Understanding the difference between corrected and uncorrected barometer
The value we see as barometric pressure is a way of measuring the pressure of the column of air above us. The higher we go in elevation, the less that pressure becomes. Therefore lower barometer values are correlated with higher altitudes. You might have heard the term density altitude. That is taking the current weather conditions and applying them to conditions at a certain altitude. Similarly, barometer values can be used to estimate altitude.
Pressure at different altitudes
Historically, barometer correction (density altitude) was used in conjunction with mercury-based meters. Changes in atmospheric pressure affect the density of mercury and so, affect the readings on the barometers. The temperature affects the readings also. Knowing the temperature value would help scientists correct the values on barometers to give a more accurate reading of the pressure.
Today we have barometers and altimeters. Barometers measure changes in atmospheric pressure in one location. Pressure altimeters measure pressure changes in order to estimate the altitude at different locations. Note that any portable device you have will need to be calibrated occasionally in order to provide accurate results over time.
Why is pressure important
For racers, pressure is not only an important variable in altitude but also important for determining how much oxygen is in the air. The lower the pressure or the higher the altitude, the less oxygen there is in the air. This has an influence on the air/fuel ratio used in engine tuning. You can read more about this in our previous article.
However, meteorologists correct the barometer value to give the pressure reading at sea level instead of at your current elevation. This is because pressure is also useful for determining pressure systems. A barometer value above 29.92 inHg is a high pressure system and usually means air is cooling, humidity is decreasing, and the pressure of that air is stabilizing the atmosphere. A barometer reading below 29.92 inHg is a low pressure system and usually means air is warming and humidity is increasing. Low pressure systems tend to bring clouds, rain and snow storms. High pressure systems tend to bring clear skies and a calm atmosphere.
Getting barometer values
If you are using the barometer value obtained from a local weather report in engine tuning, you need to uncorrect it to your current elevation. The uncorrected value is better for telling you how much pressure actually is in the air at your current location and, therefore, how much oxygen is available for your air/fuel ratio.
A good rule of thumb is to decrease the barometer value by 1 inHg for every 1000ft of altitude increase. This means that a barometer value of 29.54 inHg at sea level would be approximately 28.54 inHg at 1000ft of elevation. If you travel somewhere like Colorado, with an approximate elevation of 5000ft, it would mean the uncorrected value is around 24.54 inHg.
If you have an altimeter or a portable weather station, most likely you are already getting the uncorrected value. You can use that on this site or in your own calculations.
If you are using Pro-Calc or the free calcs on this site, you can get corrected or uncorrected barometer. If you know your barometer value is already uncorrected, you can simply enter it in the barometer field and proceed with your calculations. You should not enter the elevation.
If you need to uncorrect it, you will need to know the elevation of your current location along with the temperature, humidity, and barometer values. We have a calculator on our site that can provide the altitude.
NOAA Glossary: Sea Level Pressure
Temperature Correction for Mercury Barometers
Altitude.org: Air Pressure