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gasoline, has shown that when the ambient temperature falls to within 5 to 10F of the
dew point temperature there will be an unexpected slight fall off in performance due
primarily to fuel degradation in the intake manifold that is caused by water contamination.
By keeping accurate records as to when the performance actually falls off in relation the
outside temperature versus the dew point temperature, performance degradation can be
predicted.
Dry Barometric Pressure
The dry barometric pressure (Inches Hg) is the absolute barometric pressure with the
partial saturation water vapor pressure from humidity subtracted. The measured absolute
barometric pressure is from a combination of oxygen, nitrogen, argon, and some other
trace gases plus water vapor. The portion of the air that is water vapor will not contribute
to combustion of the fuel and therefore must be factored out. Dry barometric pressure is
used to calculate ADR, Density Altitude, and the Correction Factor.
Dry Bulb Temperature
See ‘Temperature’.
Pressure Altitude
Pressure altitude is the aircraft altimeter indicated value in feet that results from setting
the pressure adjustment window to 29.92 Inches Hg. This number relates to the
aeronautical industry defined Standard Day altitude. An indication of 0 ft is equal to an
absolute barometric pressure of 29.92 Inches Hg. Lower or negative values (i.e. –180 ft)
indicate more air pressure. Higher values (i.e. 2500 ft) show less air pressure. An altimeter
will also show absolute barometric pressure (Inches Hg) in the pressure window when the
altitude pointer is rotated to indicate 0 ft. Note that this number is pressure only. Until it is
adjusted for temperature and humidity via the density altitude process it is not an indication
of total air quality.
Relative Humidity
Relative humidity (% RH) is a value to indicate how much water vapor is present in an air
sample. Air has the ability to hold more water vapor as the temperature increases. This
phenomenon makes comprehending water vapor content difficult. By always relating the
amount of water vapor present to the maximum amount that air could hold at that
temperature, the relationship is easier to understand (i.e. 50% RH is always 50% of the
possible saturated water vapor content for air at any temperature).
Temperature
Temperature (F) is also referred to as dry bulb temperature. In order to correctly calculate
the effect of temperature on performance, the measured temperature needs to be similar
to the engine intake air temperature.
Vapor Pressure
Vapor pressure (Inches Hg) is a value calculated from the sensed temperature and relative
humidity. The RaceAir Pro uses numbers derived from mechanical engineering steam
tables to calculate the partial saturation vapor pressure of the moisture in the sampled air.
That value is displayed and used in the ‘Dry Barometric Pressure’ computation. The vapor
pressure number indicates the portion of air pressure that is actually from water vapor
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