Stack PT Schemes  

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The aim of the Gas Measurement and Gas Cylinder Proficiency Testing Scheme is to assess the performance of test houses in sampling and measuring stack gases, using test atmospheres traceable to National Standards.
The scheme involves the measurement of combustion gases (NO, NO2*, SO2, CO, O2, VOCs & CO2 **) directly from cylinders and from NPL’s stack simulator facility, at concentrations and conditions typical of plants falling under the Industrial Emissions Directive (IED).
Cylinder measurements can be used for the annual requirement in EN14792 to demonstrate NOx converter efficiency as well as the quarterly requirement to assess NO2 losses for portable measuring systems. A traceable CO2 cylinder is available to allow participants to assess their measurement capability of this increasingly important pollutant.

Measurements from the artificial stack give additional information on sampling, i.e. to demonstrate dryer performance for SOsub>2 sampling is in compliance with CEN/TS 17021.

Maximum Concentrations for Gas Measurement Proficiency Testing:







H2 O

Max Range







Note: Water vapour is not assessed as part of the gas measurement scheme, but results for NO, SO2, CO and O2 need to be reported dry.

Range of concentrations for the gas cylinder testing:

Description: Gases & Gas Analysis


Minimum concentration

Maximum concentration


5 ppm

1000 ppm

C3H8 + 10%O2 in nitrogen

1 ppm

50 ppm


5 ppm

500 ppm


10 ppm

500 ppm


2 %





NO and NO2 in nitrogen
For testing NOx converters

NOx 12 ppm
NO 8 ppm

NOx 500 ppm
NO 400 ppm

The NPL stack simulator is able to reproduce a wide range of simulated stack gases under controlled conditions. The simulator is a recirculating system which recreates a cross section of a 1.5 m duct. Four standard five-inch ports are available for fixed sampling probes or cross stack instruments. Assigned values are calculated by measurement of the gases within the simulator using the standard reference methods or proven equivalent. Standard deviation for proficiency testing is derived by reference to the relevant CEN standards to which participants are accredited.

All components of the Gas Measurement and Gas Cylinder PT Scheme are UKAS accredited (0016) to ISO 17043 (except NO2 measurements in the gas measurement component).

Participants will be able to make measurements using 2 analysers for each pollutant for both the Gas Measurement and Cylinder components, thus allowing them to test different measurement technologies and calibration techniques.

The Gas Measurement & Gas Cylinder Proficiency Testing Scheme will run in June 2023 for two weeks, subject to enough participants registering.

* NOTE: NO2 measurement from the stack simulator is outside the scope of the UKAS accreditation for this scheme.

** NOTE: CO2 is only included in the cylinder component of the PT scheme.