Three numbers, three different criteria
For particles, the class determines whether counts in specified size bands or mass concentration are assessed. For water, pressure dew point is distinguished from liquid water in other classes. Oil assessment covers the sum of liquid, aerosols and vapours. The class sequences differ by contaminant; claiming that each has only classes 0 to 6 is incorrect.
The specification applies to a particular location
Record the required classes, sampling point, pressure, operating mode and verification method. Express concentrations at the standard reference conditions of 20 °C, 100 kPa absolute and dry air. Class 0 requires individually specified stricter limits. Microbiological or other special requirements cannot automatically be inferred from the three-number designation.
What to check in practice
- Derive the requirement from the process and the consumer's specification.
- Measure every component needed to assess the specified class.
- Also consider peak flow, summer temperatures and the condition of downstream pipework.
Example scenario
The designation [1:4:1] requires class 1 particles, water with a pressure dew point no higher than +3 °C, and total oil no higher than 0.01 mg/m³. For particle class 1, the limits are 20,000, 400 and 10 particles/m³ in the bands >0.1 to 0.5; >0.5 to 1; and >1 to 5 µm. Detection of particles above 5 µm prevents classification in classes 1–5.
| Position | What is assessed | What is needed for verification |
|---|---|---|
| First | Solid particles | Counts and size bands, or mass concentration |
| Second | Water | Pressure dew point or liquid water concentration, depending on the class |
| Third | Total oil | The sum of liquid, aerosols and vapours |
What to watch out for
- Treating the oil class as the class of the entire air supply.
- Declaring compliance from oil vapour measurements alone.
- Automatically assigning the same mandatory class to every company in an industry.
Does the calculator output confirm air quality?
No. The guide helps define requirements, and the assessment helps interpret measured data. Evidence of quality requires appropriate measurements, defined conditions and an assessment of uncertainty. The calculator is not a certificate.