OIML R 99 and ISO PAS 3930: What a Petrol Exhaust Gas Analyser Has to Measure
OIML R 99 is the reference document behind most national rules for petrol exhaust gas analysers. Since 2008 its technical content has been published jointly with ISO as ISO PAS 3930, so the two numbers describe one set of requirements, not two competing ones.
One document, two reference numbers
OIML R 99-1 & 2, edition 2008, was developed jointly by OIML Technical Committee TC 16/SC 1 (air pollution) and ISO Technical Committee TC 22/SC 5 (engine tests). It is published by OIML as the Recommendation and by ISO as ISO PAS 3930:2009, and the technical requirements in the two publications are identical. The 2008 edition supersedes OIML R 99:2000 and its 2004 amendment.
The recommendation is split into Part 1, metrological and technical requirements, and Part 2, metrological control and performance tests. A separate Part 3, published as OIML R 99-3:2008, defines the report format.
This matters when you read a tender or a national regulation. A document that cites OIML R 99 and one that cites ISO PAS 3930 are asking for the same instrument, and a supplier quoting both as if they were separate approvals is padding the file.
What the instrument has to measure
The scope covers digital instruments that determine the volume fractions of components in the exhaust gas of vehicles with spark-ignition engines, used in the inspection and maintenance of in-service vehicles along the procedure defined in ISO 3929.
The components named in scope are carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC, expressed as n-hexane) and oxygen (O2) at the moisture level of the sample as analysed. Instruments may also offer a lambda calculation. The detection principle assumed by the document is infrared absorption for CO, CO2 and HC, with oxygen normally measured by a fuel cell, though the text allows other principles provided they meet the requirements and pass the tests.
On-board diagnostic equipment is explicitly outside the scope. An OBD scan tool and a gas analyser answer different questions and neither replaces the other.
Why this is a petrol document, not a general emissions one
Nothing in OIML R 99 covers oxides of nitrogen, and nothing in it covers smoke from a compression-ignition engine. A diesel vehicle is assessed by opacity and light absorption, which is a separate standard (ISO 11614) with its own instrument, the opacimeter.
This is the single most common specification error in a mixed fleet market: one bench is bought and described as an emission tester, then the lane discovers it cannot produce the reading the diesel test requires. If the station tests both petrol and diesel vehicles, the equipment list needs both instruments, and the layout needs a position for each.
What the accuracy class decides
The 2008 edition defines three accuracy classes — 00, 0 and I — with class 00 carrying the tighter requirements on CO, CO2 and HC. Older class II and analogue instruments are no longer addressed by the current edition; national authorities may set transitional arrangements for instruments already in service under the 2000 edition.
The class is not a quality label to be upgraded for marketing. It decides whether the instrument can be used at all where the national rule names a class, and it decides how much of the measurement uncertainty is left for the rest of the chain. Confirm which class the regulation in force where you operate actually requires before comparing prices, because the same model in two classes is two different instruments.
| Class | What it signals | Who decides you need it |
|---|---|---|
| 00 | Tightest requirements on CO, CO2 and HC of the classes in the 2008 edition | The national regulation, or a customer specification stricter than it |
| 0 | The middle class of the current edition | The national regulation in force at the place of use |
| I | The widest of the three classes | The national regulation in force at the place of use |
| Class II / analogue | No longer addressed by the 2008 edition | Transitional arrangements set nationally for instruments already in service |
Requirements that are not about chemistry
A working analyser is more than a bench that reads four gases. The recommendation also constrains the things that decide whether a reading survives an audit.
Electromagnetic compatibility requirements and tests were added or updated in the 2008 edition, because an analyser sitting next to a brake tester, a dynamometer and a welding set is an analyser in an electrically noisy room. Requirements for durable recording were added, so a result can be produced later without retyping it by hand. Software, and instruments powered by an internal battery, are addressed as well.
- Warm-up and stabilisation behaviour defined, so the first test of the morning is not the outlier.
- Zero and span adjustment with a documented procedure, using gases of stated composition.
- Leak and flow checks, because a sample diluted by an intake leak reads clean.
- Response time short enough for the test procedure being run.
- A record output — printer or file — that carries the identification of the instrument and the vehicle.
What Part 2 adds: control, not just capability
Part 1 tells you what the instrument must be able to do. Part 2 sets out the metrological control and performance tests: type evaluation before the model is placed on the market, and the checks that keep an individual instrument valid afterwards.
In practice this is the part that decides your operating budget. A model with an OIML certificate issued under the OIML Certification System has had its type tested against the recommendation by an issuing authority, which shortens the national approval route in many markets. A model without one may still be accurate, and still need a full local type evaluation before it can legally be used for inspection.
What to put in the specification
Write the class, the components, the detection principle and the certificate into the specification, and leave the marketing adjectives out. Then ask for the documents that prove each line.
A supplier who can answer these five items in writing is answering a technical question. A supplier who answers with a photograph of the case is answering a different one.
- Accuracy class, and the national rule that requires it.
- OIML certificate or type approval number, with the issuing authority.
- Components measured, whether lambda is included, and how HC is expressed.
- Calibration gas specification the instrument expects, and where those gases are available locally.
- Record output format, and whether results can be exported rather than only printed.
Common questions
Is ISO 3930 the same as OIML R 99?
The current technical content is the same. ISO PAS 3930:2009 and OIML R 99-1 & 2:2008 were developed jointly and their requirements are identical, so a citation of either refers to one set of requirements. Older references to ISO 3930 without a year usually mean the 2000 edition, which the 2008 text replaced.
Can one analyser test both petrol and diesel vehicles?
Not for the regulated result. OIML R 99 covers CO, CO2, HC and O2 from spark-ignition engines; diesel smoke is measured as opacity and light absorption under ISO 11614 with an opacimeter. Some benches combine both in one housing, but they are two instruments with two approvals and should be specified as such.
Does an OIML certificate replace national approval?
Not automatically. An OIML certificate is evidence that the type was tested against the recommendation, and many authorities accept it as part of, or instead of, a local type evaluation. Whether it is accepted, and for which class, is a question for the metrology authority of the market where the instrument will be used.
Working to this requirement on a live project?
Send the vehicle classes, the daily volume and the standard the tender references. We will reply with the station list, the capacity each machine needs, and the report format the authority will accept.