This calculator uses the method described by A. G. Okkes in
Hydrocarbon Processing, dated July 1987, pp 53-55. The SO3 conversion
from SO2, if calculated, is assumed to be set by equilibrium at 1000
°C. The flue gas is assumed to be at atmospheric pressure. A deviation
of 200mm H2O will result in a 0.4 °C error in the predicted dew point.