ISO 11614: What a Diesel Opacimeter Measures, and Why k Is Not a Percentage
ISO 11614 is the standard behind the smoke meter on a diesel lane. It defines two different numbers from the same light beam — opacity as a percentage and the light absorption coefficient k — and the difference between them decides whether two stations can compare results at all.
What the standard covers
ISO 11614:1999 is titled, in full, "Reciprocating internal combustion compression-ignition engines — Apparatus for measurement of the opacity and for determination of the light absorption coefficient of exhaust gas". It sets out the general requirements and the installation of the apparatus, and it is not confined to road vehicles, which is why the same instruments appear in engine test cells and in inspection lanes.
It replaces ISO 3173:1974 and ISO/TR 4011:1976. Published in 1999 and confirmed again on systematic review, it remains the current edition. The technical committee responsible is ISO/TC 70/SC 8, exhaust gas emission measurement.
Note what the title does not say: it does not define a pass limit. The standard constrains the instrument; the number a vehicle has to reach is set by the regulation of the market where it is tested.
Opacity and k are two different readings
Opacity is the share of light that does not reach the receiver, expressed as a percentage of the light emitted. It is the number a driver would recognise from a visibly smoking exhaust.
The light absorption coefficient, written k, comes from the same measurement but is derived through the Beer-Lambert relation and is expressed per unit length. Because it is normalised to the optical path length, k stays comparable between instruments built with different geometries, while a bare percentage does not.
This is the practical reason k exists. Two smoke meters with different effective path lengths can show the same percentage for the same exhaust plume while describing different amounts of soot. If a regulation or a customer asks for k, quoting a percentage instead is not a unit conversion, it is a different quantity.
Four layouts, four installation rules
The standard recognises several ways of getting the light beam through the gas, and each carries its own installation requirements. Choosing the wrong one is an installation error that no amount of calibration fixes.
Sampling or partial-flow instruments draw a sample through a measuring chamber. Full-flow in-line instruments measure across the whole exhaust stream. End-of-line or plume-type instruments measure across the plume as it leaves the tailpipe. Instruments intended for free-acceleration tests form a further group with their own requirements.
A lane that runs a free-acceleration test and a lane that runs a steady-state test at a test cell do not need the same head, even if both are described as opacity meters.
| Layout | How it samples | Typical use |
|---|---|---|
| Partial flow / sampling | A sample is drawn into a measuring chamber | Lanes with a fixed test cycle and a sample probe |
| Full flow / in-line | Measures across the whole exhaust stream | Engine test cells and steady-state work |
| End of line / plume | Measures across the plume at the tailpipe exit | Where fitting a probe is impractical |
| Free-acceleration instrument | Configured for the transient free-acceleration test | Inspection and maintenance testing of in-service diesel vehicles |
What the standard demands of the instrument itself
Beyond the optical principle, the requirements that decide whether a meter is usable in a regulated lane are about stability and about honesty in transients.
The standard addresses indicator resolution, the stability of zero and of full scale over a defined period, and the definition and control of the effective optical path length — the figure the k calculation depends on. For transient measurement it defines physical delay time, electrical and overall response time, temperature response and peak hold, because a free-acceleration test is over in seconds and a slow meter reports a number that never happened.
Verification follows three levels: manufacturer data, type verification against the standard, and performance checks with in-service conformity procedures. A meter with no verification route is a meter whose readings cannot be defended.
Where buyers get it wrong
Most of the argument about smoke meters is not about accuracy. It is about comparability, and comparability is what the standard was written to create.
- Comparing percentages between two instruments with different path lengths, then treating the difference as a fault.
- Buying a steady-state head for a lane that has to run free acceleration.
- Ignoring the conditioning of the optical chamber: a dirty lens reads high, and reads higher over a shift.
- Skipping the zero and span check at the start of the day, which is exactly the drift the stability requirement is about.
- Assuming the instrument sets the limit. It does not; the national rule does.
What to specify
Ask for the layout, the reported quantities, the path length and the verification route in writing. Those four answers separate a lane instrument from a workshop gadget.
- Which layout is being supplied, and where it is installed on the vehicle.
- Whether the instrument reports opacity, k, or both, and in which units k is presented.
- The effective optical path length and how it is maintained and checked.
- Response time and whether peak hold is implemented for free acceleration.
- The verification and calibration route available in your market, including who can perform the in-service check.
Common questions
Is a smoke meter reading in percent the same as a k value?
No. Opacity is a percentage of light blocked; k is a light absorption coefficient derived from the same beam and normalised to the optical path length. Percentages from instruments with different path lengths are not directly comparable, which is why regulations that need comparability specify k.
Does ISO 11614 set the smoke limit for a vehicle test?
It does not. The standard covers the apparatus, its installation and its verification. The limit a vehicle must meet is set by the regulation or the manufacturer specification applicable in that market, and it is usually expressed against a defined test method such as free acceleration.
Can an opacimeter be used for petrol vehicles?
Smoke is a diesel problem. Petrol vehicles are assessed on CO, CO2, HC and O2 with a gas analyser under OIML R 99, which is a separate instrument with a separate approval. A station handling both fuel types needs both.
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.