
Company
Products
Leak test
Leak testing methods
Leak test panels
Background information

Job offers
Contact
Service


Home
Sitemap	E-Mail

Background information

Automatic leak testing with air

Leak testing by the pressure measuring method

The complete test cycle of an automatic leak test consists of four stages:

Filling (Fill)
Levelling (Balance)
Measuring (Test)
Evacuation (Evacuate)

1. Filling Stage
The workpiece to be tested is pressurised. The filling stage can be stopped as a function of either time or pressure. By the pressure rise gradient during filling, coarse control of the test volume can be carried out, or the self-test valve in the leak tester, which is in the test air pipe to the workpiece, can be checked for the "open" position.

2. Levelling Stage (Balance)
In this period the test system must be stabilised. The turbulence arising during the filling process is to stop, and the temperature changes of the test air produced in the process must be levelled out: the air flowing into the test piece expands at the filler valve and cools down, and the air which is in the test piece is compressed and at the same time heated. The latter is predominant, as shown by the temperature gradient in the water jacket of a cylinder-crankcase during the four leak test stages (Fig. 4).

Fig. 4: Temperature and Pressure Gradients
of test air

Corresponding to the temperature, the pressure in the test chamber also changes. If a display of "zero" is to appear on the leak tester for a sealed part, it is necessary to wait for the isothermal state. The time in reaching this "complete levelling" is however substantially longer than the usual cycle time allowed in the mass production industry.
The levelling stage can be shortened in three ways:

Compensation Filling (Shock Filling)
In compensation filling, the workpiece during the filling stage is subject to a filling pressure slightly higher than the test pressure. On changing from the filling to the levelling stage, the filling pressure is reduced to the test pressure. In this expansion process, the air in the test chamber cools down, as a result of which "incomplete levelling" is compensated. By varying the filling pressure, in the case of a sealed part even after a short levelling stage, a display of "zero" can be produced. Moreover, by this "shock filling effect" the filling time is shortened.

Reference Chamber = Test Chamber
On the reference side of a differential pressure leak measuring system, a sealed test piece is connected, so that on both sides of the differential pressure sensor during the filling and levelling stages the same conditions prevail, cancelling each other out in their effect.

Advancing Measuring Time ("Optimised Levelling")
As a display of "zero" in the case of a sealed part is not the aim of leak testing, shortening of the levelling time is most easily obtained by advance of the measuring stage (Fig. 5). The levelling time can be reduced as long as the state in the test system before the beginning of the measuring stage is no longer reproducible. In the case of a sealed test piece, a value of X is then displayed at the end of the measuring stage. The maximum reject level must also be shifted by this offset.

Fig. 5: Shortening of the levelling time by advancing the measuring stage.

3. Measuring Stage
A quantity of escaping air produces in the test chamber a pressure drop which is measured and displayed. This is either measured with a differential pressure sensor as a deviation from a sealed reference chamber, or the absolute pressure variation is detected with a pressure sensor (Fig. 6.).

Whereas the duration of the filling and levelling stages must be determined empirically, the measuring time can be calculated according to Boyle-Mariotts gas law:

Here it is assumed that the measuring process takes place isothermally. The equation then reads:

Transferred to the conditions in leak testing, by the pressure drop measuring method, the equation reads:

The noteworthy thing about this equation is that the degree of test pressure is not in it. The test pressure has no effect on the size of the pressure variation in the test chamber. To put it another way, the pressure drop in a workpiece is always the same at the same leakage rate, whether the pressure prevailing in it is 1 bar or 10 bars. The leakage rate however, which escapes through a porous area, increases at higher pressurisation. On the correlation between test pressure and leakage rate, see Fig. 2.

For a given test specification, the duration of the measuring stage can be calculated as follows:

Fig. 6: Basic diagram of differential pressure measurement (top) and absolute pressure measurement (bottom).