Guidance

Monitoring stack emissions: maximum uncertainty values for periodic monitoring

These are the maximum uncertainty values to assess whether periodic measurement results comply with emission limit values in environmental permits (formerly part of M2).

Applies to England and Wales

This guidance provides the maximum expanded uncertainties we use to assess whether periodic measurement results comply with the emission limit values (ELVs) specified in permits. It was formerly part of the guidance document M2.

We normally use the uncertainty values that are determined by the organisation that carried out the periodic monitoring on behalf of the operator. However, if these are above the maximum values provided in this guidance, we will assess compliance using these maximum values. When uncertainties are above the maximum values, we will investigate why.

ELVs and maximum measurement uncertainties

The values in the table Maximum measurement uncertainties for periodic monitoring apply to the measurement uncertainties for results that are at or above the ELV.

If a reported result is above the ELV, we will assess whether it complies by taking account of its measurement uncertainty. We do this by subtracting the measurement uncertainty from the measured value (see How to assess compliance with an ELV).

If after completing the assessment the result is still above the ELV, we are likely to consider this as a breach of the ELV. However, if after completing the assessment the result is below the ELV, we are likely to consider this as approaching but not breaching the ELV.

If the reported result is below the ELV, we do not need to carry out a compliance assessment that takes into account measurement uncertainty (see How to assess compliance with an ELV).

This guidance describes our general approach to assessing compliance and other factors we may consider.

Maximum measurement uncertainties for periodic monitoring

The following table shows the maximum expanded uncertainties we use to assess whether periodic measurement results comply with the ELVs specified in permits.

Species % uncertainty MU source
Particulate matter 20% or 1mg/m3 for ELVs below 5mg/m3 EN 13284-1    
Total organic compounds 15% Half the value applied to continuous emission monitoring systems    
Total gaseous chlorides expressed as hydrogen chloride 30% EN 1911    
Hydrogen chloride 20% or 1 mg/m3 for ELVs below 5 mg/m3 EN 16429    
Carbon monoxide 6% EN 15058    
Oxides of nitrogen 10% EN 14792    
Sulfur dioxide 20% EN 14791    
Sulfur dioxide 15% CEN TS 17021    
Hydrogen fluoride 30% or 0.6 mg/m3 for ELVs below 2 mg/m3 CEN TS 17340    
Ammonia 20% EN ISO 21877    
Cadmium and thallium 30% Value based on uncertainties applied by monitoring organisations    
Mercury 30% Value based on uncertainties applied by monitoring organisations    
Other metals 20% Value based on uncertainties applied by monitoring organisations    
Dioxins and furans 30% EN 1948    
Speciated volatile organic compounds 25% CEN TS 13649    
Formaldehyde 40% or 2 mg/m3 for ELVs below 5 mg/m3 CEN TS 17638    
Carbon dioxide 6% expected measured concentration or 0.3% volume concentration CEN TS 17405    
Generic wet chemistry methods (not listed in this table) 25% The value is based on uncertainties applied by monitoring organisations 40% or a maximum expanded measurement uncertainty of 2 mg/m3 for ELVs below 5 mg/m3 CEN TS 17638
Generic sorbent tube methods (not listed in this table) 25% Given same value as speciated volatile organic compounds because the analysis and sampling is similar    
Generic analyser methods (not listed in this table) 25% The value is based on uncertainties applied by monitoring organisations    
Oxygen 6% measured value (dry) or 0.3% volume concentration EN 14789    
Water vapour 20% EN 14790    

How to assess compliance with an ELV

To assess compliance with an ELV you must follow these steps.

1. Determine the measurement uncertainty:

measurement uncertainty = (measured value × % uncertainty) ÷ 100

The measured value is already corrected to the permit reporting conditions.

2. Adjust the measured result by subtracting the measurement uncertainty:

adjusted value = measured value − measurement uncertainty

3. Compare the adjusted data versus the appropriate ELV to assess compliance.

Example calculation

This example is based on:

  • a measured value for particulates of 11mg/m3, at standard temperature and pressure
  • a measurement uncertainty of 15%
  • an ELV of 10mg/m3

These values are used as follows:

  1. (11 × 15) ÷ 100 = 1.65mg/m3 measurement uncertainty.
  2. 11 − 1.65 = 9.35mg/m3 adjusted value.
  3. The adjusted value is less than the ELV, so we would classify the measured value as an approach to a limit rather than a breach.

The measurement uncertainty is the expanded uncertainty at a 95% confidence interval.

Taking account of oxygen measurement uncertainty

The overall uncertainty of a measurement for combustion processes that require oxygen correction must include all of the following. The uncertainty of the:

  • pollutant being measured
  • oxygen measurement
  • correction to a fixed oxygen reference value

See the example calculation.

Example calculation for oxygen measurement uncertainty (PDF, 71.7KB, 1 page)

The reported measurement uncertainty should be below the overall uncertainty. That is, the uncertainty in the table Maximum measurement uncertainties for periodic monitoring plus the additional uncertainty from the oxygen correction.

Published 18 December 2019
Last updated 24 September 2021 + show all updates
  1. Amended table in section 'Maximum measurement uncertainties for periodic monitoring': changed particulate matter % uncertainty; an amended entry for hydrogen chloride and added determinand 'Formaldehyde'.

  2. Added a new line for sulfur dioxide in the ‘Maximum measurement uncertainties for periodic monitoring’ table.

  3. We have updated the table of maximum measurement uncertainties for periodic monitoring to take account of standards published in 2020 (CEN TS 17340 and CEN TS 17405).

  4. First published.