Depending on the value of the pressure generated inside the tested object, the method of slow pressure changes is classified as either vacuum or pressure methods. In both variants, it allows to assess the tightness of the facility, , however, without the location of a possible leak.
The sensitivity of this method in the vacuum variant depends on the measuring range and accuracy of the vacuum gauge used. For the ionization vacuum gauge (p min = 10 -6 Pa) the 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 of a few atmospheres, and after disconnecting the compressor, the rate of pressure drop –Δp / Δt resulting from the actual leakage (pressure drop method) is measured. The sensitivity of the pressure gauges used is usually approx. 100 Pa (the lowest measurement resolutions are 0.01 Pa for the most expensive measuring devices) and therefore the sensitivity of this variant is several orders of magnitude lower than in the case of the vacuum variant.
The difference in the measured pressure drop (ΔP = P 2 – P 1 ) is influenced by several factors such as:
- presence of a leak
- volume difference ΔV
- temperature difference ΔT
Therefore, when using the pressure drop method, one should take into account the influence of temperature change ΔT and the effect of volume change ΔV on the values of the measured pressure drop ΔP. When using this method on the production line, you should be aware of the influence of the above physical quantities on the final result of the measurement. To eliminate the influence of these factors, use the differential pressure drop method. In other words, the differential method compensates for volume differences ΔV and temperature differences ΔT.
In the differential method, one of the basic applicable laws is Mariotte’s law (Boyle’s law), which for ideal gases takes the form:
P V = n R T
where: P [Pa] – pressure, V [m 3 ] volume, n – number of moles (amount of matter), R – constant for ideal gases (R = 8.31 J / mol · K) , T [K] – temperature.
After taking into account the influence of temperature change and volume change, we get:
(P + ΔP) (V + ΔV) = n R (T + ΔT).
The volume V consists of: the volume of the test element, the volume of pipes used to connect the tested tank, the volume of pipes and fittings inside the measuring device.
Tightness test using the pressure drop method with a standard element.
The figure shows the concept of measurement using the differential method, in which we use a standard (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 under the same climatic conditions (temperature, pressure). Thanks to this approach to measurement, the values of ΔT and ΔV are the same for the tested and standard element and cancel each other out without contributing to the measurement indicated by the leakage measurement device.