The principle of a linearity test is to check that the gas analyser system is ‘linear’.
The linearity shall be calculated and tested using the procedure given in annex B of the standard EN14181. Linearity tests can be performed by Service Engineer, Site Engineer or Stack Testing House, although must be deemed competent to perform the checks.
The reference material concentrations shall be selected such that the measured values are at approximately 20%, 40%, 60%, 80% and 100% of the range of two times the emission limit.
To perform a linearity test for each gas and at five different percentages it would be very expensive to have separate gas bottles for each test. It would also be very time consuming to manoeuvre a large quantity of bottles.
A gas divider makes the job much easier. These five reference materials can be obtained from different gas cylinders or can be prepared by means of a calibrated dilution system from one single gas concentration. So in essence, the engineer will only need to carry as many bottles that represent each of the gases.
The gas divider from Environics offers:
> Broad range of dilution ratios (up to 1,000:1) allows the user to significantly reduce the number of cylinders needed to perform linearity tests
> Allows multi-point calibration of analysers
> Automatic calculation of dilution and span gas flows
> Internally-stored Mass Flow Controller calibration tables
> Modular design allows for additional gas circuits to be added
> User definable cylinder library allows for easy selection of frequently used gas cylinders
Each cylinder may contain an unlimited number of component gases with automatic K-factor