ATTACHMENT B TO ICAO MANUAL APPENDIX 3.
SPECIFICATION FOR CO AND CO2 ANALYSERS Note 1.- Paragraph 5.3 of Appendix 3 summarizes the ch aracteristics of the analysis sub-system to be employed for the individual measurements of CO and CO2 concentrations in the exhaust gas sample. The instruments are based on the principle of non-disper sive absorption of infra-red radiation in parallel reference and sample gas cells. The required ranges of sensitivity are obtained by use of stacked sample cells or changes in electronic circuitry or both. In terferences from gases with overlapping absorption bands may be minimised by gas absorption filters and/or optical filters. preferably the latter.
Note 2. — See Attachment D for information on calibration and test gases.
Precautions: The performance specifications indicated are generally for analyser full scale. Errors at part scale may be a significantly gr eater percentage of reading. The relevance and importance of such increases shall be considered when preparing to make measurements. If better performance is necessary, then appropriate precautions shall be taken.
Supplemental information The performance specifications for these analyzers, given in terms of full scale response, can have a significant and adverse impact on par t scale measurements. This needs to be considered when planning and executing the test and in ev aluating the accuracy of the measurements after the test. Concentrations of CO, when going from the idle mode to take off, can differ by orders of magnitude. In general, where concentrations of species to be measured are known to vary this way, it is always good practice to use a multi-range instrument and to choose ranges such as to keep the measurement in the upper 30% of scale on the range in use, where possible. A measurement made at 20% of full scale could result in an error 5 times the error specified as a percent of full scale. This is a general precaution. Some modern instruments with internal electronic ranging and calibration capability can be used over thei r entire range without penalty. Calibrations should be performed on each range used as required. Relative to the precautions mentioned above, ranges are chosen such that the instrument responds in the upper 30% of scale for the range in use. While this may not always be possible it should be a goal.
The principal performance specification shall be as follows:
CO Analyser
a) Total range: 0 to 2 500 ppm in appropriate ranges.
b) Resolution: better than 0.5 per cent of full sca le of range used or 1 ppm, whichever is greater.
c) Repeatability: better than ±1 per cent of full scale of range used, or ±2 ppm, whichever is greater.
d) Stability: better than ±2 per cent of full scale of range used or ±2 ppm, whichever is greater, in a period of 1 hour.
e) Zero drift: less than ±l per cent of full scale of r ange used or ±2 ppm. whichever is greater, in a period of 1 hour.
f) Noise: 0.5 Hz and greater, less than ±l per cent of full scale of range used or ±l ppm, whichever is greater.
g) Interferences: to be limited with respect to indicated CO concentration as follows:
1) less than 500 ppm/per cent ethylene concentration
Supplemental information It is unlikely that high concentrations of ethylene will be found in gas turbine engine exhaust. The highest concentration of hydrocarbons is found at idle, corresponding to the highest concentrations of CO. If all of the hydrocarbons were ethylene, C2H4, and the concentration were 100% of the maximum range, 5,000 ppmC, – corresponding to 2,500 ppm ethylene, the allowable in terference would be less than 125 ppm, or less than 5% of the highest CO range, 2,500 ppm. Since the interf erence limit is in absolute terms, the relative error will increase for measurements made at less than full scale. If ethy lene is present in significant concentrations then corrections to the data are required.
2) less than 2 ppm/per cent CO2 concentration
3) less than 2 ppm/per cent water vapour.* Supplemental information These two interferents, CO 2 and water vapor, are additive. Being the major products of combustion they increase and decrease together and are at their highe st levels at the highest power. Unfortunately concentrations of CO tend to be at their lowest concentrations at the highest power. This can cause significant problems in the accuracy of the measurement even if the interference limits are met. It is not unusual for tests to be conducted with the sample dried before measurement and the interference due to the remaining interferent, CO2, compensated for through use of gas or optical filters. It does bear mentioning that the contribution of high power CO concentrations to the total gross CO emissi on measured over the LTO cycle is relatively small.
If the interference limitation(s) for CO2 and/or water vapour cannot be met, appropriate correction factors shall be determined, reported and applied.
Note.- It is recommended as consistent with good practi ce that such correction procedures be adopted in all cases.
*Need not apply where measurements are on a "dry" basis.
CO2 Analyser
a) Total range: 0 to 10 per cent in appropriate ranges.
Supplemental information Although the total range specified for CO 2 is 0 to 10% concentrations most often will vary between 1% and 5%. This range is considerably narrower than that for CO. Never-the-less good practice dictates using ranges that keep the instrument response in the upper 30% of the meter scale as appropriate.
b) Resolution: better than 0.5 per cent of full scale of range used or 100 ppm, whichever is greater.
c) Repeatability: better than ±1 per cent of full scale of range used or ±100 ppm, whichever is greater.
d) Stability: better than ±2 per cent of full scale of range used or ±100 ppm, whichever is greater, in a period of 1 hour.
e) Zero drift: less than ±l per cent of full scale of range used or ±l00 ppm, wh ichever is greater, in a period of 1 hour.
f) Noise: 0.5 Hz and greater, less than ±1 per cent of full scale of range used or ±100 ppm, whichever is greater.
g) The effect of oxygen (O 2) on the CO2 analyser response shall be checked. For a change from 0 per Page 63
cent O2 to 21 per cent O2, the response of a given CO2 concentration shall not change by more than 2 per cent of reading. If this limit cannot be met an appropriate correction factor shall be applied.
Supplemental information Gas turbine engines use a considerable amount of inte rnal cooling air that mixes with the combustion products before exiting the engine. Oxygen rich exhaust samples warrant close attention.
Note.— It is recommended as consistent with good practice that such correction procedures be adopted in all cases.
CO and CO2 Analysers
a) Response time: shall not exceed 10 seconds from inlet of the sample to the analysis system, to the achievement of 90 per cent of the final reading.
b) Sample temperature: the normal mode of operation is for analysis of the sample in its (untreated) "wet" condition. This requires that the sample cell and all other components in contact with the sample in this sub-system be maintained at a temperature of not less than 50 ° C, with a stability of ±2 ° C. The option to measure CO and CO2 on a dry basis (with suitable water traps) is allowed, in which case unheated analysers are perm issible and the interference limits for H2O vapour removed, and subsequent correction for inlet water vapour and water of combustion is required.
Supplemental information Stability is defined in terms of a time interval which, because this is a temperature control set point, can be taken as the duration of th e test, or one hour to be consistent with the stability limits placed on the detection system.
The temperature quoted for the CO and CO 2 subsystems, 50 °C, is on the low end of the sample line specification, 65°C ±15°C. Good practice would suggest that the subsystem temperature be approximately the same as the sample gas temperature. If the samp les are dried and the analyzers unheated, it would be reasonable to lower the sample temperature to that of the analyzer. If water is removed prior to analysis, corrections must be applied to compensate for the loss of water of combustion and inlet water vapor.
Correction procedures are detailed in Attachment F of this Appendix.
c) Calibration curves:
i) Analyzers with a linear signal output characteri stic shall be checked on all working ranges using calibration gases at known concentrations of approximately 0, 30, 60 and 90 per cent of full scale.
The maximum response deviation of any of these po ints from a least squares straight line, fitted to the points and the zero reading, shall not exceed ±2 per cent of the full scale value. If it does then a calibration curve shall be prepared for operational use.
Supplemental information Straight line fits to sets of linear data can be arrived at graphically or analytically. If graphically, the results are subject to interpretation, i.e., best estimate by ‘eye’ by individual. If analytically, there is assurance that each case and all data are handled the same way each time. The most appropriate technique is to perform a linear regression, or ‘least-squares fit’ for a line. The calibration gas values are the independent “variables and are assumed to be correct (to have negligible error) for the purpose of this analysis. The instrument response values are the “dependent” variables and are assumed to have errors and that these errors are normally (Gaussian) distributed about the true line. The equation describing the straight line is:
yi = A + B*xi true value for response yi = A (a constant) + B (another constant) * (calibration gas value xi) Page 64
For instruments which have been adjusted such that zero input results in zero output, and where the variance in known to be proportional to the reading the slope B can be shown to be equal to the ratio of the averages and can be expressed as:
y x Often A and B are not such simple values for intercept and slope but must be calculated as if the variance were not known to be proportional to the instrument response. Again if we set the instrument to read zero for zero gas input then A = 0 which makes the calculation relatively simple. The generalized expression for B can be found in any elementary statistics or error analysis text and is:
Where xi is the calibration gas value and yi is the instrument response and N refers to the number of points used in the analysis.
With N = 4, corresponding to 0, 30, 60 and 90% of full scale, this equation can be rewritten as:
A simple table, for the four sets of values, can be used for organizing the information thereby simplifying the calculation:
Measurement number i xi cal. gas value i yi response i 2xi xiyi 1 2 3 4 N = 4 Σxi Σyi 2Σxi Σxi yi If the instrument is not set to zero-zero (zero re sponse for zero input) then A must be determined. The equation for A is (for N = 4) is:
The table shown earlier can be used to organize the elements of this equation as well.
Usually the next step would be to calculate the uncertainty in y i about this line. However, instead of controlling uncertainty about the line, ICAO chose to set an absolute limit of ±2% deviation of the full scale value for each point. This should make clear the advantage in using the upper region (top 30%) of the range for all measurements.
ii) Analyzers with a non-linear signal output characteristic, and those that do not meet the requirements of linearity given above, shall have calibration curves prepared for all working ranges using calibration gases at known concent rations of approximately 0, 30, 60 and 90 per cent of full scale. Additional mixes shall be used, if necessary, to define the curve shape properly.
Supplemental information For analyzers with a non-linear signal output characte ristic calibration curves shall be prepared, again using approximately 0, 30, 60 and 90% of full scale calibration gases. If a curve is substantially non-linear in shape, it is recommended that additional cali bration gases be used with values between the ones specified. These calibration curves can be determined analytically using a least squares fit, but in this case the fit would be to a polynomial or exponential. The equations for doing this can be found in any basic text on statistics or error analysis. It should be noted that for exponential fits it is often convenient to work with the logarithm of the expression, which reduces the problem to a least-sq uares-fit about a line as is described above. (This technique is used in analyzing smoke filters as requir ed in 3.0 of Appendix 2). Al though not stated explicitly, the presumption is that the same ±2% of full scale response deviation is true for non-linear as well as linear instruments. The use of a gas divider is an acceptable alternative to acquiring and maintaining additional gas resources.
Table A3-2 summarizes the specifications for CO and CO 2 analyzers. These are typical of those analyzers offered by major analyzer manufacturers.
Table A3-2 NDIR ANALYZER PERFORMANCE SPECIFICATIONS Parameter Value CO2 CO Total Range 0 to 10% in appropriate ranges 0 to 2 500 ppm in appropriate ranges Resolution better than 0.5% fs range used or 100 ppm whichever greater better than 0.5% fs range used or 1 ppm whichever greater Repeatability better than ±1% fs range used or ±100 ppm whichever greater better than ±1% fs range used or ±2 ppm whichever greater Stability better than ±2% fs range used or ±100 ppm whichever greater period of 1 hr better than ±2% fs range used or ±2 ppm whichever greater period of 1 hr Zero Drift < ±1% fs range used or ±100 ppm whichever greater period of 1 hr < ±1% fs range used or ±2 ppm whichever greater period of 1 hr Noise ≥0.5 Hz, <±1% fs range used or ±100 ppm, whichever greater ≥0.5 Hz, <±1% fs range used or ±1 ppm, whichever greater Interference ≤ 2% of reading for O2 between 0 and 21% <500 ppm/% ethylene <2 ppm/% CO2 <2 ppm/% water vapor Response Time ≤10 seconds from instrument inlet to 90% fs ≤10 seconds from instrument inlet to 90% fs Sample Temperature wet samples ≥50°C stability ± 2°C wet samples ≥50°C stability ± 2°C Page 67
ATTACHMENT C TO ICAO MANUAL APPENDIX 3.
SPECIFICATION FOR NOX ANALYSER Note. — See Attachment D for information on calibration and test gases.
1. As indicated in 5.4 of Appendix 3, the measurement of the oxides of nitrogen concentration. shall be by the chemiluminescent technique in which radiation emitted by the reaction of NO and O3 is measured. This method is not sensitive to NO 2 and therefore the sample shall be passed through a converter in which NO2 is converted to NO before the measurement of total NOX is made. Both the original NO and the total NOX concentrations shall be recorded. Thus by difference, a measure of the NO2 concentration shall be obtained.
2. The instrument to be used shall be complete wi th all necessary flow control components, such as regulators, valves, flowmeters, etc. Materials in co ntact with the sample gas shall be restricted to those, which are resistant to attack by oxides of nitrogen, such as stainle ss steel, glass, etc. The temperature of the sample shall everywhere be ma intained at values, consistent with the local pressures, which avoid condensation of water.
Precautions: The performance specifications indicated are generally for analyzer full scale. Errors at part scale may be a significantly greater percent age of reading. The relevance and importance of such increases shall be considered when preparing to make measurements. If better performance is necessary, then appropriate precautions shall be taken.
Supplemental information The performance specifications for these analyzers given in terms of full scale response can have a significant and adverse impact on part scale measur ements. This needs to be considered when planning and executing the test and in evaluating the accuracy of the measurements a fter the test. Concentrations of NO X, when going from the idle mode to take-off, can differ by orders of magnitude. In general, where concentrations of species to be measured are known to va ry this way, it is always good practice to use a multi-range instrument, and to choose ranges such as to keep the measurement in the upper 30% of scale on the range in use. A measurement made at 20% of full scale could result in an error 5 times the error specified as a percent of full scale. This is a general precaution. Some modern instruments with internal electronic ranging and calibration capability can be used over their entire range without penalty. Calibrations should be performed on each range used as required.
3. The principal performance specification, determi ned for the instrument operated in an ambient temperature stable to within 2 °C, shall be as follows:
a) Total range: 0 to 2 500 ppm in appropriate ranges.
b) Resolution: better than 0.5 per cent of full scale of range used or 1 ppm, whichever is greater.
c) Repeatability: better than ±l per cent of full scale of range used, or ±1 ppm, whichever is greater.
d) Stability: better than ±2 per cent of full scale of range used or ±l ppm, wh ichever is greater, in a period of 1 hour.
e) Zero drift: less than ±1 per cent of full scale of range used or ±1 ppm, whichever is greater, in a period of 1 hour.
f) Noise: 0.5 Hz and greater, less than ±1.0 per cent of full scale of range used or ±l ppm, whichever is greater, in a period of 2 hours.
g) Interference: suppression for samples containing CO2 and water vapour shall be limited as follows:
— less than 0.05 per cent reading/per cent CO2 concentration;
— less than 0.1 per cent reading/per cent water vapour concentration.
If the interference limitation(s) for CO2 and/or water vapour cannot be met, appropriate correction factors shall be determined, reported, and applied.
Note.- It is recommended as consistent with good practi ce that such correction procedures be adopted in all cases.
h) Response time: shall not exceed 10 seconds from inlet of the sample to the analysis system to the achievement of 90 per cent of the final reading.
i) Linearity: better than ±2 per cent of full scale of range used or ±2 ppm, whichever is greater.
j) Converter: this shall be designed and operated in such a matter as to reduce NO2 present in the sample to NO. The converter shall not affect the NO originally in the sample.
Supplemental information
Correction: …”in such a matter” … “should read” … “in such a manner” … .
The converter efficiency shall not be less than 90 per cent.
This efficiency value shall be used to correct the measured sample NO2 value (i.e. [NOX]c - [NO]) to that which would have been obtained if the efficiency had not been 100 per cent.
Supplemental information The last sentence should read as follows:
“This efficiency value shall be used to correct the measured sample NO 2 value (i.e. [NO X]c –[NO]) to that which would have been obtained if the efficiency had been 100 per cent.”
A procedure for determining the NO 2 to NO conversion efficiency, in the absence of any specific manufacturer’s instructions, may be found in 40 CFR Part 87, Control of Air Pollution from Aircraft and Aircraft Engines, Federal Register, Vol. 43, No. 58 — Friday, March 24, 1978. It was taken, in turn, from SAE ARP1256, "Procedure for the Continuous Sampling and Measurement of Gaseous Emissions from Aircraft Turbine Engines".
Figure A3-5: NOx Converter Efficiency Detector Schematic Figure A3-5 schematically depicts such a device. This device is intended for use with the NO X analyzer specified in Attachment C. It depends on the reaction: NO + O3 = NO2 + O2 Starting with a known concentration of NO in N 2, measurements are made through – and bypassing – the chemiluminsecence analyzer converter, the inlet to wh ich is shown as “C3” in the diagram. With the NO X converter ozonator alternately on, reducing the NO concentration by approximately 80%, and off, allowing 100% of the NO to reach the analyzer the analyzer’s conv erter efficiency can be determined. This efficiency should be used to correct test data as required.
The specific instructions for using this device, from 40 CFR part 87, are as follows:
(i) Attach the NO/N 2 supply (150-250 ppm.) at “C2”, the O 2 supply at “C1”, and the analyzer inlet connection to the efficiency detector at C3. If lower concentrations of NO are used, air may be used in place of O2 to facilitate better control of the NO2 generated during step (iv).
(ii) With the efficiency detector Variac ( Note: SAE uses the term autotransformer ) off, place the NO X converter in bypass mode and close valve “V3”. Open valve “MV2” until sufficient flow and stable readings are obtained at the analyzer. Zero and span the analyzer output to indicate the value of the NO concentration being used. Record this concentration.
(iii) Open valve V3 (on/off flow control solenoid valve for O 2) and adjust valve “MV1” (O 2 supply metering valve) to blend enough O 2 to lower the NO concentration (ii) to about 10 percent. Record this concentration.
(iv) Turn on the ozonator and increase its supply vo ltage until the NO concentration of (iii) is reduced to about 20 percent of (ii). NO is now being formed from the NO + O 2 reaction. There must always be at least 10 percent unreacted NO at this point. Record this concentration.
(v) When a stable reading has been obtained from (iv), place the NO X converter in the convert mode. The analyzer will now indicate the total NOX concentration. Record this concentration.
(vi) Turn off the ozonator and allow the analyzer reading to stabilize. The mixture NO + O 3 is still passing through the converter. This reading is the total NO X concentration of the dilute NO span gas used in step (iii). Record this concentration.
(vii) Close valve V3. The NO concentration should be e qual to or greater than the reading of (ii) indicating whether the NO contains any NO2.
Calculate the efficiency of the NO X converter by substituting the concent rations obtained during the test into the following equation:
(v) − (iv)%Eff.= ×100%(vi) − (iv) To improve the effectiveness of thermal converters, particu larly those with efficiencies of less than 90%, it is sometimes helpful to raise the temperature of the converter.
Table A3-3 NOX ANALYZER PERFORMANCE SPECIFICATION Parameter Value Total Range 0 to 2 500 ppm in appropriate ranges Resolution better than 0.5 percent full scale range used or 1 ppm whichever is greater Repeatability better than ±1 percent full scale range used or 1 ppm whichever is greater Stability better than ±2 percent full scale range used or ±1 ppm whichever is greater in a period of 2 hours Zero Drift < ±1 percent full scale range used or ±1 ppm, whichever is greater in a period of 2 hours Noise ≥0.5 Hz, <±1 percent fs range used or ±1 ppm, whichever greater period of 2 hours Interference < 0.05 percent reading/ percent CO2 < 0.1 percent reading/ percent water vapor Response Time ≤ 10 seconds from inlet of sample to achieve 90 percent of final reading Linearity better than ±2 percent full scale for each range used or ± 2ppm, whichever greater Converter Efficiency ≥ 90 percent Table A3-3 summarizes the specifications for NOx analyzers. These are typical of those analyzers offered by major analyzer manufacturers.
HC CO2 CO NOx Analyzer propane in l0 ±1 per cent O2 balance zero nitrogen propane in 21 ±1 per cent O2 balance zero nitrogen propylene in zero air HC n-hexane in zero air propane in zero air CO2 CO2 CO ATTACHMENT D TO ICAO MANUAL APPENDIX 3.
CALIBRATION AND TEST GASES Table of calibration gases Analyzer Gas Accuracy* propane in zero air CO2 in zero air CO in zero air NOx in zero nitrogen ±2 per cent or ±0.05 ppm** ±2 per cent or ±100 ppm** ±2 per cent or ±2 ppm** ±2 per cent or ±1 ppm** * Taken over the 95 per cent confidence interval.
** Whichever is greater The above gases are required to carry out the routine calibration of analyzers during normal operational use.
Gas Accuracy* HC ±1 per cent HC ±l per cent HC ±l per cent toluene in zero air ±l per cent HC ±l per cent HC ±1 per cent CO2 in zero air ±1 per cent CO2 in zero nitrogen CO in zero air ±1 per cent ±1 per cent NOx NO in zero nitrogen ±1 per cent * Taken over the 95 per cent confidence interval The above gases are required to carry out the tests of Attachments A, B, and C.
Carbon monoxide and carbon dioxide calibration gases may be blended singly or as dual component mixtures.
Three component mixtures of carbon monoxide, carbon dioxide, and propane in zero air may be used, provided the stability of the mixture is assured.