Air Pressure Measurement

There are a variety of different techniques commonly used to measure air pressure, but not all such techniques are suitably accurate for precise calculation of Density Altitude and/or Engine Horsepower. Here are some guidelines to help you choose an appropriate measurement:

1)  Pressure measurements which are generally suitable for use in calculating Density Altitude and/or Engine Horsepower:

a) Absolute Pressure:

This is the actual pressure of the air. (also called the Station Pressure).

Many of the newer portable digital weather stations can directly display the Absolute Pressure.

On a standard day, for example, the sea level absolute pressure is 29.92 in-hg, at 2000 ft the absolute pressure drops to 27.82 in-hg, at 4000 ft the absolute pressure is 25.84 in-hg, at 6000 ft the absolute pressure is 23.99 in-hg, and at 8000 ft the absolute pressure is 22.23 in-hg.

Also, note that the absolute air pressure may be directly read in the Kollsman window of a properly calibrated aircraft altimeter by turning the adjustment knob until the altimeter indicates an altitude of zero (if the altimeter has sufficient adjustment range to do so).

b) Station Pressure:

This is the actual pressure of the air. (also called the Absolute Pressure). Note that the Station Pressure is often measured a few feet above the ground to account for the height of an aircraft's air pressure measurement location.

Many airports can provide their current station pressure.

 In the USA, the National Weather service reports station pressure for many locations. See for example:

Colorado Pressure Readings

c) Altimeter Setting:

The altimeter setting is a simplified sea-level pressure value which may be dialed into the Kollsman window of an aircraft’s altimeter so that the altimeter will accurately indicate the current elevation above mean sea level.

The value of the Altimeter Setting is calculated using the Station Pressure modified by a specified pressure reduction computation based only on the station pressure and the station elevation. (Unlike sea-level corrected barometric pressure, which is temperature dependent, the altimeter setting is computed using only the station elevation and station pressure as variables.)

Using the specified pressure reduction (in accordance with the Standard Atmosphere) it is possible to accurately calculate the absolute pressure given the altimeter setting and the elevation.

Many airports can provide the current altimeter setting, and the altimeter setting is included in most US National Weather  Service reports. For example, click here for NOAA weather data  including hourly listing of dew point and altimeter setting for worldwide locations, in both English and Metric units. 

Since the Altimeter Setting is readily available in many parts of the world, and since the Altimeter Setting can be used to directly determine the absolute pressure, the Altimeter Setting is used for the ambient pressure measurement in many of my on-line calculators.

2)  Pressure Measurements which are NOT suggested for use in calculating Density Altitude and/or Engine Horsepower:

a) Absolute Barometric Pressure:

This is the actual pressure of the air, originally measured using a column of mercury. Since the density of mercury is a function of temperature, the air pressure measurements made with a mercury barometer are also a function of temperature.

And to further complicate the issue, a variety of different sensors, some using various liquids and some fabricated without liquids (aneroid), all give similar, yet slightly different readings as a function of temperature.

In some specific circumstances, a barometer may accurately indicate the actual ambient air pressure. However, in general, due to the effects of temperature, there may be some difference between a barometer reading and the actual air pressure.

Due to the uncertainty of the temperature dependence, barometric pressure readings are not generally appropriate for use in accurately calculating Density Altitude and/or Engine Horsepower.

b) Sea-level Corrected Barometric Pressure:

Sea-level pressure is a theoretical pressure at the station if the station were actually at sea level. Unfortunately, there are a variety of slightly different methods used to convert from the actual barometric pressure to the sea-level corrected pressure. 

In general, conversion from actual barometric pressure to sea-level barometric pressure includes certain effects of temperature (which are not included in the Altimeter Setting calculations).

Due to the various sensors and algorithms which are employed in calculating the sea-level barometric pressure, the accuracy of any conversion from the sea-level corrected barometric pressure to absolute pressure is unpredictable (unless the specific type of sensor and the specific algorithm used to make the sea-level correction are known).

And, as is the case far all barometric pressure readings due to the uncertainty of the temperature dependence, sea-level barometric pressure readings are not generally appropriate for use in accurately calculating Density Altitude and/or Engine Horsepower