As with many other products, gas equipment marketed in EU countries must carry the CE mark. This mark has extra significance for gas appliances since the equipment uses gases that are particularly flammable in order to operate. Therefore, safety is a key element of the testing and certification work that forms the basis for the marking scheme.

As notified body, DBI can carry out accreditation testing and certification in accordance with the EU’s Gas Appliances Directive (90/396/EEC). The directive applies to all gas appliances that are used for cooking, heating, producing hot water as well as cooling, lighting and washing.
 
“The testing that we carry out is geared towards the safety aspect of appliances, particularly the safety of the consumer, and it does not cover shortcomings in functionality or durability, for example”, explains engineer Ken T. Olesen, who is general manager of DBI’s gas laboratory.
 
Particularly LPG equipment
“AT DBI we test gas stoves, for instance, but we also carry out a considerable amount of work on LPG gas powered appliances, such as barbecue, camping and heating equipment. Almost every week we get new equipment in for testing and certification. The vast majority of our customers are from abroad since the manufacture of gas equipment has all but ceased in Denmark. But we also have Danish customers who have either imported equipment or had equipment specially made abroad.”

Ken Olesen elaborates on the testing method:

“When designing a test process for a gas appliance we assess:

variations in power supply, load and frequency.”

The result of these assessments must comply with a number of standards that describe the individual testing procedures and the appurtenant acceptance limits. Standards have been drawn up for most types of gas equipment.

In order to be able to take into account variations in gas quality and pressure, it is important that you know the size of the variations. Therefore, the gases are split up into different sub-types or categories, and the permitted variation within each category has been defined.

For the tests, each category is represented by different standardised test gases which reflect the permitted variation in the gas that will be supplied to the consumers.

The standardised test gases consist of a normal gas or reference gas, which resembles the gas that is normally supplied, and a number of extreme gases or
borderline gases that have to simulate the variations in the gas supplied. 

Safety margin testing philosophy
A principle developed by Frenchman Paul Delbourg is used for setting the composition of the standardised test gases. The principle is based on the standardised test gases lying on the borderline of the acceptable, while the gas supplied to the consumer is on the safe side of the acceptable – in other words, it has a respectable safety margin.
 
The theory is that, if an appliance sent in for testing burns satisfactorily with the standardised test gases, it will also be able to burn with the gas that is supplied to the consumer – even when taking the permitted variations in gas quality into account.

Almost all testing that is carried out on gas equipment is based on the same philosophy. When testing a specific characteristic, the most incompatible test gas is applied to the appliance at the most incompatible pressure, concurrently ith another effect if necessary. If the gas appliance the test under these conditions it will be able to handle the variations in gas quality and environments that it may come up against when it is used by the consumer.

By incorporating these safety-determined buffers into the testing procedure, account is taken of uncertain and unforeseen conditions such as wear and tear and lack of maintenance.

Delbourg’s gas theory When testing gas appliances, a number of standardised gases are used. The  gases are composed on the basis of a theory developed by French researcher Paul Delbourg. The results of his research can be used for determining which characteristics the various test gases should have, and from that, the composition that will give the desired characteristics can be calculated. The resultant knowledge is used when testing gas equipment. Delbourg conducted a number of tests in which he observed what happened when gases with different compositions were applied to various gas burners. In particular, he studied the following three conditions: combustion quality flash back flame lift. The combustion quality is an expression of the burner’s ability to burn the gas applied completely. When it burns in-completely, carbon monoxide (CO) - an  odour-free and highly toxic gas - in particular is produced. Additionally, the tips of the flames will often have yellow melanising tips. Flash back and flame lift are both expressions of a lack of balance between the combustion flame velocity speed and  the gas velocity. If the gas is burning quicker than it is being discharged from the burner, there will be flach back where the gas burns either partially or totally inside the burner (fig. 1): If, on the other hand, the gas is burning slower than it is being discharged, the flame will sway slightly above the burner or totally disappear (fig. 2). Both of these conditions can give rise to safety problems.  The correct balance between the speeds is illustrated in fig. 3. Delbourg-diagram Combustion patterns can be illustrated graphically using the so-called Delbourg diagram. The outer triangle of the diagram defines the gas types whereby all appliances will burn acceptably. Incomplete burning, flach back or flame lift has been observed for gases outside this area. Where tests for CE certification are involved, it has been decided that gas appliance standardised test gases should be used, these are marked with x in the diagram. From the acceptable area, a safety margin has been plotted so that the gas that is supplied to the consumer lies within the inner triangle.