Pressure Decay measurement is one of the oldest and best known, quantifiable leak testing methods around today. From the simple use of a pressure gauge or water manometer, the differential transducer measurement device has increased the sensitivity to another level. Its simple operation makes it the most versatile and widely used technique the world over. Modern instruments require only air and electrical supplies together with a ‘leak tight’ way of connecting to the component, to enable a leak test to be made. ION SYSTEMS recognizes the high/fast production demands, and offers a range of solutions using this technique, from the very basic to the very complex (with a whole host of add-ons).
Principle of Operation
The operator loads the part onto a purpose designed nest, and the test started. Prior to any testing being performed, component features, bar code label scanning, and sub assembly work can all be carried out automatically.
Ready to test, the part is clamped into position (to ensure the part is stable, and avoid false rejects). Once clamped, the part is sealed by the most appropriate method (to mimic where possible the connections made during its operational life). This may by use of specially designed QUICK CONNECTORS, or by specially developed sealing heads.
Once sealed the part is pressurized to the required test pressure, when it undergoes the standard Pressure Decay leak test sequence. Should the pressure decay be greater than the permissible limit, the part is rejected. Like all our systems, rejected parts remain clamped until the operator presses the ‘accept Leak’ button to ensure that product status is identified.
Passed parts can be automatically marked (using a number of MARKING techniques), or a label printed, for later identification.
Differential Pressure Decay Technique
This method utilises a differential transducer to measure the pressure difference between the component, and a non-leaking side, thus the zero point for measurement is placed at the test pressure itself, thereby maintaining higher sensitivity at any test pressure.
THE TEST CYCLE
Fill Phase: During this phase, the test air pressure is allowed to enter both sides of the transducer assembly, as well as the product itself. The time required for this phase is purely the time it takes for the system to reach the desired test pressure.
Stabilisation Phase: In this phase we shut off the air supply, whilst leaving the test and reference sides of the system connected together. During this phase adiabatic effects are allowed to subside, creating stable conditions before the test phase commences. The Stabilisation time will depend upon a number of factors such as the size and shape of the component, the ambient temperature, and test air pressure. Test Phase: In this phase of the cycle, we isolate the test and reference sides of the system so that the test side of the transducer is connected to the component under test, and the reference side blanked / sealed off. Any drop in pressure in the test side of the system will be measured by the transducer, as a difference in pressure between the component and the blanked side of the system.
Exhaust Phase: This is the final part of the sequence during which time the test air from the component, and the rest of the system, is released through the valve arrangement. Problems can occur as a result of this phase, as the air from the component may well be contaminated with dirt, dust, or moisture. As the air travels back past the transducer some of the moisture etc could enter the sealed chambers and onto the diaphragm. Due to the very small internal dimensions of this type of transducer, a tiny droplet of moisture creates a large imbalance in the test and reference measurements (appearing as a large leak).
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