Metrology of the leak detection – Practical guide

Leaks have not always been a major issue in vacuum technology where they are a limiting factor to the ultimate pressure and the purity of a process gas that can be reached in a vacuum vessel, but also in any other container, be it for operational reasons (e.g. engines, air bags or pace makers), security reasons (e.g. for poisonous or radioactive materials) or environmental reasons. Limitations of refrigerant gas leakages were pointed out in regulations to meet recommendations of the Kyoto Protocol (1997). The above mentioned examples are far from being exhaustive and the leak measurement is a crucial concern in many industries.

This practical guide deals with tools and theory in the field of the gas leak detection under the angle of metrology, in a  range from 1 x 10-10 Pa·m3·s-1 to 1 x 10‑4 Pa·m3·s-1, considering leaks towards atmosphere or vacuum.

Leaks are, in general, very low gas flows.
The Chapter I introduces the gas flow metrology and contains definitions and current units found in industry and research. The primary standards for low gas flows in the European National metrology institutes, which took part in the project EMRP IND12, with the associated measurement uncertainties are presented.

To ensure leak measurements traceability, people in industry need tools more practical than primary standards: leak artefacts are the secondary standards. Depending on the artefact type (permeation or conductance), the downstream pressure where the gas flows (vacuum or atmosphere), the behaviour of the instruments differs.
The Chapter II gives theory and characteristics of the different leak artefacts.

The Chapter III delves into helium leak detection, a method used world-wide to detect and quantify leaks. After a complete description of helium leak detectors and their principle, the metrological characteristics of three of these detectors assessed during the project EMRP IND12 are presented.

The Chapter IV focuses on the refrigerant leak detection. The environmental issue of greenhouse gases emissions was pointed out during the Kyoto protocol in 1997 and it was then decided to limit their emission. In Europe regulations were published afterwards, where it is in particular mandatory for owners of air conditioning equipment to have a suitable refrigerant leak detector. In this chapter, principle of the refrigerant leak detectors and the evaluation of their performance according to the standard EN 14624:2012 are presented. Evaluation of eight commercially available instruments was performed in the frame of the project EMRP IND12 which has resulted in a series of recommendations for using the instruments as properly as possible.

Finally, the reader will find in the Annexes some additional information about the vocabulary of metrology, some uncertainty assessment general considerations and a simple model to predict the gas flow in capillary leaks from few measurements with one gas species.

This work underlying this guide and also the development of the guide itself was supported by the European Metrology Research Programme (EMRP) in the designated project IND12 « Vacuum for production environments ». The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Introduction

CHAPTER I. Gas flow metrology: general concepts
I.1. Definitions and units
I.2. Conversion table for flow rate units
I.3. Techniques for leak detection
I.4. Metrological traceability of leak detection
I.5. Primary standards
I.6. Secondary standards

CHAPTER II. Leak artefacts
II.1. Operating principles of leak
II.2. Influence parameters

CHAPTER III. Helium leak detection
III.1. Introduction
III.2. Why helium is used in leak detection?
III.3. Leak detector operation principle
III.4. System configuration
III.5. Study of leak detector performance
III.6. Influence of the inlet pressure on the leak detector signal
III.7. Leak detector calibration
III.8. Industrial application
III.9. Existing standards for calibration

CHAPTER IV. Refrigerant leak detection
IV.1. Operating principle
IV.2. European standard EN 14624:2012
IV.3. Standard tests on commercially available detectors
IV.4. Good practices for industrial using

References
Annex A- Table of fluid isothermal data for helium
Annex B- Uncertainty: main definitions
Annex C- Uncertainty assessment general consideration
Annex D- Simple model to predict the gas flow from a leak
Annex E- International community acceptance of calibration certificate