SAE Aerospace Recommended Practice ARP1533A is being revised to resolve two discrepancies and to correct several typographical errors. The first discrepancy concerned the third column in Table 1 that listed the molecular weights for a variety of fuels. In both ARP1533A and ICAO Annex 16, the molecular weight of fuel used in the calculation of the emission indices and air to fuel ratio is defined as m(M_{C} + α M_{H}). The third column in Table 1 has been eliminated to avoid confusion with other listings of fuel weights.
The second discrepancy concerned the units of the interference coefficients and the form of the interference correction equations. The original AIR1533 described the interference effects as either a “zero shift” or a “concentration factor” effect. ICAO Annex 16 and ARP1256 specify the maximum limits of each interference coefficient with units for each parameter:
The interference effects on the CO measurement were described as “zero shift” effects with the form:
The “zero shift” descriptor is consistent with the units of L and M in the table above. Some number of ppm CO is added to the measured CO concentration for each mole percent CO_{2} or H_{2}O that was present. The interference effects on the NOx measurement were described as a “concentration factor” effect with the form:
The “concentration factor” descriptor is consistent with the units of L’ and M’ in the table above. The measured NOx concentration is multiplied by a factor representing the percent reduction in the NO reading for each mole percent CO_{2} or H_{2}O that was present. The “concentration factor” descriptor is also consistent with the units of J in the table above. The measured CO_{2} concentration is multiplied by a factor representing the percent reduction in the CO_{2} reading for each mole percent O_{2} that was present.
Therefore the units for the interference coefficients and the forms of the interference correction equations must be compatible. ARP1533A incorrectly specified all of the interferences to be zero shifts. Equations 15, 16, 17, 19, 20, 21, and 25 in ARP1533A were modified by leaving out the [NOx]_{ms}, [NO]_{ms}, [CO_{2}]_{ms} terms to make them compatible with the “P” terms (moles). However Equations 35, 36, 37, 40, and 41 (the equations used in the matrix A) retained the [NOx]_{ms}, [NO]_{ms}, [CO_{2}]_{ms} terms which is consistent with the ARP1256 units on the interference coefficients. Both of the sample calculations in the ARP1533A used the correct units for the interference coefficients and the correct forms of the equations.
With this ARP1533 revision comes a caution that if in the future the form of interference coefficients are changed, then so too must the form of the correction equations. Every attempt should be made for consistency between ARP1533, ICAO Annex 16 and ARP1256.