Depending on the value of pressure generated inside the tested object, the method of slow pressure changes is classified as vacuum or pressure methods. In both variants it allows to assess the tightness of the object, however, without the location of the leak.
The sensitivity of this method in the vacuum variant depends on the measuring range and accuracy of the vacuum gauge used. For ionization vacuum gauge (pmin = 10-6 Pa), recorded leaks reach the level of 10-6 mbar · l/s. The principle of the pressure variant differs only in that the tested object is inflated to a pressure usually in the order of several atmospheres and after disconnecting the compressor the pressure drop rate is measured – Δp / Δt resulting from the occurrence of real leaks (pressure drop method). The sensitivity of the pressure gauges used is usually around 100 Pa (the lowest measurement resolutions are 0.01 Pa for the most expensive measuring devices) and hence the sensitivity of this variant is several orders of magnitude lower than in the case of the vacuum variant.
Several factors influence the difference in measured pressure drop (ΔP = P2 – P1), such as:
- presence of leakage
- volume difference ΔV
- temperature difference ΔT
Therefore, when using the pressure drop method, the impact of temperature change ΔT and the impact of volume change ΔV on the measured pressure drop ΔP, should be taken into account. When using this method on the production line, the impact of the above physical quantities on the final result of the measurement should be taken into account. To eliminate the impact of these factors, a differential pressure drop method should be used. In other words, the differential method compensates for volume differences ΔV and temperature differences ΔT.
In the differential method, one of the basic laws used is Mariotte’s law (Boyle’s law), which for ideal gases takes the form:
P V = n R T
where: P [Pa] – pressure, V [m3] volume, n – amount of moles (amount of matter), R – constant for ideal gases (R = 8,31 J/mol·K), T [K] – temperature.
After taking into account the impact of temperature changes and volume changes, we get:
(P+ΔP)(V+ΔV) = n R (T+ΔT).
The volume V consists of: the volume of the test item, the volume of tubes used to connect the test tank, the volume of tubes and fittings inside the measuring device.
Fig. 10. Leak testing by pressure drop using a reference element
The figure shows the idea of differential measurement, in which we use a pattern (an element with an acceptable level of tightness) made of the same material with the same dimensions (volume, structure) as the tested element. Both elements are placed in the same climatic conditions (temperature, pressure). Due to this approach to measurement, the ΔT and ΔV values are the same for the tested and reference element and cancel each other out without participating in the measurement indicated by the leakage measurement device.