Appendix 4

This appendix describes the procedure for measurement of ammonia (NH₃). For non-linear analysers, the use of linearising circuits shall be permitted.

Three measurement principles are specified for NH₃ measurement and either principle may be used provided it meets the criteria specified in points 2.1, 2.2 or 2.3, respectively. Gas dryers shall not be permitted for NH₃ measurement.

The FTIR employs the broad waveband infrared spectroscopy principle. It allows simultaneous measurement of exhaust gas components whose standardized spectra are available in the instrument. The absorption spectrum (intensity/wavelength) is calculated from the measured interferogram (intensity/time) by means of the Fourier transform method.

The FTIR shall be installed in accordance with the instrument manufacturer's instructions. The NH₃ wavelength shall be selected for evaluation. The sample path (sampling line, pre-filter(s) and valves) shall be made of stainless steel or PTFE and shall be heated to set points between 383 K (110 °C) and 464 K (191 °C) in order to minimize NH₃ losses and sampling artefacts. In addition, the sampling line shall be as short as practically possible.

The spectral resolution of the NH₃ wavelength shall be within 0,5 cm^{– 1} in order to minimize cross interference from other gases present in the exhaust gas.

The NDUV is based on a purely physical principle, no auxiliary gases or equipment is necessary. The main element of the photometer is an electrode-less discharge lamp. It produces a sharply structured radiation in the ultraviolet range, enabling the measurement of several components such as NH₃.

The photometric system has a dual beam in time design set up to produce a measuring and a reference beam by filter correlation technique.

In order to achieve a high stability of the measuring signal the dual beam in time design is combined with a dual beam in space design. The detector signals processing fosters an almost negligible amount of zero point drift rate.

In the calibration mode of the analyser a sealed-off quartz cell is tilted into the beam path to obtain an exact calibration value, since any reflection and absorption losses of the cell windows are compensated. Since the gas filling of the cell is very stable, this calibration method leads to a very high long term stability of the photometer.

The analyser shall be installed within an analyser cabinet using extractive sampling in accordance with the instrument manufacturer's instructions. The analyzer location shall be capable of supporting the weight specified by the manufacturer.

The sample path (sampling line, pre-filter(s) and valves) shall be made of stainless steel or PTFE and shall be heated to set points between 383 K (110 °C) and 464 K (191 °C).

In addition, the sampling line shall be as short as possible. Influence from exhaust gas temperature and pressure, installation environment and vibrations on the measurement shall be minimized.

The gas analyzer shall be protected from cold, heat, temperature variations, and strong air currents, accumulation of dust, corrosive atmosphere and vibrations. Adequate air circulation shall be provided to avoid heat build-up. The complete surface shall be used to dissipate the heat losses.

An appropriate spectral range shall be chosen in order to minimize cross interferences of accompanying gases. Typical components causing cross sensitivities on the NH₃ measurement are SO₂, NO₂ and NO.

Additionally, further methods can be applied to reduce the cross sensitivities.

1. (a)

   Usage of interference filters;

2. (b)

   Cross sensitivity compensation by measuring cross sensitivity components and using the measurement signal for compensation.

An infrared laser such as a tunable diode laser (TDL) or a quantum cascade laser (QCL) can emit coherent light in the near-infrared region or in mid-infrared region respectively where nitrogen compounds including NH₃ have strong absorption. This laser optics can give a pulsed-mode high resolution narrow band near-infrared or mid-infrared spectrum. Therefore, laser infrared analyzers can reduce interference caused by the spectral overlap of co-existing components in engine exhaust gas.

The analyser shall be installed either directly in the exhaust pipe (in-situ) or within an analyser cabinet using extractive sampling in accordance with the instrument manufacturer's instructions. If installed in an analyser cabinet, the sample path (sampling line, pre-filter(s) and valves) shall be made of stainless steel or PTFE and shall be heated to set points between 383 K (110 °C) and 464 K (191 °C) in order to minimize NH₃ losses and sampling artefacts. In addition, the sampling line shall be as short as practically possible.

Influence from exhaust gas temperature and pressure, installation environment and vibrations on the measurement shall be minimized, or compensation techniques be used.

If applicable, sheath air used in conjunction with in-situ measurement for protection of the instrument, shall not affect the concentration of any exhaust gas component measured downstream of the device, or sampling of other exhaust gas components shall be made upstream of the device.

If NH₃ is measured using laser infrared analyser, the amount of interference shall be verified after initial analyser installation and after major maintenance.

Interference gasses can positively interfere with certain laser infrared analyzer by causing a response similar to NH₃. If the analyzer uses compensation algorithms that utilize measurements of other gases to meet this interference verification, these other measurements shall be simultaneously conducted to test the compensation algorithms during the analyzer interference verification.

Good engineering judgment shall be used to determine interference gases for laser infrared analyzer. Note that interference species, with the exception of H₂O, are dependent on the NH₃ infrared absorption band chosen by the instrument manufacturer. For each analyzer the NH₃ infrared absorption band shall be determined. For each NH₃ infrared absorption band, good engineering judgment shall be used to determine interference gases to use in the verification.

Prior to the emissions test, the analyser range shall be selected. Emission analysers with automatic or manual range switching shall be permitted. During the test cycle, the range of the analysers shall not be switched.

Zero and span response shall be determined, if the provisions set out in point 3.4.2 do not apply for the instrument. For the span response, a NH₃ gas that meets the specifications set out in point 4.2.7, shall be used. The use of reference cells that contain NH₃ span gas is permitted.

At the start of the test sequence, the NH₃ data collection shall be started, simultaneously. The NH₃ concentration shall be measured continuously and stored with at least 1 Hz on a computer system.

At the completion of the test, sampling shall continue until system response times have elapsed. Determination of analyser's drift in accordance with point 3.4.1 shall only be required if the information required in point 3.4.2 is not available.

As soon as practical but no later than 30 minutes after the test cycle is complete or during the soak period, the zero and span responses of the analyser shall be determined. F1The difference between the pre-test and post-test results shall be less than 2 % of full scale.

Determination of analyser drift is not required in the following situations:

1. (a)

   If the zero and span drift specified by the instrument manufacturer in points 4.2.3 and 4.2.4 meets the requirements of point 3.4.1;

2. (b)

   The time interval for zero and span drift specified by the instrument manufacturer in points 4.2.3 and 4.2.4 exceeds the duration of the test.

The analyser shall comply with the linearity requirements specified in Table 6.5 of this Annex. The linearity verification in accordance with point 8.1.4 of this Annex shall be performed at least at the minimum frequency set out in Table 6.4 of this Annex. With the prior approval of the approval authority, less than 10 reference points are permitted, if an equivalent accuracy can be demonstrated.

For the linearity verification, a NH₃ gas that meets the specifications set out in point 4.2.7 shall be used. The use of reference cells that contain NH₃ span gas shall be permitted.

Instruments, whose signals are used for compensation algorithms, shall meet the linearity requirements specified in Table 6.5 of this Annex. Linearity verification shall be done as required by internal audit procedures, by the instrument manufacturer or in accordance with ISO 9000 requirements.

The analyser shall have a measuring range and response time appropriate for the accuracy required to measure the concentration of NH₃ under transient and steady state conditions.

The analyser shall have a minimum detection limit of < 2 ppm under all conditions of testing.

The accuracy, defined as the deviation of the analyser reading from the reference value, shall not exceed ± 3 % of the reading or ± 2 ppm, whichever is larger.

The drift of the zero response and the related time interval shall be specified by the instrument manufacturer.

The drift of the span response and the related time interval shall be specified by the instrument manufacturer.

The system response time shall be ≤ 20 s.

The rise time of the analyser shall be ≤ 5 s.

A gas mixture with the following chemical composition shall be available.

NH₃ and purified nitrogen.

The true concentration of the calibration gas shall be within ± 3 % of the nominal value. The concentration of NH₃ shall be given on a volume basis (volume per cent or volume ppm).

The expiration date of the calibration gases shall be recorded.

The interference verification shall be performed as follows:

1. (a)

   The NH₃ analyzer shall be started, operated, zeroed, and spanned as it would be before an emission test;

2. (b)

   A humidified interference test gas shall be created by bubbling a multi component span gas through distilled H₂O in a sealed vessel. If the sample is not passed through a sample dryer, the vessel temperature shall be controlled to generate an H₂O level at least as high as the maximum expected during emission testing. Interference span gas concentrations shall be used at least as high as the maximum expected during testing;

3. (c)

   The humidified interference test gas shall be introduced into the sample system.

4. (d)

   The water mole fraction, x _H2O, of the humidified interference test gas shall be measured, as close as possible to the inlet of the analyzer. For example, dew point, T _dew, and absolute pressure, p _total, shall be measured to calculate x _H2O;

5. (e)

   Good engineering judgment shall be used to prevent condensation in the transfer lines, fittings, or valves from the point where x _H2O is measured to the analyzer;

6. (f)

   Time shall be allowed for the analyser response to stabilize;

7. (g)

   While the analyser measures the sample's concentration, its output shall be recorded for 30 s. The arithmetic mean of this data shall be calculated;

8. (h)

   The analyzer meets the interference verification if the result of paragraph (g) of this point meets the tolerance in this section.

9. (i)

   Interference procedures for individual interference gases may also run separately. If the interference gas levels used are higher than the maximum levels expected during testing, each observed interference value may be scaled down by multiplying the observed interference by the ratio of the maximum expected concentration value to the actual value used during this procedure. Separate interference concentrations of H₂O (down to 0,025 mol/mol H₂O content) that are lower than the maximum levels expected during testing may be run, but the observed H₂O interference shall be scaled up by multiplying the observed interference by the ratio of the maximum expected H₂O concentration value to the actual value used during this procedure. The sum of the scaled interference values must meet the tolerance for combined interference as specified in paragraph (j) of this point.

10. (j)

    Analyser shall have combined interference within ± 2 % of the applicable mean value of ammonia (NH ₃ ) specified in point 3.4 of Annex IV.

Other systems or analysers may be approved by the approval authority, if it is found that they yield equivalent results in accordance with point 5.1.1 of this Annex. In this case, ‘Results’ in that point shall refer to mean NH₃ concentration calculated for the applicable cycle.