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Differential method
  • Leak & Flow
  • Leak & Flow
  • Leak & Flow

Differential method

To detect a leak, ATEQ devices measure a small pressure difference or increase in this difference between two volumes:
the volume of the test part and the volume a tight reference part of the same type.

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Advantages of differential leak testing using a reference part

The pressure difference (ΔP = P2 – P1) is influenced by several factors such as:

  • leak presence
  • volume difference: ΔV
  • temperature difference: ΔT
 P V = n R T,
where P (absolute pressure) [Pa] x V [m3] = k x T [K]
(P+ΔP)(V+ΔV) = n R (T+ΔT)

Since we only want to detect and measure the size of the leak, we need to compensate the differences in volume ΔV and temperature ΔT.

We do this by using a master reference part – of the same material, dimensions, volume and internal construction as the test part, placed in the same environmental conditions, so that they are subject to the same phenomena that cancel each other out during a differential measurement. In other words, the values ΔT and ΔV are the same for the reference and test parts and are not indicated by the leak testing equipment.

Measuring the amount of leakage

The relationship between the pressure drop ΔP measured by the ATEQ leak detector and the amount of leakage is defined by the following formula:

F [cm3/min] = 0.0006 x V [cm3] x ΔP/Δt [Pa/s]

where the leak F is specified in [cm3/min], and the pressure drop ΔP is measured in Pascals [Pa] during the time Δt. This formula comes from Mariotte’s law (Boyle’s law), which for ideal gases takes the form:

P V = n R T

where:

P – pressure in Pa
V – volume in m3
n – number of moles (amount of matter)
R – ideal gas constant (R=8.31 KJ/mol/K)
T – temperature in Kelvin [K] (0°C corresponds to 273 K)

Note: The volume V consist of:

  • volume of the tested part
  • volume of the pipes connecting the tooling
  • volume of the pipes and assemblies inside the measuring equipment

Stabilization

The measurement cycle of ATEQ leak detection and measurement devices consists of phases including the stabilisation time – the time for equalization of pressure fluctuations occurring immediately after filling the test and reference parts with air.

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Reject level

Perfectly airtight items do not exist, but it is possible to achieve a very high level of tightness specified by safety standards, which is particularly important in domestic appliances using natural gas. The size of the leak can also be monitored and repairs or adjustments can be made during measurement to obtain an acceptable level of gas leak. A reject level can be programmed into the ATEQ leak detector so that it automatically classifies the tested parts as “good” or “bad”.