The process of differential pressure testing
The differential pressure test is available in different variants. For most tests, however, the measurement against a reference volume is used. This method allows the most accurate measurement under the pressure change method, but is also the most complex in the implementation. The test object is filled with a defined pressure. This pressure is compared to the pressure of a reference volume. The reference volume is in most cases an internal volume of the tester of a few milliliters. Only with large volumes to be tested is a reference body used. That is, as a reference volume, a test piece is connected, the tightness is established. As a rule, this procedure can only be considered for a volume of 5 liters or more. A comparison body can also be used for test specimens with special properties, for example, when the body behaves elastically, which is the case with some plastic parts. As a result, the influences of these special factors are compensated as far as possible or ideally eliminated altogether.
During the differential pressure test, the test object and the reference volume are slowly filled to the same pressure. If both volumes are filled to the defined pressure, the two are separated with a shut-off valve after a rest phase. The reference volume now maintains pressure, while it decreases in the sample under leaks. This pressure drop is analyzed by measuring the differential pressure between the two volumes. The prerequisite for this method is the tightness of the measuring system and the test device.
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The differential pressure measurement
In the measurement of the differential pressure can be proceeded differently. The simplest and most cost-effective differential pressure measurement is performed with just one pressure sensor. This determines the pressure before starting the measurement and after completion of the measurement phase. No reference volume is needed, as only the change in the test cycle is registered. This can be done with absolute or relative pressure sensors.
A more accurate variant is to measure the pressure of the test object and the reference volume by means of two pressure sensors in the two circuits.
The most complex but also the most exact measurement is possible by means of a differential pressure sensor. This is arranged between the two circuits and registered via, for example, a membrane changes in pressure balance.
With the differential pressure test, a higher accuracy can be achieved with a higher test pressure than with the absolute / relative pressure method, since the resolution of the measurement signal is independent of the height of the test pressure. The measuring accuracy is determined by the measuring range of the differential pressure transducer. In addition, temperature influences on both sides of the measuring system, the influence is thus less than in the leak test using the absolute pressure method.
In the case of pressure drop measurement (or pressure rise with negative pressure) according to the differential pressure method, the later operating state in the test object is simulated. The test conditions are similar to the later operating conditions.
Hood or bell method
With the pressure inrease measurement with negative pressure (vacuum) disturbing influences on the component to be tested can be kept low. Fixtures for vacuum tests are generally simpler and less expensive. The workpiece is placed under a bell. The bell is subjected to negative pressure. Leakage in the workpiece causes air to escape and increase the pressure under the bell.
In pressure rise measurement with overpressure, the workpiece is sealed in a closed test bell. The workpiece sealed to the bell is pressurized with test pressure. The leakage air emerging from the workpiece is measured in the test bell. With this method, the compensation time can be chosen very small, often it can be completely eliminated. The detection of very small leakage quantities is possible.
The test sequence of a leak test by means of differential pressure measurement is subdivided into 4 phases:
The first 3 phases of differential pressure measurement in detail:
1. Filling phase of the differential pressure test
The workpiece to be tested and the reference volume are pressurized. The filling phase is terminated depending on time and / or pressure. Care must be taken that the pressure in both tanks is always the same, otherwise the sensitive differential pressure sensor may be damaged.
When the filling and separating valve is open, the test pressure set on the pressure regulator simultaneously reaches the reference volume, the transmitter and the test object. The filling time is determined by the performance of the pressure regulator, the total volume of the device including the test and reference volumes, the line resistance and the cross section of the valves. The size of the filling pressure also has an influence.
2. Stabilizing phase of the differential pressure test
At the beginning of the compensation sphase, the filling valve is closed and the test system can stabilize. The filling process creates turbulences and temperature changes that need to calm down or even out. [Nbsp]
In this phase, therefore, there is a pressure and temperature compensation in the entire test system. Air entering the test specimen expands at the fill valve and thereby cools. At the same time, the air in the test specimen is compressed and thereby heated.
The temperature also changes the pressure in the test system. It is ideal if the leak tester displays the value 'zero' in the case of a leakproof test piece. So the isothermal state has to be awaited. However, the achievement of this ideal state often takes much longer than the usually available cycle time in a mass production.
There are several ways to shorten the stabilizing phase:
The pressure when filling the workpiece is slightly above the test pressure. With the transition from the filling to the balancing phase, the filling pressure is reduced to the test pressure. Here, the air cools in the test part.
By experimentally changing the filling pressure, the compensation phase can be gradually shortened until the display shows 'zero' in the case of a part known to be tight. As a desired side effect, the filling time of the differential pressure test can be shortened.
Reference volume = test volume
On the reference side of a differential pressure leak testing system, a tight sample (a pressure tight original workpiece) is connected.
The reference volume serves to produce the greatest possible symmetry between the test circuit and the reference circuit. This makes it possible to reduce the compensation time, since any disturbing influences act on both circuits of the system and thus reduce the influence on the measurement result. Experience shows that at volume >5000 ccm a reference volume can usually be dispensed with.
The aim of the leak test is not a 'zero' indication for a dense part but the safe distinction between dense and leaky parts. Therefore, the measuring phase can be advanced and thus the compensation time can be shortened. The compensation time is thereby reduced until the condition in the test system at the beginning of the measurement phase is no longer reproducible. At the end of the measuring phase, a certain value is then displayed for a dense test specimen. This value is subtracted as an offset from the permissible upper limit.
In use, a combination of the individual methods is usually used.
3. Measurement phase of the differential pressure test
After the stabilizing time has elapsed, the actual measuring process begins. Closing the shut-off valve separates the measuring and reference path. If there is a leak in the test specimen, the pressure in the measuring branch is reduced relative to the reference path. This pressure drop is measured with a differential pressure transducer as a deviation from the dense reference chamber. [Nbsp]
The leak tester displays the pressure difference at the transmitter on the measured value display. If the pressure difference during the measuring phase exceeds the preset limit value, a signal UNTIGHTis output and the test procedure of the differential pressure measurement is ended.
Duration of the phases
The duration of the filling and stabilizing phase is determined empirically. The duration of the measurement time can be calculated according to the gas law of Boyle Mariotte. From the equations to the gas law surprisingly results that the test pressure has no influence on the height of the pressure change in the test volume. This means that the pressure difference in a workpiece with the same leakage is always the same, no matter which test pressure is used. It should be noted, however, that the rate of leakage that escapes through a given leak also increases with increasing pressure.
Advantages and disadvantages
The advantages and disadvantages of differential pressure measurement
The differential pressure test is particularly suitable for use in series production, because the differential pressure measurement can be well automated with sufficient accuracy and used in leak testing systems . Furthermore, the test is carried out without contamination. Compressed air is used as the test medium, so that relatively low operating costs are incurred and the environment is protected. Due to its low viscosity, it can penetrate existing porosities or other leaks sufficiently quickly. The cleaning and drying as well as anti-corrosion measures of the test items are eliminated. The continuous maintenance of the measuring system is also problem-free.
The measurements themselves, however, give no statement about the location of the leak. Thus, another method is needed for leak detection unless the device under test is either not repairable or a relatively low value product that can be discarded as scrap.
The differential pressure test in serial production
In the case of automatic leak testing of series components, leakage testing using the differential pressure method has become widespread. The reasons for using the differential pressure measurement for leak testing are:
- high inspection reliability
- objective and qualified quality statement
- short test cycles
- low investment and operating costs
- sufficient accuracy
- gentle workpiece handling
- easy integration into linked production processes
- easy calibration
The automatic leak test with differential pressure testing of series components will continue to gain in importance. Especially in connection with the requirements of the following areas:
- ISO 9000 ff
- Product liability
- Documentation requirements
- Environmental protection
- Consumer protection
- Workplace design
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