8.2 NOx analyser quench checks
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8.2 NOx analyser quench checks

 The two gases of concern for CLD (and HCLD) analysers are CO2 and water vapour. Quench responses to these gases are proportional to their concentrations, and therefore require test techniques to determine the quench at the highest expected concentrations experienced during testing.

Parent topic: 8 Interference effects with CO, CO2, NOx and O2 analysers

8.2.1 CO2 quench check

  8.2.1.1 A CO2 span gas having a concentration of 80 to 100% of full scale of the maximum operating range shall be passed through the NDIR analyser and the CO2 value recorded as A. It shall then be diluted approximately 50% with NO span gas and passed through the NDIR and (H)CLD, with the CO2 and NO values recorded as B and C, respectively. The CO2 shall then be shut off and only the NO span gas shall be passed through the (H)CLD and the NO value recorded as D.

  8.2.1.2 The quench shall be calculated as follows:

(2)
where:
A = Undiluted CO2 concentration measured with NDIR %
B = Diluted CO2 concentration measured with NDIR %
C = Diluted NO concentration measured with (H)CLD ppm
D = Undiluted NO concentration measured with (H)CLD ppm

and shall not be greater than 3% of full scale.

  8.2.1.3 Alternative methods of diluting and quantifying of CO2 and NO span gas values, such as dynamic mixing/blending, may be used.

8.2.2 Water quench check

  8.2.2.1 This check applies to wet gas concentration measurements only. The calculation of water quench shall take into consideration the dilution of the NO span gas with water vapour and scaling of water vapour concentration of the mixture to that expected during testing.

  8.2.2.2 A NO span gas having a concentration of 80 to 100% of full scale of the normal operating range shall be passed through the (H)CLD and the NO value recorded as D. The NO span gas shall then be bubbled through water at room temperature and passed through the (H)CLD and the NO value recorded as C. The analyser's absolute operating pressure and the water temperature shall be determined and recorded as E and F, respectively. The mixture's saturation vapour pressure that corresponds to the bubbled water temperature (F) shall be determined and recorded as G. The water vapour concentration (in %) of the mixture shall be calculated as follows:

(3)

and recorded as H. The expected diluted NO span gas (in water vapour) concentration shall be calculated as follows:

(4)

and recorded as D e. For diesel exhaust, the maximum exhaust water vapour concentration (in %) expected during testing shall be estimated, under the assumption of a fuel atom hydrogen/carbon (H/C) ratio of 1.8/1, from the undiluted CO2 span gas concentration (A, as measured in 8.2.1 above) as follows:

(5)

and recorded as H m.

  8.2.2.3 The water quench shall be calculated as follows:

(6)
where:
D e = Expected diluted NO concentration ppm
C = Diluted NO concentration ppm
H m = Maximum water vapour concentration %
H = Actual water vapour concentration %

and shall not be greater than 3%

Note It is important that the NO span gas contains minimal NO2 concentration for this check, since absorption of NO2 in water has not been accounted for in the quench calculations.

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