The Resolution That Takes Time

Some time ago we looked at Gage R&R. In that post we suggested that most repeatability studies require a 10% or better repeatability. This is perhaps better expressed as an example. If the leak rate specification for a part calls for a leak rate of 10 sccm then the system must be repeatable to 10 sccm + 1sccm.

Because the 10% repeatability encompasses the complete system – part, seals, air lines, valves etc the instrument itself should have a resolution of at least 10 times better than the repeatability of the system. In our example that equates to 0.1 of 1 sccm or 0.1sccm.

In reality we are often dealing with leak rates well below 10 sccm, maybe in the order of just 1.0 sccm. In that case the leak tester resolution should be 10 times better than 0.1 sccm or 0.01 sccm.

This formula is used to calculate the necessary pressure resolution of the leak tester.

Leak rate resolution (sccm) x Test time (sec) x 14.7 psi

Pressure resolution = ———————————————————————

Part volume (cc) x 60 sec/min

So let’s add a part volume of 20 cc to our example along with a test time of 5 seconds:

                 0.01 sccm x 5 sec x 14.7 psi

Pressure resolution = ——————————————————————— = 0.000613 PSI

          20 (cc) x 60 sec/min

If we have chosen a leak tester with a resolution of only 0.001 sccm we would need to increase the test time to 8.2 seconds:

                 0.01 sccm x 8.2 sec x 14.7 psi

Pressure resolution = ——————————————————————— = 0.001005 PSI

          20 (cc) x 60 sec/min

From this it should become clear that instrument resolution not only determines the measurable leak rate but that it has a significant effect on cycle time also.

All Uson instruments utilize 24 bit analog to digital converters to maximize the resolution. When the sensor in the instrument tester is spanned, the tester is able to convert the voltage from the sensor to roughly six million A/D counts.   For example if the sensor full scale = 30 psig, then, 30/6000000 »0.000005 psi per A/D count.  This is the tester resolution.