Cars, motorbikes and trucks emit pollutant gases in exhaust emissions from internal combustion engines. To ensure that these gases are emitted at acceptably low levels, they must be measured using instrumentation that requires calibration with specialty gas mixtures.
For testing of roadside heavy duty vehicle emissions monitoring equipment the following two gas mixtures are common. The first mixture is a low level mix and the second mixture a high level mix at around 90% of full-scale deflection on most emissions measurement instruments used in this application.
The two mixtures above will also be known as:
The above mixtures are generally required to be traceable to national standards and are therefore generally supplied in Australia with a NATA-accredited ISO/IEC 17025 certificate.
Coregas is also able to manufacture a range of gas mixtures for calibration of the instrumentation used for vehicle type testing in automotive emissions testing laboratories. Similar mixtures will be used to calibrate the instrumentation used for exhaust gas monitoring on engine test benches. The concentrations of the pollutant gases for the calibration point will generally be selected to be close to the measured value. These concentrations will be higher if tests are being done on the engine emissions pre-catalytic converter. The concentrations for calibration gas mixtures and for measurement post the exhaust gas after treatment system will generally be low. Some example gas mixtures for this application are listed below. For additional options please contact the Coregas specialty gases customer service team.
It is essential to use a stainless steel regulator for all calibration gas mixtures with corrosive components such as NO. It will also often be the case that a stainless steel regulator is specified for the other ppm level gas mixtures containing non corrosive chemicals such as propane or methane. This is best practice, but alternatively a chrome-plated brass regulator may also be used with these non-corrosive components. For the introduction of calibration gas mixtures a single-stage regulator will be adequate. The additional flow stability of a two-stage regulator is not required for this 'one shot' type of calibration application where the end user is often present to adjust the regulator pressure, if required. For further advice on gas control equipment selection, please e-mail our specialty gases technical experts.
The total hydrocarbons (THC) such as butane and propane in the exhaust gas are generally measured using a flame ionisation detector (FID). For the flame in the FID to operate it will typically be fed with the following two instrumentation gases:
For these two instrumentation gases, where continuous flow of the gas is required, use of a two-stage chrome-plated brass regulator is recommended. A stainless steel regulator is not required for these non-corrosive gases.
In many petrochemical processing applications a GC with an FID detector is used for VOC or hydrocarbon measurement. Interestingly, in those applications it is more common to use pure hydrogen not the 40% hydrogen in helium mixture which is the standard for automotive emissions testing applications.
The air for the FID is sometimes supplied from a zero air generator. This can be acceptable in some cases. However, for the highest purity of air supply and the best stability and sensitivity of measurement use of a specialty gases instrument-grade air cylinder is always recommended.