Computech Systems, Inc.
29962 Killpeck Creek Ct.
Charlotte Hall, MD 20622
301-884-5712
RACEAIR
TM
WEATHER STATION TERMS
INTRODUCTION
This guide is intended to explain the terms displayed by the RaceAir series of competition
weather stations. Understanding these terms and how they impact engine performance will
assist you in making informed tuning decisions as well as expected changes in performance.
DEFINITIONS
Air Density Ratio
Air density ratio (ADR) is the ratio produced by dividing the calculated density of the air being
sampled by the Standard Day air density. Standard Day air is defined as a temperature of 60F,
a relative humidity of 0%, and an absolute barometric pressure of 29.92 In. Hg (inches of
mercury). The ADR for those conditions is 1.00. The RaceAir system expresses this ratio as a
percent, Standard Day = 100%. Air Density Ratio is used to assist in determining required
engine fuel tuning changes. Typically, as the ADR goes up or down the amount of fuel being
metered into the engine must be increased or decreased by the same percentage to maintain
the same air/fuel ratio.
Absolute Barometric Pressure
Absolute barometric pressure, measured in inches of mercury (In. Hg.), is the pressure that is
present at a location from the atmosphere only. The term ‘barometric’ comes from the weather
instrument, the mercury barometer, which is used as a laboratory precision measuring device.
The term ‘absolute’ refers to absolute zero, where without the earth’s atmosphere, the
measured pressure would be 0 In. Hg. Normal pressure gages have a scale that sets the
atmospheric pressure of 14.7 psi (29.92 In. Hg.) as the zero point. This is referred to as gage
pressure. The RaceAir system provides an accurate absolute barometric pressure that is used
in the calculations for ADR, Density Altitude, and the Correction Factor. Air pressure has the
biggest influence on air quality. Absolute barometric pressure normally varies between 24 and
31 In. Hg. depending on the elevation above seal level where the measurement is being taken.
The Standard Day absolute barometric pressure is defined as 29.92 In. Hg., at sea level.
Absolute barometric pressure decreases approximately 1 In. Hg. per 1000 ft. increase in
elevation.
Altitude
Altitude equals the elevation above mean sea level (MSL) in feet.
Altitude – Density
See ‘Density Altitude’.
Altitude – Pressure
See ‘Pressure Altitude’.
Barometric Pressure - Absolute
See ‘Absolute Barometric Pressure’.
Barometric Pressure - Corrected to Sea Level
In order to provide a standard method of stating barometric pressure, the National Weather
Service (NWS) adjusts the absolute barometric pressure observations to sea level elevation (0
feet). Without this standard the barometer readings would be meaningless at different
geographic elevations around the world. This correction adds about .001 In. Hg. per foot of
elevation to the absolute barometer reading. Corrected barometric pressure cannot be used for
performance calculations without subtracting the local elevation from the NWS value. Even then
it would be an approximation because the barometer is at location different from your own.
Barometric Pressure - Dry
See ‘Dry Barometric Pressure’.
Correction Factor
The correction factor is a Society of Automotive Engineers (SAE) equation that uses the
absolute barometric pressure (In. Hg.), the temperature (F), and relative humidity (%) of the
air. The correction factor is used to compensate for changes in observed dynamometer
horsepower readings due to air density differences. The correction factor is also utilized in the
following Computech calculations: Standard Dial In, Throttle Stop, Test and Tune, and
Estimated Performance.
Density Altitude
Density altitude is a value in feet, above or below seal level, that uses the absolute barometric
pressure (In. Hg.) or pressure altitude (feet), the temperature (F), and relative humidity (%) of
the air. Density altitude is a value typically used in the aeronautical industry, and a standard
day is defined as 0 feet, which is calculated based on an absolute barometric pressure of 29.92
In. Hg. or a pressure altitude of 0 ft., a temperature of 60F, and a humidity of 0%. A good air
day in the spring or fall might be below 0 ft. (i.e. –231 ft.). Conversely, a hot summer day at the
same location might have a density altitude of 4500 ft. (equivalent to 4500 ft. of altitude on a
standard day). Density Altitude is used in the Computech Density Altitude Dial-In calculation.
Dew Point
The dew point temperature is the temperature at which water vapor in the air will turn to liquid
and form dew. Moisture can cause inconsistent traction and, in certain circumstances, effect
engine performance. The warmer the air is the more water vapor it can hold. See ‘Relative
Humidity’ for more information. An air sample has a specific amount of water vapor present. As
the temperature drops and reaches the dew point, the air can no longer suspend the water
vapor, so it condenses and turns to water. This condensation will typically occur on cooler
surfaces first.
Computech research with methanol fuel, and to a lesser extent with gasoline, has shown that
when the ambient temperature falls to within 5 to 10F 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 ambient temperature versus dew point temperature,
you should be able to estimate when you should anticipate this fall off in performance and
change your dial in.
Dry Barometric Pressure
The dry barometric pressure (In. Hg.) is the absolute barometric pressure with the partial
saturation water vapor pressure subtracted from it. The measured absolute barometric pressure
is from a combination of oxygen, nitrogen, argon, other trace gases, and water vapor. The
portion of the measured barometric pressure that is water vapor will not contribute to
combustion of the fuel, and therefore must be subtracted out. Dry barometric pressure is used
to calculate ADR, Density Altitude, and the Correction Factor.
Pressure Altitude
Pressure altitude is a term from the aeronautical industry. It is the aircraft altimeter indicated
value in feet that results from setting the pressure adjustment window to 29.92 In. Hg. This
number relates to the industry defined Standard Day altitudes. An indication of 0 ft. is equal to
an absolute barometric pressure of 29.92 In. Hg. Lower or negative values (i.e. –180 ft.)
indicate better air pressure. Higher values (i.e. 2500 ft.) show poorer air pressure. An altimeter
will also show absolute barometric pressure (In. 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. The RaceAir system calculates and displays this information as additional air data but
does not use it for other calculations.
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 temperature increases. This
phenomenon makes thinking of 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
Also known as dry bulb temperature (F). For accurate atmospheric calculation the sensor
measured temperature needs to be similar to the engine intake air temperature.
Vapor Pressure
Vapor pressure is a value (In. Hg.) calculated from the sensed temperature and the relative
humidity. The RaceAir system 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 is used in the ‘Dry Barometric Pressure’ computation. This is one of the
more complex but very necessary operations the RaceAir does. The vapor pressure number
indicates the portion of the available air pressure that is actually water vapor, which could alter
the air/fuel ratio in the engine.
Water Vapor
Water vapor is a value expressed in grains (Gr.). It is the same information as ‘Vapor Pressure’
expressed in a different unit of measure. Many race teams use this unit of measure for making
fuel and timing tuning adjustments.
Wet Bulb Temperature
Wet bulb temperature refers to using a mercury thermometer with a water soaked cotton wick
over the sensing bulb. This technique produces a lower temperature indication than the dry bulb
temperature due to evaporation of moisture from the cotton wick. The wet bulb temperature and
the dry bulb temperature can be used to calculate the relative humidity. The RaceAir system
does not use this method to determine the RH %, but does display the data as information.
GOOD AIR / BAD AIR
The atmospheric conditions that make air quality better for racing are: lower temperature, lower
humidity, and higher air pressure. Conditions that make the air worse for racing purposes are:
higher temperature, higher humidity, and lower air pressure. Lower Correction Factor numbers,
lower Density Altitude numbers and higher Air Density Ratio numbers all mean better air quality.
The variables that cause the Correction Factor numbers and the Density Altitude numbers to
decrease (smaller numerically), as well as the Air Density Ratio numbers to increase (larger
numerically), are: a lower temperature, a lower relative humidity, a higher barometric pressure
(as measured with a barometer), or a lower pressure altitude (as measured with an altimeter).
Typically the Correction Factor will be between .98 (better) and 1.15 (worse); the Density
Altitude will be between -300 ft. (better) and 8,000 ft. (worse); the Air Density Ratio will be
between 85% (worse) and 103% (better).
For example, if the air temperature is 65F, the relative humidity is 45% and the absolute
barometric pressure is 30.15 In. Hg., which equals a pressure altitude (from an altimeter with a
29.92" barometric pressure standard) of -262 ft., the RaceAir system will calculate a Correction
Factor of 1.0065, a Density Altitude of 387 ft., and an Air Density Ratio of 98.9%. The system
will also provide the Water Vapor Pressure of .280 In. Hg., the Dry Barometric Pressure of
29.87 In. Hg. and the Dew Point Temperature of 43F. Conversely, with a temperature of
95F, a relative humidity of 65%, and a barometric pressure of 29.70 In. Hg., which equals a
pressure altitude (from an altimeter) of 252 ft., the Correction Factor would be 1.0800, the
Density Altitude would be 3,687 ft., and the Air Density Ratio would be 89.7%. The Water
Vapor Pressure would be 1.079 In. Hg., the Dry Barometric Pressure would be 28.62 In. Hg.,
and the Dew Point Temperature would be 81F.
You may see changes like this when racing at one track on one weekend and traveling to
another track the following weekend. You may also encounter these same types of changes at
the same track between spring and summer weather conditions. The typical change in air
quality from the heat of the day to the cool of the night is usually not very large.
Remember, as the Correction Factor and the Density Altitude increase (larger numerically),
performance will decrease resulting in slower elapsed times. As the Correction Factor and
Density Altitude decrease (smaller numerically), performance will increase causing faster
elapsed times. The lower the Air Density Ratio (smaller numerically), the leaner your jetting
needs to be and the higher the Air Density Ratio (larger numerically), the richer your jetting
needs to be.