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PRINCIPLE OF AIR LEAK TEST

What is Atmospheric Pressure (pressure of gas)?
The pressure used in the air leak test is the atmospheric pressure (or air pressure). Atmospheric pressure is the pressure generated by the weight of air. As we live on earth, atmospheric pressure is constantly applied to us and everything around us. The atmospheric pressure at sea level is 101325Pa (1013hPa) which means that we are pressed by a force of 1013hPa.The level of atmospheric pressure changes depending on the altitude. The uncomfortable feeling we get on our eardrum when we ride in an elevator to the highest floor of a high-rise building is the effect of the atmospheric pressure.
Flow of Air by Difference in Pressure
When the atmospheric pressure is high, the amount of air per unit volume is high and when the atmospheric pressure is low, the amount of air per unit volume is low.Air flows from where the atmospheric pressure is high to where it is low and tries to maintain a balanced condition of atmospheric pressure.The air leak test is performed using this transfer of air by difference in pressure.
Pressure Change by Leakage
When air is charged to the work (tested work) and pressure inside the work increases, the pressure difference between the inside and outside of the work will be generated.When the work has leak by a hole, the air inside the work will flow outside and the pressure inside the work will decrease.The change of pressure inside the work is monitored and a leak is detected through the air leak test.

PRINCIPLE OF DIRECT PRESSURE (DROP) TYPE AIR LEAK TEST

In this method, a leak is detected by charging test pressure to the work and measuring the change in the test pressure in the work after a given time.
Relationship of pressure change and leak inside the work

Pressure change inside the work during the air leak test is shown in the right figure. Pressure inside the work sharply increases during the pressurization process. This is because the temperature inside the work increases by pressurization and affects the pressure. The pressure change by temperature and other effects is stabilized (balancing process) and then the leak is measured (detection process). Constant value will be maintained and no pressure change in work will be detected when the work has no leak. However, the value will decrease at a constant rate due to decrease of pressure inside the work when the work has a leak. The direct pressure type air leak tester detects leak of work through this pressure change and judges the result.

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PRINCIPLE OF DIFFERENTIAL PRESSURE TYPE LEAK TEST

In the field of medicine, the use of a balancing scale is often used as a precise method for calculating arbitrarily small amounts of substance. The instrument is used to precisely measure a weight close to the balancing weight (a reference value) by increasing the weight value on the opposite side. The differential pressure type air leak tester uses the same principle as the balancing scale. The same air pressure is charged to both the work (tested work) and the master work (work with no leak) and the change in pressure balance within a fixed time is checked for the presence of a leak.

Basic Operation of Differential Pressure Type Leak Test
For the differential pressure type leak test, the leak test is performed through four processes; pressuring, balancing, detection and exhaust.
Pressurizing Process Balancing Process Detection Process
Charge air in the tested work and master work. Close the valve and wait until pressure inside the tested work stabilizes.When there is a leak, air flows out and the pressure inside the work decreases Detect and measure the pressure difference between the tested work and master work through the differential pressure sensor.
Relationship of pressure change and leak inside work
Pressure change inside the work during the air leak test is shown the below figure. Pressure inside the work and master work sharply increases during the pressurization process. This is because the temperature inside the work and master work increases by pressurization and affects the pressure. The pressure change by temperature and other effects is stabilized (balancing process) and then the leak is measured (detection process). After the balancing process, the constant value will be maintained with no pressure change in the master work since it has no leak. However, when the work has a leak, the value will decrease at a constant rate due to decrease of pressure inside the work. Also, when the work does not have any leak, the constant value will be maintained with no pressure change inside the work. A leak is detected by calculating the difference of pressure change between the master work and work.
FUKUDA provides various models of differential pressure type air leak testers. FUKUDA also has a lineup of products with additional functions according to specifications and applications requested by customers.
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MEASUREMENT METHOD OF AIR LEAK TEST

Pressure Change Method  Internal Pressure Type

Method to detect a leak from the difference of pressure change before and after a certain amount of time after applying pressure inside the work and master work. It is an air leak test method optimal for work that has an air charge port and small amount of deformation.

Pressure Change Method  External Pressure Type

Method to detect a leak by placing the work and master work in the chamber, applying pressure in the volume between the chamber and work and then check the difference of pressure change due to intrusion of air inside the work. It is an air leak test method optimal for work that does not have any air charge port and has small amount of deformation.

Tank Pressuring Type

Method to detect a leak from the test pressure after applying pressure inside the work and master work using the compressed air stored in a tank. It is optimal for measuring work that does not have any pressurization port and has relatively large internal volume.

Tank Partial Pressure Type

Method to detect a leak by placing the work and master work in the chamber, applying pressure in the volume between the chamber and work and then check the difference of pressure change due to intrusion of air inside the work. It is an air leak test method optimal for work that does not have any air charge port and has small amount of deformation.

Work – Work Comparison Test

Method to measure leak from the difference of pressure change of two works without using the master work. External effects other than leaks are eliminated with this method since items with the same characteristics are measured and noise is offset. It is optimal for measuring work with large deformation or in conditions (work, ambient temperature, etc.) susceptible to heat.

Leak Side Measurement

Method to perform a leak test by placing the work in a chamber, applying a minute amount of pressure inside the chamber, applying test pressure inside the work and comparing the pressure change due to leak from the work to chamber with that of the master work. It is optimal for measuring work with porous filter element inside the work.

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