Barometric Pressure

Barometric pressure is the total outside air pressure measured with reference to a perfect vacuum. The pressure varies depending on geographical location, altitude and local weather conditions. For weather reporting purposes the barometric pressure is normally adjusted to a sea level value so that all locations can be compared independent of the altitude at each location.

The measurement of ambient air pressure is carried out in many applications throughout research and industry.

The most well known use is as a barometer for weather monitoring but there are a number of other measurements that incorporate barometric pressure as well, some of the most common are explained below.

Meteorology

There are many measurements made to monitor weather conditions at a particular location but the measurement of barometric pressure has to be the most important since it can be used as an indication of general conditions:

  • High Pressure – Clear skies with very warm temperatures in the Summer and very cold temperatures in the Winter.
  • Low Pressure – Cloudy skies with milder temperatures in Summer and Winter.

A significant change in barometric pressure can also predict the likelihood of precipitation since it maybe because of cold or warm front passing through.

For meteorological purposes the measured pressure is always adjusted to its equivalent value at mean sea level height so that all readings at any location can be compared to a common datum altitude.

Gas Analysis

The accurate concentration of a discreet chemical that is present in a sample of mixed gas is dependent on a few physical parameters, one of which is the atmospheric pressure.

A gas analyser will typically indicate the presence of a chemical by measuring the strength of the signal returned by the gas sensor.  To more accurately determine the concentration of the chemical, other secondary measurements are necessary, one of these is an air density correction which can be determined from the measurement of ambient pressure, temperature and humidity.   The higher the pressure of the air sample is, the lower the corresponding concentration of the chemical will be, given that all other measured parameters are constant.

Water Level Measurement

If using hydrostatic pressure to monitor water depth or level the reading needs to be compensated for the atmospheric air pressing down on the water surface.  One way to do this is to feed a vent path from the reverse side of the pressure sensing diaphragm to the water surface so that the reading is compensated automatically and continuously.

In some applications it is not possible to provide a vent path, in these cases the surface air pressure will need to be measured independently via a barometer at set intervals or approximated at the location of water level sensor via a regional meteorological service.

Altimetry

Surrounding air pressure is generated by the pull of gravity on the air molecules that exist in the Earth’s atmosphere.   If the altitude is increased, the air pressure will reduce due the reduced pull of Earth’s gravity the further away it is measured from the surface of the Earth.  The relationship between aiir pressure and height is defined by the Barometric formula.

This method of altitude measurement is used extensively in aviation and all aircraft include an altimeter instrument in the cockpit which uses this measurement technique.

Altitude measurement is one of the required parameters for testing aircraft components and is carried out on the ground inside environmental or climatic test chambers and during flight tests, since many internal parts of an aircraft are exposed to outside air pressure.

Relative Humidity

The percentage of moisture in the air is measured by calculating the relative humidity which is defined as the proportion of water vapour partial pressure to the saturated vapour pressure of air.  The point at which the air is saturated is called the dewpoint and the relative humidity at this point is always 100%.  Any change in air pressure or temperature will have a direct effect on the relative humidity.   If the temperature is maintained at a constant value, a lowering of air pressure will increase the relative humidity.

Interferometry

Laser Michelson interferometers are used for measuring length extremely precisely to calibrate machine tools and other devices which must measure length very accurately.   The interference fringe pattern generated by the inferometer is directly related to the wavelength of the laser light source used.  Since laser light is monochromatic with a very narrow spread in wavelength it is possible to produce a length measurement scale of a very high precision.

The wavelength of light will vary depending on the type and density of medium it is radiating through, therefore the measurement precision of a laser interferometer is affected by the changes in laser wavelength due to air denisity variation.  As part of the air density calculation, the changes in ambient air pressure are constantly monitored to aid the wavelength correction

Engine Inlet Pressure

The performance of a combustion engines are affected by changes in barometric pressure.  This is because the power that an engines delivers is dependent on the amount of air/fuel mixture being injected into the engine’s combustion chamber.  If the air density or pressure increases, more fuel can be mixed with the air and therefore more power can be generated by the engine.

This is why engine performance is reduced at higher altittudes which have a relatively lower air pressure and why turbo charged engines which pressurise the inlet air generate more power.




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