Study of standard leaks performance for different gas, in the transition regime

Project Description

The leak detection is nowadays widely used in various fields from the industrial production to research ambient, from automotive or refrigeration industries to particle acceleration. The gas flow rate, of course, depends on the considered apparatus or component. For example in the refrigeration industry, compressors or heat exchangers are critical elements, which are subjected to regulation, which requires leak tests. The required sensitivity of measurement is of 0.5 g/y of refrigerant (R134a, R410, R600) or less, with an acceptable expanded uncertainty up to 10%.
The leak test may be carried out in two different ways:

• the considered component is placed inside a vacuum chamber connected to a leak detector and it is pressurized up to 1 MPa with tracer gas (generally He or a mixture of He in N₂)
• the component is pressurized with tracer gas (He or a mixture of He in N2) and the leak is detected with a sniffer
   

In both cases the leak is measured in terms of helium throughput whose value must be converted into the refrigerant throughput. The conversion (from He to refrigerant) requires the knowledge of the gas regime inside the leak; in molecular regime the conversion from a gas specie to another is well know. The situation is more complex when the gas flow inside the leak is in the transition regime.
One the most diffused system to detect and measure leaks is based on mass spectrometry (mass spectrometry leak detector-MSLD) and needs to be calibrated with reference leaks.

The aim of the project is to describe the gas-flow through a reference leak (generally geometrical leak of the capillary type) when the transition regime takes place in it with a simple equation that allows converting the throughput of one gas (He) specie to that of another gas.
With the purpose of better understanding the behavior of capillary leaks, the project may be developed as follows:

• the laboratories exchange information about the facilities they developed, and the results already available
• the coherency between the measurements carried out in the participating laboratories is checked for few measurements
• a complete program will be established from these data, in order to optimize the efforts.

Final Report 2013-04-10

Primary standard flow meters are developed and maintained for the calibration of leak artefacts used in many applications in which the quantification of leakage is required. The INRIM and LNE maintain primary flowmeters based on different methods in order to measure small gas flow of R134a referred to atmosphere (ambient pressure). In the framework of the project the cooperation gives the opportunities to check and improve the primary standard flow meters. The measurements have been performed over the range of 3.5 g/y to 60 g/y with R134a using a permeation leak and a capillary leak as transfer standards supplied by LNE. The results of the comparison show that the laboratories agree within their expanded uncertainty over the measured range of gas flows.

Furthermore the gas flow from capillary was measured in different conditions: reference to vacuum or atmospheric pressure and using different gas species (helium and refrigerant) with the aim to verify that in most of cases concerning with useful applications of gas flow measurements with outlet to atmosphere or vacuum with high inlet pressure, the gas flow released by a reference capillary leak is, or mainly, in viscous regime.

The temperature coefficient of the capillary leak for various gas species and in the range from 15 °C to 30 °C was determined at LNE for He, N2, Ar in vacuum and R134a at atmosphere. At INRIM the temperature coefficient was also determined in the same temperature range and for the same gas species. The results showed that the ratio between the throughput at temperature ti and at 20 °C and the ratio of the corresponding viscosity,  neglecting the  effects due to slightly different (upstream and downstream) pressures and material thermal expansion coefficient, are in agreement inside the measurements iuncertaity and the viscous regime seems to be confirmed.

Progress Report 2010-02-22

Primary standard flow meters are developed and maintained for the calibration of leak artefacts used in many applications in which the quantification of leakage is required. The INRIM and LNE maintain primary flowmeters based on different methods in order to measure small gas flow of R134a referred to atmosphere (ambient pressure).

In the framework of the project the cooperation gives the opportunities to check and improve the primary standard flow meters. The measurements have been performed over the range of 3.5 g/y to 60 g/y with R134a using a permeation leak and a capillary leak as transfer standards supplied by LNE. The results of the comparison show that the laboratories agree within their expanded uncertainty over the measured range of gas flows. Furthermore the gas flow from capillary was measured in different conditions: reference to vacuum or atmospheric pressure and using different gas species (helium and refrigerant) with the aim to found an easier way to convert the helium gas flow value in refrigerant gas flow value. As preliminary result a conversion coefficient from helium gas flow rate and R134a was found with a relative expanded uncertainty of 10%.