e) Zero drift: less than ±1 per cent of fu ll 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 practice 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 …”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 NO 2 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 (ie. [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 C ontinuous Sampling and Meas urement of Gaseous Emissions from Aircraft Turbine Engines".
Figure A5-7: NOx Converter Efficiency Detector Schematic Figure A5-7 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 N2, measurements are made through – and bypassing – the chemiluminsecence analyzer converter, the inlet to which is shown as “C3” in the diagram. With the NOX converter ozonator alternately on, reducing the NO concentration by approximately 80%, and off, allowing 100% of the NO to reach the analyze r the analyzer’s conver ter efficiency can be determined. This efficiency should be used to correct test data as required.
Note: it has been suggested that NO be mixed with Argon to avoid possible formation of NO in the ozonator.
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 lo wer 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 NOX 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 s upply voltage 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 b een obtained from (iv), place the NO X converter in the convert mode. The analyzer will now indicate the total NO X concentration. Record this concentration.
(vi) Turn off the ozonator and allow the analy zer 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 equal 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 concentrations obtained during the test into the following equation:
v − iv%Eff.= ×100%vi − iv To improve the effectiveness of thermal converters, particularly those with efficiencies of less than 90%, it is sometimes helpful to raise the temperature of the converter.
Table A5-3 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 A5-3 summarizes the specifications shown are typical of those analyzers offered by major analyzer manufacturers.
HC CO2 CO NOx Analyser 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 5.
CALIBRATION AND TEST GASES Table of calibration gases Analyser 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 analysers 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.
Zero gas as specified for the CO, CO2 and HC analysers shall be zero air (which includes "artificial" air with 20 to 22 per cent O2 blended with N2). For the NO X analyser zero nitrogen shall be used as the zero gas. Impurities in both kinds of zero gas shall be restricted to be less than the following concentrations:
1 ppm C 1 ppm CO 100 ppm CO2 1 ppm NOx
The applicant shall ensure that commercial gases supplied to him do in fact meet this specification, or are so specified by the vendor.
Supplemental information Calibration and test gases are readily available, with traceability to the National Institute of Standards and Technology (NIST), from commercial suppliers. Gases with an accuracy of ± 1% are considered “primary” standards and intended for use where the highest accuracy and reliability are needed. Very tight controls over cylinder material, age, and histor y are considered good practice, if not essential.
Periodic checks of the accuracy of assay for each cylinder is advisable. Gases such as CO, CO2, and propane with a certified accuracy of ± 2% can be obtained as multi- component mixtures. This will reduce the number of gas cylinders needed for test ing. The importance of this becomes obvious when considering the calibration requirements for each instrument and each range. Managing just the minimum number of calibration and test gase s can be challenging. For example, an average certification test, when backup gases are counted, can require 100 two component cylinders, or more.
The stability of the components in each mixture should be checked periodically. If the assay no longer agrees with the certified analysis, the cylinder should be returned to the supplier or discarded as appropriate.
Because of the accuracy required and the sophistication of the techniques necessary to produce and analyze gases to these standards, most testing laboratories rely on the commercial suppliers analysis and certification for primary accuracy and use in -house checks via instrument response for consistency of assay. It is good practice to recheck all calibration and working standard gases immediately prior to a certification test.
All calibration gas monitoring data should be kept and made available to the certificating agency if required.
Although not a calibration gas or, strictly speaking, a test gas, the FID analyzer fuel and oxidant gases should also meet a hydrocarbon specification. For hydrogen/nitrogen or hydrogen/helium fuel mixtures, total hydrocarbons present should be < 1 ppmC. The oxidant should be hydrocarbon free grade air, containing < 1 ppmC hydrocarbon. The SAE in ARP 1256 restricts the FID fuel gas to a 40% hydrogen, 60% helium mixture which is required to meet this hydrocarbon level specification.
sample per mole of fuel sample per mole of fuel ATTACHMENT E TO ICAO MANUAL APPENDIX 5.
THE CALCULATION OF THE EMISSIONS
PARAMETERS - BASIS, MEASUREMENT CORRECTIONS AND
ALTERNATIVE NUMERICAL METHOD
1. SYMBOLS P7 number of moles of NO2 in the exhaust sample per mole of fuel AFR air/fuel ratio, the ratio of the mass flow rate of dry air to that of the fuel P8 number of moles of NO in the exhaust sample per mole of fuel El emission index; l0 3 X mass flow rate of gaseous emission product in exhaust per unit mass flow rate of fuel PT P1 + P2 + P3 + P4 + P5 + P6 + P7 + P8 K ratio of concentration measured wet to R concentration of O2 in dry air, by volume = 0.2095 normally S concentration of N2 + rare gases in dry air, by volume = 0.7902 normally T concentration of CO2 in dry air, by volume = 0.0003 normally P0 number of moles of air per mole of fuel in initial air/fuel mixture Z symbol used and defined in 3.4 MHC molecular mass of exhaust [CO2] mean concentration of CO2 in exhaust that measured dry (after cold trap) L, L' analyser interference coefficient for interference by CO2 M, M' analyser interference coefficient for interference by H2O MAIR molecular mass of dry air = 28.966 g or, where appropriate, = (32 R + 28.156 4 S + 44.011 T) g MCO molecular mass of CO = 28.011 g hydrocarbon, taken as CH4 = 16.043 g sample, vol/vol MNO2 molecular mass of NO2 = 46.008 g [CO] mean concentration of CO in exhaust sample, vol/vol MC atomic mass of carbon = 12.011 g [HC] sample, vol C/vol mean concentration of HC in exhaust MH atomic mass of hydrogen = 1.008 g [NO] sample, vol/vol mean concentration of NO in exhaust P1 number of moles of CO2 in the exhaust [NO2] mean concentration of NO2 in exhaust sample, vol/vol P2 number of moles of N2 in the exhaust [NOX] mean concentration of NO and NO2 in exhaust sample, vol/vol P3 number of moles of O2 in the exhaust [NOX]c mean concentration of NO in exhaust sample per mole of fuel sample. after passing through the N02/NO converter, vol/vol P4 sample per mole of fuel number of moles of H2O in the exhaust [NO2] mean = ([NOX]c -[NO]) η P5 number of moles of CO in the exhaust [ ] d mean concentration in exhaust sample after cold trap, vol/vol [ ] m mean concentration measurement sample per mole of fuel P6 number of moles of CxHy in the exhaust Page 114
sample per mole of fuel indicated before instrument correction applied, vol/vol h humidity of ambient air, vol water/vol dry air hd humidity of exhaust sample leaving "drier" or "cold trap", vol water/vol dry sample rn number of C atoms in characteristic fuel molecule n number of H atoms in characteristic fuel molecule x number of C atoms in characteristic exhaust hydrocarbon molecule y number of H atoms in characteristic exhaust hydrocarbon molecule η efficiency of NO2/NO converter
2. BASIS OF CALCULATION OF El AND AFR PARAMETERS
2.2 It is assumed that the balance between the original fuel and air mixture and the resultant state of the exhaust emissions as sampled can be represented by the following equation:
CmHn + P0[R(O2) + S(N2) + T(CO2) + h(H2O)] = P1(CO2) + P2(N2) + P3(O2) + P4(H2O) + P 5(CO) + P6(CxHy) + P7(NO2) + P8(NO) from which the required parameters can, by definition, be expressed as ⎡ 103 MCO ⎤EI CO P( ) = 3 ⎢ ⎥mM + nM⎣ C H ⎦ ⎡ 103 MHC ⎤EI HC xP( ) = 6 ⎢ ⎥mM + nM ⎣ C H ⎦ expressed as methane equivalent ⎡ 103 MNO ⎤ EI NO = PP 2( X ) ( 7 + 8 ) ⎢ ⎥mM + nM⎢ C H ⎥⎣ ⎦ ⎡ MAIR ⎤AFR = P0 ⎢ ⎥mM + nM ⎣ C H ⎦ expressed as NO2 equivalent Supplemental information This is a slightly different formulation for AFR than that stated in 7.1.3, “Basic parameters”. In this formulation “m”, the number of C atoms in characteristic fuel molecule is placed within the bracket. There is no particular advantage to using one formulation over the other.
2.2 Values for fuel hydrocarbon composition (r n, n) are assigned by fuel specification or analysis. If only the ratio n/m is so deter mined, the value m = 12 may be assigned. The mole fractions of the dry air constituents (R, S, T) are normally taken to be the recommended standard values but alternative values may be assigned, subject to the restriction R + S + T = 1 and the approval of the certificating authority.
2.3 The ambient air humidity, h, is as meas ured at each test condition. It is recommended that, in the absence of contrary evidence as to the characterization (x, y) of the exhaust hydrocarbon, values of x = 1 and y = 4 are assigned.
2.4 Determination of the remaining unknowns requires the solution of the following set of linear simultaneous equations, where (1) to (4) derive from the fundamental atomic conservation relationships and (5) to (9) represent the gaseous product concentration relationships.
(1) M = TP0 = P1 + P5 + xP6
(2) n + 2hP0 = 2P4 + yP6
(3) (2R + 2T + h)P0 = 2P1 + 2P3 + P4 + P5 + 2P7 + P8
(4) 2SP0 = 2P2 + P7 + P8
(5) [CO2]PT = P1
(6) [CO]PT = P5
(7) [HC] PT = xP6
(8) [NOX]CPT = ηP7 + P8
(9) [NO]PT = P8
(10) PT = P1 + P2 + P3 + P4 + P5 + P6 + P7 + P8 The above set of conditional equations is for the case where all measured concentrations are true ones, that is, not subject to interference effects or to the need to correct for sample drying. In practice, interference effects are usually present to a significant degree in the CO, NOX and NO measurements, and the option to measure CO2 and CO on a dry or partially dry basis is often used. The necessary modifications to the relevant equations are described in 2.5 and 2.6.
2.5 The interference effects are mainly caused by the presence of CO2 and H2O in the sample which can affect the CO and the NOX analysers in basically different ways. The CO analyser is prone to a zero-shifting effect and the NOX analyser to a sensitivity change, represented thus:
[CO] = [CO]m + L[CO2] + M[H2O] and [NOX]c = [NOX]cm(1 + L'[CO2] + M'[H2O]) which transform into the following alternative equations to (6), (8) and (9), when interference effects require to be corrected, Page 116
(6A) [CO]m PT + LP1 + MP4 = P5 (8A) [NOX]cm (PT +L'P1 +M'P4) = ηP7 + P8 (9A) [NO]m (PT + L'P1 + M'P4) = P8 The option to measure CO2 and CO concentrations on a dry or partially dry sample basis, that is, with a sample humidity reduced to h,, require s the use of modified conditional equations as follows:
(5A) [CO2]d(PT - P4)(1 + hd) = P1 and [CO]d (PT - P4)(1 + hd) = P5 However, the CO analyser may also be subject to interf erence effects as described in 2.5 above and so the complete alternative CO measurement concentration equation becomes (6B) [CO]md (PT - P4)(1 + hd) + LP1 + Mhd (PT - P4) = P5
3. ANALYTICAL FORMULATIONS
3.1 General Equations (1) to (10) can be reduced to yi eld the analytical formulations for the EI and AFR ameters, as given in 7.1 to this appendix. This reduction is a process of progressive elimination of the roots P0, P1 through P8, PT making the assumptions that all concentration measurements are of the "wet" sample and do not require interference corrections or the like. In practice the option is often chosen to make the CO2 and CO concentration measurements on a "dry" or "semi-dry" basis; also it is often found necessary to make interference corrections. Formulations for use in these various circumstances are given in 3.2, 3.3 and 3.4 below.
3.2 Equation for conversion of dry concentration measurements to wet basis Concentration wet = K x concentration dry; that is.
[ ] = K [ ] d The following expression for K applies when CO and CO2 are determined on a "dry" basis:
4+ nm T / + | / | − h N O 2 − 2 H C / x + 2+ h | / | − nm H C 1 + hd{ ( ) ( nmT 2 )([ ] ( [ ] )) ( )( yx [ / ])[ ]}( )K = ( + h){ +(n m ) ( + hd )([ 2 ] )} −( n m T − h)( − + hd [ ] )2 2 / 1 C O | / | 2 1 | 1 | C O d d
3.3 Interference corrections The measurement of CO and/or NO may require corrections for interference by the sample CO2 and water concentrations before use in the above analytical equations. Such corrections can normally be expressed in the following general ways:
[CO] = [CO]m + L[CO2} + M[H2O] Page 117
⎛ hd ⎞CO = CO + L CO M[][] [ ] 1 hd md 2 + ⎜ ⎟ ⎝ + d ⎠ [NO] = [NO]m (1 + L'[CO2] + M'[H2O]) η[NO2] = ([NOX]cm - [NO]m)(1 + L'[CO2] + M'[H2O]
3.4 Equation for estimation of sample water content Water concentration in sample [] nmh (|/ 2 | + | P 0 m |)([C O 2 ] +[C O ] +[H C ])HO = m) − ( yx2 H C )[ ]2 1+T (P0 Where 2Z n m− ( ) m =P0 ( +− [41 hT Z 2]) and 2C O − | 2 x | | − y x )[ ] [ 2 ]2 | H C + N O −[ ] ( CO + CO + HC Z = [ ] [] [] 2 It should be noted that this estimate is a functi on of the various analyses concentration readings, which may themselves require water interfer ence correction. For better accuracy an iterative procedure is required in these cases with successive recalculation of the water concentration until the requisite stability is obtained. The use of t he alternative numerical solution methodology (4) avoids this difficulty.
4. ALTERNATIVE METHODOLOGY -NUMERICAL SOLUTION
4.1 As an alternative to the analytical procedur es summarized in 3 above, it is possible to obtain readily the emissions indices, fuel/air ratio, corrected wet concentrations, etc., by a numerical solution of equations (1) to (10) for each set of measurements, using a digital computer.
4.2 In the equation set (1) to (10) the act ual concentration measurements are substituted using whichever of the alternative equations (5A), (6A), etc. applies for the particular measuring system, to take account of inte rference corrections and/or dried sample measurements.
4.3 Suitable simple two-dimensional array e quation-solving computer programmes are widely available and their use for this purpose is convenient and flexible, allowing ready incorporation and identification of any sample drying options and interference or other corrections.
Supplemental information Details explaining various calculation procedures , can be found in ARP 1533, “Procedure for the Calculation of Gaseous Emissions from Aircraft Turbine Engines.” ARP 1533 includes, among other things, the combustion chemical equation, the bas ic matrix solution wi th computer programs, derivation of equations and a sample calculation.
ATTACHMENT F TO ICAO MANUAL APPENDIX 5.
SPECIFICATIONS FOR ADDITIONAL DATA As required in 3.2 of Appendix 5, in addition to the measured sample constituent concentrations, the following data shall also be provided:
a) inlet temperature: measured as the total tem perature at a point within one diameter of the engine intake plane to an accuracy of ±0.5 ° C;
b) inlet humidity (kg water/kg dry air): measured at a point within 15 m of the intake plane ahead of the engine to an accuracy of ±5 per cent of reading;
c) atmospheric pressure: measured within 1 km of the engine test location and corrected as necessary to the test stand altitude to an accuracy of ±100 Pa;
d) fuel mass flow: by direct measur ement to an accuracy of ±2 per cent;
e) fuel H/C ratio: defined as n/m, where CmHn is the equivalent hydrocarbon representation of the fuel used in the test and evaluated by reference to the engine fuel type analysis;
f) engine parameters:
1) thrust: by direct measurement to an accuracy of ±1 per cent at take-off power and ±5 per cent at the minimum thrust used in the certification test, with linear variation between these points;
2) rotation speed(s): by direct measurement to an accuracy of at least ±0.5 per cent;
3) gas generator airflow: determined to an accuracy of ±2 per cent by reference to engine performance calibration.
The parameters a), b), d) and f) shall be determi ned at each engine emissions test setting, while
c) shall be determined at intervals of not less than 1 hour over a period encompassing that of the emissions tests.