Guidance

Monitoring stack emissions: maximum uncertainty values for periodic monitoring

Formerly part of M2, maximum uncertainty values used to assess whether periodic measurement results comply with emission limit values in environmental permits.

This guidance provides the maximum standard 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 standard expanded uncertainties we use to assess whether periodic measurement results comply with the ELVs specified in permits.

Species % uncertainty MU source
Particulate matter ±20%. We may apply a maximum expanded measurement uncertainty of 1 mg/m3 for ELVs below 5 mg/m3 BS EN 13284-1
TOC ±15% Half the value applied to CEMS
HCl ±30% BS EN 1911
CO ±6% BS EN 15058
NOx ±10% BS EN 14792
SO2 ±20% BS EN 14791
SO2 ±15% CEN/TS 17021
HF ±30% Given the same uncertainty as HCl due to similar reactive nature and measurement technique
NH3 ±20% BS EN ISO 21877
Cd and Tl ±30% Value based on uncertainties applied by monitoring organisations
Hg ±30% Value based on uncertainties applied by monitoring organisations
Other metals ±20% Value based on uncertainties applied by monitoring organisations
Dioxins and furans / PCBs ±30% BS EN 1948
Speciated VOCs ±25% CEN/TS 13649
Generic wet chemistry methods (not listed in this table) ±25% The value is based on uncertainties applied by monitoring organisations
Generic sorbent tube methods (not listed in this table) ±25% Given same value as speciated VOCs because the analysis and sampling is very similar
Generic analyser methods (not listed in this table) ±25% The value is based on uncertainties applied by monitoring organisations
O2 ±6% measured value (dry) or 0.3% volume concentration BS EN 14789
H2O ±20% BS 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 11 mg/m3, at standard temperature and pressure
  • a measurement uncertainty of 15%
  • an ELV of 10 mg/m3

These values are used as follows:

  1. (11 × 15) ÷ 100 = 1.65 mg/m3 measurement uncertainty.
  2. 11 – 1.65 = 9.35 mg/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, 173KB, 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