5.5.1.2 The calibration curve is calculated by the Method of least squares. If the resulting polynomal degree is greater than 3, the number of calibration points (zero included) shall be at least equal to this polynomial degree plus 2.
5.5.1.3 The calibration curve shall not differ by more than ±2% from the nominal value of each calibration point and by more than ±1% of full scale at zero.
5.5.1.4 From the calibration curve and the calibration points, it is possible to verify that the calibrtion has been carried out correctly. The different characteristic parameters of the analyser shall be indicated, particularly:
.1 the measuring range,
.2 the sensitivity, and
.3 the date of carrying out the calibration.
5.5.2 Calibration below 15% of full scale
5.5.2.1 The analyser calibration curve shall be established by at least 10 calibration poi8nts (excluding zero) spaced so that 50% of the calibration points are below 10% of full scale.
5.5.2.2 the calibration curve shall be calculated by the method of least squares.
5.5.2.3 Alternative methods
If it can be shown that alternative technology (e.g., computer, electronically controlled range switch, etc.) provides equivalent accuracy, then these alternatives may be used.
6 Verification of the calibration
Each normally used operating range shall be checked prior to each analysis in accordance with the following procedure:
.1 the calibration shall be checked by using zero gas and a span gas whose nominal value shall be more than 80% of full scale of the measuring range; and
.2 if, for the two points considered, the value found does not differ by more than ±4% of full scale from the declared reference value, the adjustment parameters may be modified. If this is not the case, a new calibration curve shall be established in accordance with 5.5% adove.
7 Efficiency test of the NOx converter
The efficiency of the converter used for the conversion of NO2 into NO shal be tested as given in 7.1 to 7.8 below.
7.1 Test set-up
Using the test set-up as shown in figure 1 below (see also 3.4 of appendix 5 of this Code) and the procedure below, the efficiency of converters shall be tested by means of an ozonator.
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7.2 Calibration
The CLD and the HCLD shall be calibrated in the most common operating range following the manufacturer's specifications using zero and span gas (the NO content of which should amount to about 80% of the operating range and the NO2 concentration of the gas mixture to less than 5% of the NO concentration). The NOx analyser must be in the NO mode so that the span gas does not pass through the converter. the indicated concentration shall be recorded.
7.3 Calculation
The efficiency of the NOx converter shall be calculated as follows:
where:
(a) = NOx concentration according to 7.6 below
(b) = NOx concentration according to 7.7 below
(c) = NO concentration according to 7.4 below
(d) = NO concentration according to 7.5 below
7.4 Adding of oxygen
7.4.1 Via a T-fitting, oxygen or zero air shall be added continuously to the gas flow until the concentration indicated is about 20% (minimum 10%) of the calibration concentration given in 7.2 above. The indicated concentration (d) shall be recorded (the analyser must be in the NO mode).
7.4.2 The indicated concentration (c) shall be recorded. The ozonator must be kept deactivated throughout the process.
7.5 Activation of the ozonator
The ozonator shall now be activated to generate enough ozone to bring the NO concentration down to about 20% (minimum 10%) of the calibration concentration given in 7.2 above. The indicated concentration (d) shall be recorded (the analyser must be in the NO mode).
7.6 NOx mode
The NO analyser shall then be switched to the NOx mode so that the gas mixture (consisting of NO, NO2, O2 and N2) now passes through the converter. The indicated concentration (a) shall be recorded (the analyser must be in the NOx mode).
7.7 Deactivation of the ozonator
The ozonator shall now be deactivated. The mixture of gases described in 7.6 above passes through the converter into detector. The indicated concentration (b) shall be recorded (the analyser must be in the NOx mode).
7.8 NO mode
Switched to NO mode with the ozonator deactivated, the flow of oxygen or synthetic air shall also be shut off. The NOx reading of the analyser shall not deviate by more than ±5% from the value measured according to 7.2 above (the analyser must be in the NOx mode).
