4 Air Monitoring
Stability and Traceability of Calibration Gases for EN 14181
The terms ‘traceable’ and ‘stable’ used to describe gas mixtures have long been a topic of debate, even confusion, since the EN 14181 standard was fi rst published back in 2002. The objective of this article is to provide an explanation of these two terms as they relate to gas mixtures, and to enable users to select the correct calibration gases for their application with confi dence.
The calibration of analysers using binary mixtures can often lead to errors in the analysis of stack gases due to cross- sensitivity effects occurring within analysers, hence the value of these products in all aspects of EN14181 compliance.
By explaining the factors that affect mixture stability, and the various routes of traceability that exist for gas mixtures, the aim is to provide the required knowledge for those responsible for sourcing gas mixtures for emissions monitoring work; ultimately, to provide users with an ability to recognise the difference between a gas mixture which fulfi ls legislative obligations, and one that does not. The costs and the penalties of making a mistake in this area can be severe and have caught out many organisations over the years.
Gas Mixture Stability
The term ‘stability’ is perhaps the easier one to comprehend. It is a characteristic of a gas mixture. Stability is basically a declaration of how long the concentrations of components within a gas mixture will remain within the stated measurement uncertainty after they are originally certifi ed by the manufacturer. In accordance with the ISO 6141 standard describing the requirements for certifi cates for calibration gases, it is mandatory that the stability of a gas mixture is stated on its accompanying certifi cate of analysis. It might also be expressed as a ‘shelf life’ or ‘use by date’.
Accuracy and traceability of measurement is worth little if the analyst has no confi dence in the stability of the standard they are using to calibrate their analytical equipment.
Most manufacturers of calibration gas mixtures invest heavily in the research and development of techniques that allow them to declare long mixture shelf lives. The factors that infl uence stability are many and include the following:
•Selection of optimum cylinder and valve materials •Treatment of the internal surfaces of both the cylinder and the valve •Segregation of cylinders into specifi c gas service
Stephen Mellor, Product Specialist – Analytical, Air Products Plc, UK
The Source Testing Association (STA) Unit 11 Theobald Business Centre, Knowl Piece, Wilbury Way, Hitchin, Herts SG4 0TY United Kingdom
Tel: +44(0) 1462 457535 Fax: +44(0) 1462 457157
Email: General enquiries
sam@s-t-a.org Web:
www.s-t-a.org
IET Annual Buyers’ Guide 2014/15
www.envirotech-online.com
•Cylinder preparation before fi lling (heating, purging, evacuation) •Selection of ultra high purity components •Advanced fi lling technology •Special analytical techniques •Stability tests to prove stability
No single step leads to stable calibration gas mixtures. It is a combination of all these aspects and others that results in signifi cantly improved mixture stability.
No matter what the traceability route is for the gas mixture, all reputable suppliers of calibration gas mixtures will declare a shelf life, albeit some longer than others. The EN 14181 standard demands that gas mixtures are used within their stated shelf life for all aspects of compliance. It’s important to look carefully for this information to make sure the calibration gas being used has not expired.
Traceability of Gas Mixtures
Traceability of measurement requires an unbroken chain of comparisons to stated references, each with a stated uncertainty. The objective is to deliver national and international comparability, so it is easy to appreciate why there is such importance attached to this property of a gas mixture when it is used as part of EN 14181 compliance.
Traceable calibrations automatically provide users with the means to obtain consistent data over time, even when one mixture is replaced by another, and it allows full comparability of data obtained from similar measurements taken around the world.
Gas mixtures are traceable either to certifi ed reference gas materials provided by national measurement institutes, or to the national mass standards.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148
