Barometric Pressure

Read about determining the correct barometric pressure to enter for correcting dynamometer horsepower data

Basics | 07.20.20

How do I determine my shop’s “absolute barometric pressure?

An accurate absolute barometric pressure entry, on DYNO-MAX’s Run Information – Weather tab, is required for applying atmospheric correction standards – to internal-combustion engine test data. This information must be entered as absolute, rather than altitude-compensated, barometric pressure. Typical weather-forecasting barometers, are calibrated to report relative (altitude compensated) pressures. This applies to most home barometers (if they are even accurate at all).

Note: Even if you use the optional DYNOmite Weather Station to automatically enter your atmospheric data, it must still be (periodically) calibrated with the same pressure information.

One way to get an accurate barometric pressure reading is to call a (very) local airport. Ideally, the person you reach will understand the difference between absolute and altitude-compensated, barometric pressure. It is important for you to know which reading they give you. If the person can tell you both readings, they likely understand the difference.

If an airport (or other source) can only supply you one reading, it will probably be the relative (altitude compensated) pressure. All is not lost though, it is easy to convert a local altitude-compensated, barometric pressure reading into absolute terms – as long as you have the altitude that local reading was taken at. The airport-tower people should know their altitude. If not, try this link:

DYNO-MAX includes a tool (on its same Weather tab) for adjusting relative barometric-pressure readings – to remove their altitude compensation. Just input your Local Barometer reading and your Altitude (under Calculate Absolute Pressure). Then click the Enter Absolute From Local Pressure button to have DYNO-MAX calculate and insert the absolute pressure for you.

Important: If you attempting to use the optional DYNOmite Weather Station module, but its Barometric Pressure is wrong by a factor of two, it has probably been miscalibrated using PSI instead of Inches of Mercury (or vice versa).

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