/min.

 

We are not doing any impregnation and it is not the machined individual part, it is a product - hydraulic motor...