# The DASI System, Barometric Pressure Measurement & the FAA

Barometric altimeters calculate and display altitude by measuring the local barometric pressure and converting that measurement into altitude. This conversion is done using the Standard Atmosphere Table, an international standard representing the barometric pressure at various altitudes on a so-called “standard day” which assumes a consistent non-linear relationship between altitude and pressure.

In reality, the relationship between pressure and altitude is not consistent. As weather patterns move, high and low pressure fronts change the barometric pressure seen at a fixed location. Thus, an uncorrected altimeter located at an airport runway will indicate that the runway is moving up and down with respect to sea level as it sees daily variations in barometric pressure! The variation seen can be significant. A 1″ Hg change in barometric pressure (which can occur when a major storm front moves in, for example) is equivalent to about 1,000 feet in altitude.

Did you know…
Before digital electronics were available, the system devised for adjusting an altimeter for variations in local barometric pressure was called Altimeter Setting Indicator (ASI). Now it’s the DASI system.

In the Digital Altimeter Setting Indication (DASI) system, the first step is to measure true barometric pressure at a location or “station” (typically an airport) whose exact height above sea level, called “station elevation,” is known. That local barometric pressure measurement is then converted to local sea level pressure, ie, the pressure that would be measured if the station were dropped to seal level and all other conditions (latitude, longitude, weather) remained the same. Seal level pressure is needed because altimeters are typically referenced to sea level. The jargon for this adjustment process is “reduction of station pressure to sea level pressure” even though the adjusted pressure reading would be higher than the true pressure reading if the station were above sea level.

This local sea level pressure is the DASI. This number, measured in units of pressure, is broadcasted by local National Weather Services and by airport control towers. For example, as a pilot approaches an airport, he gets the current DASI reading from the airport and enters it into his on-board altimeter, thereby “correcting” the altimeter for the local variations in barometric pressure. The pilot then knows what his true altitude is relative to the runway.

The Federal Aviation Administration (FAA) recently approved a digital pressure gauge for use in the Digital Altimeter Setting Indicator (DASI) system for airport control towers.

This digital pressure gauge is the Setra Model 370. It can be used by airports, weather services, and in other applications to measure local barometric pressure and calculate a current DASI number automatically and with exceptional accuracy. Once the fixed “station elevation” of the Model 370 is entered, the Model 370 will continuously convert true local barometric pressure to local sea level pressure, which is the DASI number given to pilots to correct altimeters. Without the Model 370, the process is either more difficult or far more expensive. The output of a barometric pressure transducer must be converted manually (with greater risk of error) or by computer using equations and tables. The Model 370 does it all simply, very accurately, and in one box.

Setra Model 370 Digital Pressure Gauge

This highly accurate (0.02% FS) self-contained instrument has a 6-digit LCD used to display a variety of altitude measurements. It may be used as an altimeter calibrator, a standard altimeter, a true corrected altimeter (self corrected or remote), or as a relative altimeter. It may also be used to indicate small relative changes in altitude through the use of the zero/tare function.

The Model 370 has a numeric keypad for access to engineering unit conversions and it’s also equipped with a bidirectional RS-232 interface which allows communications between the gauge and other devices, such as computers and printers. These devices must also be equipped with an RS-232 serial port.*

One of the great features of this particular model is that it can be operated as a portable instrument with the battery/recharger option. The battery option is designed to allow a standard 8 hour work-shift, followed by a 16 hour recharge cycle.

The battery option is a rechargeable 12 VDC Nicad battery, and an automatic internal battery charger which functions whenever the gauge is plugged into an AC outlet. Also included in the battery option is an on/off switch and the required internal wiring.

To ensure year over year accuracy, calibration services are offered at Setra’s facility in Boxborough, MA.  Qualified service technicians provide calibration services in compliance with ANSI/NCSL Z540–1–1994 and certified per NIST traceable primary standards.