Guidance

Technical guidance and equations: pollution inventory reporting

Updated 21 August 2024

Applies to England

• Publication for Northern Ireland
• Publication for Scotland
• Publication for Wales

Technical guidance, equations, and examples to help operators of installations and other industrial sectors with their pollution inventory submission.

This guidance is particularly relevant to operators of installations in the following sectors:

• paper and pulp
• food and drink
• combustion
• chemicals
• metals
• refineries
• fuel production

Operators in other industrial sectors may also find it useful.

We recommend you read this alongside the general and sector-specific reporting guidance.

Quantification of emissions to air

Sampling data

To use sampling data to estimate emissions, you need to know the flow rate and pollutant concentration. To work out annual emissions accurately for pollution inventory reporting, your sampling should be performed under conditions representative of your annual operations. Ideally you will do the sampling in accordance with methods or standards that we have approved.

Take care if you are relying on the results of one spot sample to report annual emissions. If you do this, you need to be certain that the process conditions are representative of your actual annual average operations. Where a process has several steady state conditions, you might need to take samples under each operating condition and average the result according to the length of time the process operates at each condition. Similarly, where process conditions at the time of the spot sampling are uncertain, you may need to take several samples and to average the results to work out the final annual emission estimate. Use good engineering judgement to select the most appropriate sampling time and data to use. You need to be able to justify the sampling programme selected.

Sampling as part of a permit condition may require that you do the monitoring at maximum load (that is, higher than annual operating conditions) and this should be considered in the annual emission estimates. When in doubt, the proposed sampling protocols should be confirmed with us.

To estimate annual emissions from sampling data, multiply the measured emission concentrations by the volumetric flow rates of the emission source at the time of the test. Assuming that representative sampling has been undertaken, you can aggregate these emission rates for the annual operating period.

Take care to ensure that the emission concentration and flow rate are compatible. For example, normalised emission concentrations should be multiplied by normalised volumetric flow rates. Actual, measured emission concentrations should be multiplied by actual, measured volumetric flow rates.

Normalised emission rates are quoted in terms of a standard oxygen concentration, and are usually dry gas, at a temperature of 273.15 Kelvin (0° Celsius) and a pressure of 101.325 kPa (1 standard atmosphere). It is always good practice to confirm the basis of measured data.

Sampled emission concentrations are also often reported in parts per million (ppm). To estimate annual emissions, you need to convert these to mg per m³. The emission temperature needs to be the same as for the volumetric flow rate. Formulae for converting ppm to mg per m³ are contained in the ‘Conversion factors’ section of this guidance.

The following section shows how to calculate emissions based on stack sampling data, expressed in mg per m³. We have also given an example for calculating PM10 emissions. You can apply the same general methodology for most of the substances listed on the PI.

Equation 1: E = C × Q × 0.0036 × [Op hours]

Where:

• E = emission rate of pollutant, kg per year
• C = pollutant concentration, mg per m³
• Q = volumetric flow rate of the emission, m³ per second
• 0.0036 = the conversion factor, from mg per second to kg per hour
• Op hours = the operating hours of the activity, per year

Where the pollutant concentration is consistent over the averaging period (that is, one year), Equation 1 can be written as:

Equation 2: E = C × M × V × 10⁻⁶

Where:

• E = emission rate of pollutant, kg per year
• C = average pollutant concentration, mg per m³
• M = mass of feedstock in one year, kg feedstock
• V = standard volume of flue gas per tonne of feedstock, m³ per kg feedstock

Example A: PM10 emissions using Equation 1

• Operating hours = 24 hours per day, 280 days per year
• PM10 emission concentration = 20 mg per m³, normalised to 273K, dry, 11% oxygen
• Emission volumetric flow rate = 10 m³ per second, normalised to 273K, dry, 11% oxygen

E = C × Q × 0.0036 × 24 × 280
= 20 × 10 × 0.0036 × 24 × 280
= 4,838 kg per year
= 4.84 te per year

Continuous emission monitoring systems data

The use of continuous emission monitoring systems (CEMS) follows the same general principles as using spot sampling data.

It’s possible to use CEMS to report real-time emissions over a variety of time periods automatically. However, sometimes you might need to work out annual emissions manually from this data, especially for periods where data may be out of tolerance. If you use CEMS software to calculate annual emissions, it is good practice to check the data manually to ensure that the automatic calculations are accurate.

You must not subtract confidence intervals from the average values generated from the raw emissions data before you calculate annual mass emissions. The only exception is if you have a prior written agreement with us to do something else. Before you use CEMS to determine emissions, we would like you to agree the methodology for collecting and averaging the data with us.

The basic equation for determining emissions is Equation 1, adjusted for the appropriate time period of the measurement. It must be applied for each time period for which emission measurements are available in the year, following the guidance given above. Normally, the measurement time periods are the same, so you can multiply the average emission rate by the operating time per year to obtain the annual emission. If the measurement time periods vary, then Equation 3 should be used. Example B uses Equation 3.

Equation 3: E = _|n∑ (E_i × t)

Where:

• E = emission rate of pollutant, kg per year
• E_i = emission rate of pollutant over time period t
• t = time period for emission measurement

Example B: SO₂ emissions calculated using Equation 3 based on the average CEMS data for 6 days of a week

It is assumed that the process operates for 24 hours per day, 48 weeks per year and that the CEMS data are representative of annual operations.

• E1 = 13.2 kg per hour
• E2 = 12.6 kg per hour
• E3 = 11.2 kg per hour
• E4 = 12.2 kg per hour
• E5 = 14.0 kg per hour
• E6 = 13.4 kg per hour

E = [(13.2 × 24) + (12.6 × 24) + (11.2 × 24) + (12.2 × 24) + (14 × 24) + (13.4 x24)] kg per week
x 48 weeks per year
88,243 kg per year
88.2 t per year

Emission factors

Emission factors can be used to estimate emissions to the environment. In this guidance, they relate to the quantity of substances emitted from a source by some common activities associated with those emissions. General emission factors have been developed from a variety of sources, but this guidance draws upon UK information in particular.

Provided that unit operations remain consistent, representative monitoring data can be used to generate site-specific emission factors. The emission factor will be the ratio of the measured or calculated pollutant emission to the process activity (for example, per tonne of pulp produced). Site-specific emission factors should be verified periodically to ensure their continued validity, especially where raw material quality varies throughout the year.

Where an emission factor, or other release estimation technique, is not available for a particular substance you may review published information or use the emission factors referred to in this guidance. You need to be careful when selecting appropriate emission factors, to ensure that the conditions under which the emission factor has been determined are representative of your operations.

Emission factors are usually expressed as the mass of a substance emitted multiplied by the unit mass, volume or duration of the activity emitting the substance.

Emission factors are used to estimate an activity’s emissions by using Equation 4:

Equation 4: E = [A × Op hours] × EF

Where:

• E = emission rate of pollutant, kg per year
• A = activity rate of process, tonnes per hour or m³ per hour
• Op hours = operating hours per year of activity, hours per year
• EF = controlled emission factor of pollutant per activity, kg per t or kg per m³

When using Equation 4, remember that EF is the emission factor for the pollutant released to atmosphere. That is, after the emission has been abated.

Depending on the information you have available, Equation 4 can be rewritten as:

Equation 5: E = M × EF

Where:

• E = emission rate of pollutant, kg per year
• M = activity rate in terms of mass of product produced in the year, t per year
• EF = controlled emission factor of pollutant per activity, kg per t of product

Example C is an estimate of annual emissions using Equation 4.

Example C: Estimating (SO₂) emissions from a recovery system per tonne of air-dried unbleached pulp produced

This system typically includes recovery furnace, evaporator, acid fortification tower, and scrubbers. It is assumed that the pulping mill operates for 8,000 hours per year and that 5 tonnes of pulp is produced per hour during the reporting year. The SO₂ emission factor (assuming scrubber control) is 4.5 kg of SO₂ per tonne of air-dried unbleached pulp produced.

• A = 5 tonnes per hour
• Op hours = 8,000 hours per year
• EF = 4.5 kg per tonne

E = 5 × 8,000 × 4.5
= 180,000 kg per year
= 180 t per year

Site-specific emission factors can sometimes be used to estimate emissions at other sites. To do this, the processes need to be comparable in size and operation. We advise that you ask your local Environment Agency officer to review and approve any emission factor before you use it for pollution inventory submissions.

In the case of new or modified processes, you can get initial emission factors from manufacturers’ data. The manufacturer or you should do sampling to confirm the assumed values during commissioning.

Carbon dioxide factors

The European Commission established guidelines for the monitoring and reporting of greenhouse gas emissions in line with European Directive 2003/87/EC (Emissions Trading Scheme (EU ETS)). The EU ETS has since been incorporated into UK law as the UK ETS.

These guidelines set out the approved methodology for estimating CO₂ emissions based on emissions from regular operations and abnormal events. Abnormal events include start-up, shutdown and emergency situations over the reporting period.

Read more about the energy efficiency standards for industrial plants to get environmental permits (formerly H2 guidance).

Under the UK ETS guidelines, estimation of CO₂ emissions from combustion can be obtained using Equation 6:

Equation 6: E_CO2 = A_e × EF_CO2 × OF_CO2

Where:

• E_CO2 = emission of CO₂ t per year
• A_e = activity rate, either in terms of annual energy consumption, TJ per year, or mass/volume consumption, t per year or m³ per year 
• EF_CO2 = emission factor for CO₂, tCO₂ per TJ, or tCO₂ per t, or tCO₂ per m³
• OF_CO2 = oxidation factor for CO₂

The guidelines refer to tiers for activity data, emission factors and oxidation factors. The higher the tier, the greater the accuracy.

The oxidation factor considers the fact that when energy is consumed not all of the carbon in the fuel oxidises to CO₂. The oxidation factor expresses the proportion of carbon in the fuel that is oxidised to CO₂.

Under the guidelines, operators can choose the most appropriate oxidation factor tier:

• Tier 1 – an oxidation factor of 1.0 is used
• Tier 2 – the operator applies oxidation factors for the respective fuel as reported in the UK Greenhouse Gas Inventory (country-specific factors)
• Tier 3 – for fuels activity specific factors are derived by the operator using approved methodologies

In some cases, the emission factor may already consider the proportion of carbon in the fuel that is oxidised, and you should verify if this is the case. If so, the oxidation factor in Equation 6 is equal to 1.

It is possible to use an emission factor based on the amount of fuel used in energy terms, or mass/volume. This is Equation 5.

The UK ETS provides specific guidance regarding the determination of activity-specific emission factors for defined fuels. In the absence of activity-specific factors, general emission factors for the combustion of various fossil fuels are provided in the UK ETS guidance.

By contrast with combustion sources of CO₂ emissions, the UK ETS allows process emissions (emissions of CO₂ from non-combustion activities) to be derived from activity data based on raw material inputs, throughputs, and outputs. The basic calculation equation remains multiplication of activity data by appropriate emission factors, but it uses conversion factors in place of oxidation factors.

Biomass is considered to be CO₂-neutral in terms of the UK ETS, but the mass of CO₂ emitted from its combustion or use is a pollution inventory reporting requirement. Emissions of CO₂ attributable to biomass should be reported in the qualification box of the pollution inventory reporting form.

The UK ETS allows CO₂ emissions to be determined by 2 methodologies: a calculation methodology or a measurement methodology. Where a measurement methodology is used, you need to verify the measured emissions by calculation. In both cases however, it is likely that measurements of fuel burned or material flow will be available. These can be used to calculate other emission quantities when combined with appropriate emission factors.

Fuel analysis, process stream data, and normalisation

The basic equation used in fuel analysis emission calculations is:

Equation 7: E = Q_f × [Op hours] × [PC_f ÷ 100] × (MM_p ÷ EM_f) 

Where:

• E = emission of pollutant, kg per year
• Q_f = fuel use, kg per hour
• PC_f = pollutant concentration in the fuel, %
• Op hrs = operating hours per year, hours per year
• MM_p = molar mass of pollutant as emitted after combustion
• EM_f = relative atomic mass of polluting element as present in fuel

Equation 7 is the method usually used for calculating SO₂ emissions where it is normally assumed that all the sulfur in the fuel is converted to SO₂.

Where the pollutant concentration in the fuel is consistent over the averaging period (that is, one year), Equation 7 can be written as:

Equation 8: E = M × (PC_f ÷ 100) × (MM_p ÷ EM_f) 

Where:

• E = emission rate of pollutant, kg per year
• M = mass of fuel burnt in one year, kg per year
• PC_f = pollutant concentration in the fuel, %
• MM_p = molar mass of pollutant as emitted after combustion
• EM_f = relative atomic mass of polluting element as present in fuel

Example D: SO₂ emissions calculated from oil combustion, based on fuel analysis results and fuel flow information

It is assumed that the facility operates burning waste oil for 150 hours per year and that abatement of SO₂ does not occur.

• Q_f = 2,000 kg per hour
• PC_f = 1.17%
• MM_p = 64
• EM_f = 32
• Op hours = 150 hours per year

E = Q_f × PC_f × (MM_p ÷ EWf) × [Op hours]
= [(2000) × (1.17 ÷ 100) × (64 ÷ 32) × 150] kg per year
= 7.0 × 103 kg per year

You can also use Equation 7 for volatile elements such as fluorine and chlorine, and for trace metallic pollutants. Some of these latter species are retained in the plant, either in the ash or in abatement equipment. You need to apply appropriate retention factors for this.

When you use Equation 7 or Equation 8, you need to be aware that the amounts of pollutants present in the fuel or process stream can vary significantly.

Take care in all calculations to ensure that the emission concentration and flow rate are compatible. For example, normalised emission concentrations should be multiplied by normalised volumetric flow rates, or actual measured emission concentrations multiplied by actual measured volumetric flow rates.

Help to estimate your notifiable releases of biogas

Changes in gas temperature affects gas volumes. Therefore, you need to normalise the gas volume when calculating releases.

If you don’t know the Nm³ per hour, then use the following rough guide for gas temperature to convert gas at one temperature to an approximate normalised gas flow:

• at 20° Celsius, a volume of 1,000m³ per hour of bio/landfill gas = 932Nm³ per hour
• at 30° Celsius, 1,000m³ per hour of bio/landfill gas = 901Nm³ per hour
• at 40° Celsius, 1,000m³ per hour of bio/landfill gas = 872Nm³ per hour
• at 50° Celsius, 1,000m³ per hour of bio/landfill gas = 845Nm³ per hour

The density of methane is 0.716kg per Nm³. To convert to kg of methane multiply the normalised volume of methane component of the gas by 0.716.

Example E: Power outage at a gas producing site

All the gas control measures and backup generator to the flare fails at a gas producing site such as a landfill or anaerobic digestor. The flare did not operate so gas built up and could not be used or flared and so vented for 6 hours. The site produces 1,100m³ per hour. You calculate that 6,280Nm³ of biogas or landfill gas over 6 hours was generated and lost through venting because it had nowhere else to go. At your site your gas quality was known to be 55% biomethane at a normal operating temperature of 40° Celsius. Report this loss to air in the ‘Notifiable releases’ column.

• 6 hours at 1,100m³ per hour × 6 hours = 7,200m³
• At 40° Celsius = 7,200 × 0.872 = 6,278 Nm³
• 55% by volume of 6,278Nm³ = 3,420 Nm³ of methane
• 3,420 × 0.716 = 2,448.65kg of methane over the 6-hour period was lost

Quantification of emissions to water

There are fewer techniques available to work out emissions to water than for emissions to air. The most appropriate method is to use direct measurement. You may use other release estimation techniques, particularly mass balances, or site-specific emission factors, where these are appropriate.

Mass balances can often be used when emissions to water are very complex and difficult to quantify with other approaches. If you use a mass balance calculation, you will probably still need to do direct measurement of emissions from some of the water pathways in order to verify the calculations. You can determine site-specific emission factors from the ratio of the measured or calculated pollutant emission to the water discharge flow rate.

This guidance advises on the use of direct measurement techniques as these are likely to be applicable to the majority of operators.

Using the direct measurement technique requires information on both the flow rate and pollutant concentration. Measurement of flows and pollutant concentrations need to be carried out at the same time during representative operating conditions. Take particular care when relying on the results of one spot sample in order to report annual emissions unless you can be certain that the process conditions are representative. Where a process has a number of operating conditions, you might need to take samples at each condition and average the results according to the length of time the process operates at each condition. Similarly, where process conditions at the time of the spot sampling are uncertain, then you might need to take several samples and average the results in order to provide the final annual emission estimate.

The frequency of sampling needed depends on the variability of the data. Initially, it may be necessary to take several samples and average the results to yield an annual result. If the initial results indicate that a concentration and flow are reasonably constant, then you might be able to reduce the frequency of sampling up to a practical minimum of once per year. You need to be able to justify the sampling regime selected. This can be supported by a history of previous measurements. The sampling requirements will usually be set out in your EPR permit. It is important to note that no additional sampling and monitoring is required solely for the purposes of reporting to the PI.

The background load of a reportable substance in water may be considered. Some sites collect water (for example, for cooling process) for the installation from a neighbouring river, lake, or sea, then release used water into the same source. The ‘release’ caused by the background load of that substance can be subtracted from the total release of the installation. Your measurements of pollutants in collected inlet water and in released outlet water must be carried out in a way that ensures that they are representative of the conditions occurring over the reporting period.

You might also need to take account of the fact that evaporation of water from the process will lead to an increase in the pollutant concentration. This can be done by using Equation 9:

Equation 9: PC = OC – [IC × VF]

Where:

• PC = the pollutant emission concentration from the process, mg per l
• OC = the measured pollutant concentration in the discharge, mg per l
• IC = the measured pollutant concentration in the feedwater, mg per l
• VF = the ratio of volume of water entering the process to volume of water discharged

If the additional load results from the use of extracted groundwater or drinking-water, it should not be subtracted. These releases would increase the load of the pollutants in the river, lake, or sea.

To estimate the mass emission to water, multiply the appropriate pollutant concentration by the flow rate for that particular discharge point. Then, aggregate together these representative discharges based on the time for which the water is discharged at that rate. Finally, add together the estimated mass emissions from all discharge points to either controlled waters or in wastewater transfers for each individual pollutant for each reporting medium.

For emission points fitted with continuous monitors, you can make automatic calculations of mass emissions from a particular discharge point. For cooling water, it may also be necessary to adjust the measured data to take account of input pollutant concentrations as described above. It is good practice to check automatic calculations manually to ensure that they are accurate.

Quantification of waste

The basic equation used in waste feedstock analysis emission calculations is the same as Equation 7.

Equation 10: E = Q_f × [Op hours] × [PC_f ÷ 100] × (MM_p ÷ EM_f)

Where:

• E = emission of pollutant, kg per year
• Q_f = waste feedstock, kg per hour
• PC_f = pollutant concentration in the waste feedstock, %
• Op hrs = operating hours per year, hours per year
• MM_p = molar mass of pollutant as emitted after combustion
• EM_f = relative atomic mass of polluting element as present in waste feedstock

Equation 10 is the method usually used for calculating SO₂ emissions where it is normally assumed that all of the sulfur in the waste feedstock is converted to SO₂.

Where the pollutant concentration in the waste feedstock is consistent over the averaging period (that is, one year), Equation 10 can be written as:

Equation 11: E = M × [PC_f ÷ 100] × (MM_p ÷ EM_f)

Where:

• E = emission rate of pollutant, kg per year
• M = mass of waste feedstock burnt in one year, kg per year
• PC_f = pollutant concentration in the waste feedstock, %
• MM_p = molar mass of pollutant as emitted after combustion
• EM_f = relative atomic mass of pollutant as present in waste feedstock.

You can use Equation 10 for volatile elements such as fluorine and chlorine. It can also be used for trace metallic pollutants. Note that some of these species are retained in the plant, either in the ash or in abatement equipment. You should apply appropriate retention factors.

When using Equation 10 or Equation 11, you should be aware that the amounts of pollutants present in the waste feedstock can vary significantly.

Substances ‘reported as’

Certain substances on the pollution inventory return are required to be ‘reported as’ the main, or most environmentally significant, constituent. For instance, chloride salts are reported in terms of their chloride content under the heading ‘Chlorides – total as Cl’. Another example is ‘Nitrogen oxides, NO and NO₂ as NO₂’.

When you need to do a conversion, you should multiply the emission concentration or emission rate by the relative atomic/molar mass of the ‘reported as’ substance and divide by the molar mass of the emitted substance. This is illustrated as examples F and G.

Example F: An effluent is calculated to contain 50mg per l of sodium chloride and the total volume released is 1,000m³

• Relative atomic mass of sodium = 23
• Relative atomic mass of chlorine = 35
• Molar mass of sodium chloride = 58

Concentration of sodium chloride as chloride = 50 × 35 ÷ 58
= 30.2 mg per l
= 30.2 g per m³

The mass as chloride released is (30.2 × 10 − 3) × 1,000
= 30.2 kg (which is below the reporting threshold)

Example G: Assume a discharge concentration of NO is 50 mg per m³ − using the molar masses of NO and NO₂, the equivalent discharge concentration of NO₂ can be determined

• Molar mass of NO = 30
• Molar mass of NO₂ = 46

Concentration of NO as NO₂ = 50 × 46 ÷ 30
= 76.7 mg per m³

You can then work out the mass of NO₂ equivalent released by using Equation 1.

Conversion factors

Converting air emissions: ppm to mg per m³ and vice versa

Converting between ppm and mg per m³ is dependent on both the molar mass of the substance and the temperature and pressure at which the conversion is made. We tend to assume that the pollutant behaves as an ideal gas and as such, 1 mole of the substance occupies 22.4 litres at standard temperature (273K) and pressure (101.3 kPa). This is consistent with normalised concentrations, so we don’t normally need to take account of the temperature or pressure difference in the conversion. When converting ppm to mg per m³ under actual discharge conditions it is important to take account of these factors.

To convert from ppm to mg per m³, you should use Equation 12a. To convert from mg per m³ to ppm, use Equation 12b.

Equation 12a: mg per m³ = ppm × (MM ÷ 22.4) × (273 ÷ T) × (P ÷ 101.3)

Equation 12b: ppm = mg per m³ × (22.4 ÷ MM) × (T ÷ 273) × (101.3 ÷ P)

Where:

• MM = the molar mass of the substance (in grams)
• T = the temperature at which the conversion is to be made (degrees Kelvin)
• P = the pressure at which the conversion is to be made (kPa)

Converting emissions to water

To convert between ppm and mg per l for water, you can normally assume that water has a density of 1,000 kg per m³.

On this basis, 1 ppm = 1 mg per l = 1 g per m³ = 1 mg per kg.

Equations for normalisation of emission concentrations

In many cases, pollutant emission concentrations to air are reported as normalised concentrations. The actual measured emission concentration will have been adjusted to a normalised temperature (273K), oxygen, pressure and/or water vapour concentration. In calculating mass emissions to air, it is important that either the actual release concentration is multiplied by the actual volumetric flowrate, or the normalised concentration is multiplied by the normalised volumetric flowrate.

The following equations can be used to correct measured concentrations and flow rates for temperature, oxygen, pressure, and water vapour content.

Correcting emission concentrations

To correct for moisture concentration to dry (0% oxygen), use Equation 13.

Equation 13: C_d = C_m × (100 ÷ (100 − %H₂O))

Where:

• C_d = the dry concentration
• C_m = the measured concentration
• %H₂O = the measured water vapour percentage

To correct the % oxygen to dry basis (if required, it may already be measured dry), use Equation 14.

Equation 14: O_2(dry) = O_2m × (100÷ (100 −%H₂O))

Where:

• O_2(dry) = the dry oxygen percentage
• O_2m = the measured oxygen percentage

To correct to normalised oxygen concentration use Equation 15.

Equation 15: C_corr = C_d × (20.9 − O_2norm) ÷ (20.9 − O_2(dry))

Where:

• C_corr = the corrected concentration for oxygen concentration
• O_2norm = the stated normalised oxygen percentage

To correct for temperature use Equation 16.

Equation 16: C_normT = C_corr × ((273 + T_m) ÷ 273)

Where:

• C_normT = the normalised concentration for temperature
• T_m = the measured temperature in degrees centigrade

To correct for pressure use Equation 17.

Equation 17: C_norm = C_normT × (101.3 ÷ P_m)

Where:

• C_norm = the normalised concentration
• P_m = the measured pressure in kPa

Correcting volumetric flowrates

To correct for moisture concentration to dry (0% oxygen) use Equation 18.

Equation 18: Q_d = Q_m × (100 − %H₂O) ÷ 100)

Where:

• Q_d = the dry volumetric flowrate
• Q_m = the measured volumetric flowrate
• %H₂O = the measured water vapour percentage

To correct the % oxygen to dry basis use Equation 19. You don’t need to do this if % oxygen is already measured dry.

Equation 19: O_2(dry) = O_2m × (100 ÷ (100 − %H₂O))

Where:

• O_2(dry) = the dry oxygen percentage
• O_2m = the measured oxygen percentage

To correct to normalised oxygen concentration use Equation 20.

Equation 20: Q_corr = Q_d × (20.9 − O_2(dry)) ÷ (20.9 − O2_norm)

Where:

• Q_corr = the corrected volumetric flowrate for oxygen concentration
• O_2norm = the stated normalised oxygen percentage

To correct for temperature use Equation 21.

Equation 21: Q_normT = Q_corr × (273 ÷ (273 + T_m))

Where:

• Q_normT = the normalised volumetric flowrate for temperature
• T_m = the measured temperature in degrees centigrade

To correct for pressure, use Equation 22.

Equation 22: C_norm = C_normT × (Pm ÷ 101.3)

Where:

• C_norm = the normalised volumetric flowrate
• P_m = the measured pressure in kPa