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Leak detection – interesting basics
1. Leak rate
2. Formula symbol und measurement unit
3. Correlation between leak rate and leak size
3.1. Correlation between leak size and leak rate
4. Example of the relation hole diameter / leak rate
5 Types of leaks
1. Leak rate
The leak rate (also: leakage rate) is a measurement for the amount of substance (mass) that passes through a leak.
The leak rate is the ratio of the pV-value of a gas that passes through a cable cross-section, and the span of time. Here, the pV-value is the product of pressure and volume of a certain amount of gas at the respective prevailing temperature. The leak rate is dependent on type of gas, pressure difference and temperature.
Very small leaks are often detected by means of a helium leak detector. Usually the following conditions exist: type of gas Helium, pressure difference 1013 hPa, temperature 20°C. These conditions are also called helium standard conditions.
2. Formula symbol and unit of measurement
The formula symbol most often used for leak rate is Q or QL. The following units of measurement are usually used for leak rate.
A leak rate of 1 Pa*m³/s is given, when the pressure increases by 1 Pascal within one second in a locked, evacuated box having a volume of one cubic meter.
3. Correlation between leak rate and leak size
The following rough estimation gives an idea about the interrelationship between geometric leakage size and the according leak rate:
Given is a big box with a circular leakage with a diameter of 1mm. Outside of the box there is atmospheric pressure, on the inside there is a vacuum. In this case, all gas molecules, that are located in a cylinder with 1 mm diameter and 330 m height „above“ the leakage, would „fall“ Into the leakage/hole with sound velocity (330 m/s). This equates to the following leak rate (= pV-value per second).
Based on this estimation and the size of viruses and bacteria, it is possible to assign limit leak rates to the term “bacteria leak-proof” and “virus leak-proof”
Bacteria leak-proof: diameter of bacteria approximately. 0,5 µm -> Q < 10-5 Pa*m3/s
Virus leak-proof: diameter of small viruses approximately: 10 nm -> Q < 10-9 Pa*m3/s
Up-to date Helium leak detectors can detect leak rates up to 5*10-13 Pa*m3/s. According to the above estimation, this would be a hole diameter in the range of an atomic radius.
3.1 Correlation between leak size and leak rate
In a technical sense, leak tightness means, “free of leaks in accordance with the technical application area
4. Examples of the relation hole diameter / leak rate:
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Hole diameter
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Leak rate in
mbar*l/s
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Leak description
(∆p=1bar)
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Gas leak description
(∆p=1bar)
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≈ 1,0 mm
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102 = 100
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Wasser running
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≈ 0,3 mm
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101 = 10
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Wasser running
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≈ 0,1 mm
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100 = 1
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TDripping Water-tap
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≈ 1 cm3 Gas los in 1 sec.
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≈ 0,03 mm
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10-1 = 0,1
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TDripping Water-tap
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≈ 1 cm3 Gas los in 10 sec.
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≈ 0,01 mm
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10-2 = 0,01
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Water-tight (no dripping)
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≈ 1 cm3 Gas los in 100 sec.
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≈ 3 μm
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10-3
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Vapor-tight (condensation)
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≈ 1 cm3 Gas los in 16 minutes
(approx.. 1 Gas bubble pro sec.)
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≈ 1 μm
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10-4
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Bacteria-tight
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≈ 1 cm3 Gas los in 160 minutes
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≈ 0,3 μm
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10-5
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Fuel-tight and oil-tight
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≈ 1 cm3 Gas los in 26 hours
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≈ 0,1 μm
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10-6
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Virus-tight
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≈ 1 cm3 Gas los in 11 days
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≈ 0,03 μm
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10-7
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Gas-tight
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≈ 1 cm3 Gas los in 110 days
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≈ 0,01 μm
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10-8
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Virus-tight (secured)
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≈ 1 cm3 Gas los in 3 years
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≈ 3 nm
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10-9
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Gas-tight (secured)
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≈ 1 cm3 Gas los in 30 years
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≈ 1 nm
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10-10
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hermetically sealed (technical)
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≈ 1 cm3 Gas los in 300 years
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5. Types of leaks
Leaks are divided nto he following groups, based on their characteristics:
Hole-leak (clearly visible)
- Turbulent-leak ( wheezing gas leak)
- Laminar-leak (leakage in detachable and non-detachable connections, such as flanges or welded seams)
- Molecular-leak (also called pinhole-leak; leakage that develops from tiny pores or cuts in the polycrystalline structure.)
- Cold / warm leak – (reversible leakage, that only occurs with extreme thermal stress)
- Valve-leak (or fold-leak) – leakage with a preferred flow direction
- Lambda leak (super leak) – Leakage that occurs with the condensation (liquefaction) of Helium
- The material specific permeation of a substance/material is not considered leakage in the traditional sense.
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