The Importance of Fast Responding Hydrogen Sensors in the Detection of Hydrogen Leaks
Lois Brett, European Commission, DG JRC - Institute for Energy

The Institute for Energy (IE) is one of 7 scientific research institutes of the Joint Research Centre (JRC) of the European Commission. Its mission is to provide independent support to European Community policies related to energy in order to ensure sustainable, safe, secure and efficient energy production, distribution and use. 

At the JRC-IE we have been involved in impartial hydrogen sensor performance testing since 2001. In the execution of our activities we collaborate with sensor developers, manufacturers, end users, renowned research institutions (e.g. NREL) and international standards organisations to support the safe and effective use of hydrogen. During the past decade, we have witnessed a large increase in the number of hydrogen detection devices available on the commercial market, as well as significant improvements in their performance. Sensors are now capable of measuring hydrogen concentration with greater accuracy and under more challenging ambient operating conditions. Furthermore, both sensor size and price have decreased over this period. 
In recent times there has been a change in the focus of external interest in our work. Enquiries now relate more to the practical application and correct deployment of hydrogen sensors rather than to the details of the various types of sensing technology as in the past. 

Traditionally performance evaluation of chemical sensors, including hydrogen sensors, has focussed on their selectivity and sensitivity to the species of interest, as well as their operating life stability. However, the recognised importance of hydrogen sensor response time in safety applications means that this property is also receiving much attention, and increasingly stringent targets are being set [1]. Rapid detection of hydrogen gas is essential to provide an early warning of an unwanted hydrogen release. In this respect the shorter the sensor's response time (RT) the better (here we define response time, t(90), as the interval between the time when an instantaneous variation from clean air to the standard test gas, with a given hydrogen concentration, is produced at the inlet of the hydrogen sensor and the time when the response reaches 90% of the maximum indication). 

While a short RT is a critically important performance parameter of any hydrogen safety sensor, the total time of response to a leak depends not only on the RT of the sensor itself, but also on the location of the sensor relative to the point of release. Consequently correct sensor placement is another equally if not more important consideration in facilitating rapid leak detection and promoting safe hydrogen use. 

At present there are several commercially available hydrogen sensors with a claimed response time of >5 s. Developments in micro- and nano-structured sensor technologies promise smaller, cheaper and even faster hydrogen sensors in the future. The emergence of progressively faster hydrogen sensors demands improved methods to accurately measure, validate and compare the response time of these devices.

However accurate measurement of this property is not trivial, as it can be difficult to separate the response time of the sensor from the time taken for the hydrogen to reach it. As defined, a sensor's RT is measured relative to an instantaneous gas change and so any experimental method requires an instantaneous, step-wise gas change from clean air to the desired hydrogen concentration at the sensor inlet. In reality however such a stepwise gas exchange is not possible and the hydrogen concentration at the sensor inlet will always increase in a transient manner. Consequently any experimental measurement of a sensor's RT includes not only the time taken for the sensor to react with and respond to hydrogen but also a time period required to complete gas exchange at the sensing element.

Various sensor RT measurement methods exist and the JRC-IE has evaluated the two methods which are described in ISO 26142:2010 - Hydrogen detection apparatus. The RTs of two different commercial hydrogen sensors were measured using these methods. The purpose of the work was to assess the suitability of these different methods for the measurement of sensor response time. The methods were evaluated in terms of the measurement repeatability and accuracy when compared with the sensor manufacturer's specifications. We have shown that the measured RTs depend highly on the method used to evaluate them. Based on the observations and results new RT measurement methods were developed and evaluated yielding significant improvements in measurement accuracy and repeatability. 

The two methods suggested in ISO 26142 employ different principles for measuring hydrogen sensor RT. One method uses diffusion and the other dynamic flow to transport gas to the sensor's sensing element. In our realisation of the flow-based method the hydrogen sensor is fixed to the side of a copper pipe, down-stream of a 3-way valve. Switching of this valve allows either air or the hydrogen test gas mixture to be selectively flowed through the pipe to the sensor. Following switching of the valve from air to test gas the response of the sensor is recorded to yield the RT. This flow-based method proved to have a reasonable repeatability however the accuracy of the measurements was not very good. The main advantages of this method include its ease of execution, simple set-up and the ability to also measure sensor recovery time, which was not always possible using the other methods. 

In the diffusion based method or "Membrane method", the sensor is placed inside a small holder cell which is sealed at the top with a latex membrane. The sensor holder is then placed inside a 30L diffusion chamber which is subsequently filled with hydrogen in air. At the start of the measurement the membrane is ruptured using a scalpel, allowing the hydrogen gas mixture to diffuse to the sensor. The sensor response is recorded to yield the RT. Extensive measurements of RT were made using this method however the measured RTs were much longer than those reported by the sensor manufacturer and showed a very large variation. In addition execution was cumbersome and time consuming, large volumes of gas were required and sensor recovery time measurements were not possible. 

The low accuracy and repeatability of measurements made using the Membrane method was largely attributable to inconsistent rupturing of the membrane, diffusion of hydrogen through the membrane and uncertainty regarding the exact start time. For these reasons this method was modified by replacing the latex membrane with a latched aluminium lid. All other aspects of the set-up and method remained the same. At the start of the measurement the lid was removed by releasing the latch holding it in place thereby exposing the sensor rapidly to the test gas mixture. Results obtained using this “Lid method” showed very significant improvements in both the accuracy and repeatability of the RT measurements. This was achieved with minor, but nonetheless effective, modifications to the original method suggested in ISO 26142. Similar to the Membrane method, execution of this method was time consuming, large volumes of gas were required and sensor recovery time measurements were not possible. 

The final method tested and evaluated used a fast acting gate valve to separate the sensor under test from the hydrogen gas mixture. The "Gate valve method" was designed to allow measurement of not only the sensor response time but also the recovery time. This was made possible by locating the sensor holder outside the 30L diffusion chamber. The gate valve separated the two volumes, each of which could then be independently flushed with either air or hydrogen test gas mixture. In this way, both sensor response and recovery time measurements were theoretically possible. Following opening of the gate valve the sensor response was recorded and the RT measured. The measurements were quite accurate and repeatability was good however this method was technically complex, costly and recovery time measurement was not always possible under certain conditions. 

The full results and conclusions from this work have recently been published [2] and they were also presented during the NHA Hydrogen Conference and Expo 2010 [3]. While these results were not available for consideration during the preparation of the current version of ISO 26142:2010 the JRC-IE will continue to provide independent scientific and technical support for any future revisions of this standard. 

Sources

US DoE Multi-Year Research, Development and Demonstration Plan: Planned Program Activities for 2005-2015
http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/safety.pdf

L. Boon-Brett, G. Black, P. Moretto, J. Bousek, A comparison of test methods for the measurement of hydrogen sensor response and recovery times, International Journal of Hydrogen Energy, Volume 35, Issue 14, July 2010, Pages 7652-7663

Conference details and proceedings available fromhttp://www.hydrogenconference.org

Citing Normative Standards in ISO or IEC Documents
Karen Hall, National Hydrogen Association

As an active member of several working groups involved in developing documents for ISO/TC 197, I have witnessed many discussions regarding the appropriateness of citing existing standards in developing International Standards. There are times, after all, when an existing standard is widely used or has been developed to meet a specific need covered by the document under development. This article is provided to inform the reader about the rules that ISO and IEC have in this regard.

These rules can be found in the ISO/IEC Directives, Part 2 - Rules for the structure and drafting of International Standards (2004, 5th edition)(ISO/IEC, 2004), which is available from ISO online athttp://isotc.iso.org/livelink/livelink?
func=ll&objId=4230456&objAction=browse&sort=subtype. 

Both ISO and IEC have published guides for the preparation of documents prepared in accordance with the ISO/IEC Directives. These guides are available on the ISO web site (http://www.iso.org) and IEC web site (http://www.iec.ch).

ISO has written a specific guidance document on the subject:  Policy concerning normative references in ISO publications, which is available from ISO at http://www.iso.org/iso/policy_concerning_normative_references.pdf
Please note that this policy guidance was written to a previous version of the ISO/IEC Directives, Part 2, (ISO/IEC, 2001) although the information is still valid.

Firstly, to make sure we understand the terminology, one must understand the difference between "international standards" and "International Standards".  An "international standard" is a standard that is adopted by an international standardizing/standards organization and made available to the public [ (ISO/IEC, 2004), definition 3.2.1.1]. An "International Standard" is an international standard where the international standards organization is ISO or IEC.

Secondly, let’s be certain we understand the difference between "normative references" and "informative references" as used by ISO and IEC. 

Normative references are documents that are indispensable for the application of the document that contains them. For ISO and IEC documents, both dated and undated references may be used. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. 

Informative references are supplementary, and are typically contained in the bibliography. Elements of the ISO or IEC document that are supplementary, including informative references, are elements that provide additional information intended to assist the understanding or use of the document.

Now that we understand the terminology used, let’s review the ISO rules for normative references in ISO publications. Section 6.2 of theISO/IEC Directives, Part 2 - Rules for the structure and drafting of International Standards(ISO/IEC, 2004), provides this information explicitly, as shown below. It should be noted that it is possible to reference enquiry or final drafts as normative documents if noted by the term "to be published".

In principle, the referenced documents shall be documents published by ISO and/or IEC. Documents published by other bodies may be referred to in a normative manner provided that
  • the referenced document is recognized by the ISO and/or IEC committee concerned as having wide acceptance and authoritative status as well as being publicly available,
  • the ISO and/or IEC committee concerned has obtained the agreement of the authors or publishers (where known) of the referenced document to its inclusion and to its being made available as required — the authors or publishers will be expected to make available such documents on request,
  • the authors or publishers (where known) have also agreed to inform the ISO and/or IEC committee concerned of their intention to revise the referenced document and of the points the revision will concern, and
  • the ISO and/or IEC committee concerned undertakes to review the situation in the light of any changes in the referenced document.

The list shall be introduced by the following wording:

"The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies."
The above wording is also applicable to a part of a multipart document. 

The list shall not include the following:
  • referenced documents which are not publicly available;
  • referenced documents which are only cited in an informative manner;
  • referenced documents which have merely served as bibliographic or background material in the preparation of the document.
Such referenced documents may be listed in a bibliography (see ‎6.4.2). (ISO/IEC, 2004).

The ISO document "Policy concerning normative references in ISO publications" goes into greater detail on the requirements as follows: 

It needs to be recalled that the provision for making normative reference to, rather than reproducing text from, another document has a number of benefits such as:
  • reducing unnecessary and duplicative expenses to standards developers, participants, experts and users of standards,
  • eliminating redundancy,
  • increasing the speed of development while preserving the scope and global relevance of ISO documents.

The above rules are based, therefore, on the understanding that, whenever it is necessary to make normative reference to another publication, that reference shall preferentially be to the appropriate ISO or IEC publication if it exists. The purpose of this requirement is to ensure the overall consistency and coherence of the full set of ISO and IEC publications. If a committee wishes to make reference to another publication in preference to the corresponding ISO or IEC standard, an exemption must be requested from the Technical Management Board. In this context, it should be noted that when a document is developed under CEN-lead under the Vienna Agreement, and the normative references clause consequently refers to ENs or prENs, a normative annex shall be added indicating the references of the technically equivalent ISO or IEC publications where they exist. The ISO or IEC references take precedence over the ENs or prENs for the application of the ISO document.

When an appropriate ISO or IEC publication does not exist, publications of other bodies may be referenced normatively. Normative references to such non-ISO or IEC publications that are accepted and used globally may:

  • provide greater flexibility to best serve the specific needs of particular communities or sectors;
  • establish a worldwide implementation of ISO standards in cooperation and concert with standards of other bodies, rather than in competition with them.

The goal is for ISO documents to be market relevant and thus, in the examination of work needing to be done, ISO committees should consider whether the ISO document may normatively refer to one or more documents already in existence that meet the needs of global users and whose developers agree to any necessary requirements of the IEC/ISO Directives to support such normative references. The above clause of the Directives stipulates a number of conditions that need to be fulfilled. The first condition is that the referenced document is recognized by the responsible ISO or IEC committee as having wide acceptance and authoritative status as well as being publicly available. It needs to be stressed that for the end-user it is imperative that any normatively referenced document be publicly available. For this reason, draft documents which have not reached the enquiry stage in ISO or IEC, or an equivalent stage in the case of documents of other organizations, may not be normatively referenced.

The second condition is that the authors/publishers have given their agreement to the document being referenced and to its being made available to support the committee’s standards development efforts as required. It needs to be recalled that while particular standards may be widely used within particular regions, there may well be cases in which other member bodies would wish to study the document before they are able to agree to it being normatively referenced in the proposed ISO publication. For this reason the committee secretariat should obtain a copy of the referenced document and make it available upon request to P-members wishing to review the document to confirm its suitability for normative referencing. Working group conveners and secretariats should endeavour to ensure that such referenced documents are available as soon as possible once the decision has been taken to make reference to them, ideally simultaneously with the authors/publishers giving their agreement to their document being referenced. 

The authors/publishers of such referenced documents need also to be aware that, if an ISO member body wishes to adopt an ISO publication as a national publication, then the member body may need to make available a version of any normatively referenced documents in its national language. The authors/publishers of referenced documents will consequently be expected to cooperate with ISO member bodies in such cases. 

The third condition requires the authors/publishers of the referenced document to inform the ISO or IEC committee concerned whenever it is proposed to amend or revise the referenced document, as well as of the points that will be revised or amended. This is to allow the committee (fourth condition) to confirm that the document as revised/amended is still appropriate for referencing in the ISO or IEC publication and if necessary to allow amendment of the ISO or IEC publication to refer to the revised/amended edition of the referenced document.

As a matter of policy, ISO does not require normatively referenced non-ISO or non-IEC documents to be transposed into ISO or IEC publications. Nevertheless, when it is proposed to revise a document that is normatively referenced in an ISO publication, the authors/publishers are encouraged to consider the possibility of offering the revision to ISO. If, in other circumstances, a committee sees merit in transposition, rather than normative referencing, of another document, it shall seek the agreement of the authors/publishers of the referenced document. Any unilateral action would essentially constitute copyright infringement which could potentially lead to litigation. 

Finally, in the past, a number of ISO committees are reported to have adopted policies of making normative reference ONLY to ISO or IEC publications. The Technical Management Board has now ruled that such policies shall be withdrawn. (ISO, 2001) 

From this ISO policy, we can see that it is possible to cite non ISO/IEC documents as normative references, providing the above conditions are met. When these conditions cannot be met, it is still possible to cite the document as an informative reference.

Works Cited

ISO. (2001). Policy concerning normative references in ISO publications.

ISO/IEC. (2004). ISO/IEC Directives: Part 2 - Rules for the structure and drafting of International Standards. Fifth Edition.

ISO/IEC. (2001). ISO/IEC Directives: Part 2 - Rules for the structure and drafting of International Standards. Fourth Edition.

September 2010 Codes and Standards Meetings
Karen Hall, National Hydrogen Association

September promises to be a very busy time for those involved in the development of hydrogen and fuel cells codes and standards. At the time of publication, all the meetings listed below had been announced. A calendar of upcoming meetings is available by clicking "Meetings/Events" on the left. You can also find a comprehensive calendar that includes due dates for key codes and standards development cycles at www.fuelcellstandards.com/calendar_new.html.

  • ISO/TC 197 WG 14: September 13 and 14. Vancouver, B.C.  Canada

  • SAE Fuel Cell Vehicles Standards Committee and Working Groups: September 14, 15 and 16. Troy, Michigan

  • NFPA 52 Pre-Report on Proposals (ROP) Meeting: Sept 15-16. Las Vegas, Nevada

  • NextEnergy Codes & Standards Conference: September 21. Detroit, Michigan (tentative)

  • ISO TC 197 WG 8: September 21, 2010. Tokyo, Japan

  • ISO TC 197 WG 11: September 22, 2010.  Tokyo, Japan

  • CSA America: September 27-30.  Orlando, Florida
    Sept 27th: Fuel Cell Technical Committee
    Sept 28th: HGV3 - Fuel System Components for H2 Vehicles
    Sept 28th: HGV4.9 - Fueling System Guidelines
    Sept 29th: HGV4.3 - Fueling Parameters for H2 Dispensing
    Sept 29th: HPRD - H2 Relief Devices for H2 Vehicle Fuel Containers
    Sept 30th: HGV2 - Hydrogen Vehicle Fuel Containers