Application Of Leak Testing Machines
PRESSURE DECAY LOSS DETECTORS
There are essentially two different types for Δp/Δt measure: absolute systems and differential systems. Both of them execute a test cycle that is based on three fundamental phases:
Filling, which allows to pressurize the test cell, Settlement that establishes the volume of the introduced air, and Test where is analyzed the pressure development in order to measure an eventual decay of the same one during the time.
The absolute system, represented in FIG. 1, is the theoretical method most immediate, economical and evident in order to execute that measurement.
The differential system, represented in figure 2, today is employed in those cases where it is necessary to have the same sensibility with very different pressures, or where are executed high pressures tests (>20 Bar), even if we will see later on, that the interception systems are in any case better and safe thanks to the elevated pressures which are brought into play.
The system limits are the following:
Major pneumatics complexity
Non positive safety pneumatics
Double pressure measurement section ( filling and test )
Lower measurement repeatability
Longer test times
Major instrumentations costs
We can analyze the FIG. 2 in order to understand the differences between the two systems and considering the differential application in a symmetrical way, that is to say with a airtight standard piece and a test piece; it is easy to understand that, between the first day test and the following tests, we will see the standard piece with a settlement cumulus both thermal and mechanical equal to “n”, while the piece in test equal to “0″, because it is substituted test after test. It is for that reason that the system is non comparable with a direct measure in repeatability terms.
Furthermore, what is really important to consider going from a system to another, is that the pressure values measured in Δp/Δt do not often coincide. In fact, in the absolute system, this is the real pressure decay, that is comparable with a certified precision manometer, while the differential measurement is the measure of the difference between two pressures. According to the tested elements, to the symmetrical or not symmetrical usage and to the settlement times, we can consider a proportion from 1:0,8 to 1:0,1 between absolute measure and differential measure: in other terms, the millibar in a second that is measured with an absolute or manometer system, can be considered as 0,8 – 0,1 mb/s on the basis of a differential system. This thing does not mean that the differential system does not work correctly, but it simply means that they are two different types of measure among themselves, and this fact must be considered during the installation phase.