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Barometric Pressure

barometric pressure mbar to inhgA guide to barometric pressure measurement including explanations, applications and choice of products for measuring 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.

Applications

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 altitudes which have a relatively lower air pressure and why turbo charged engines which pressurise the inlet air generate more power.

Questions & Answers

Formula for non-vented water depth reading

I have a baro sensor with an accuracy of 0.1 mbar, and a pressure sensor calibrated in mH2O for measuring water depth, which is non vented and provides 20 mA at 20 mH2O and 4 mA at around 10 mH2O. What is the formula to correct the mH2O level for the baro reading, and will the level increase or decrease with a rise in barometric pressure?

The formula for compensating a non-vented pressure reading to true level reading is as follows:

[non-vented level rdg] – [baro rdg] = [equivalent vented level rdg]

This assumes you are using the sames units for both level and baro when making the calculation. If not you can use this pressure converter to change the values so they are in the same units.

In theory the true level reading should be unaffected by changes in barometric pressure, but since there will be slight differences in accuracy between the two sensors, the true level calculation may vary by a small amount even if the actual level is the same. The non-vented level reading will increase as barometric pressure increases by the same amount and vice versa if it decreases.

For more background info on non-vented pressure measurement to determine level readings we would suggest the following articles :

Two different barometric readings at the same location

One of my barometer shows 24.50″ and the other 30.15″, they are in the same location, so why the difference? One is electronic and it is the one showing the very low pressure, and a spring-based barometer reads the higher pressure. We live in Denver, Colorado, and the elevation is about 5,500′ above msl. It doesn’t make sense to me that the analog barometer measures close to what is reported by the weatherman on TV, around 30″, and the electronic one measures the very low pressure, around 24.5″.

It sounds like the electronic barometer is showing local atmospheric pressure (5,500 ft elevation) and the analogue one is corrected to sea level (0 ft elevation), so both are measuring correctly but are referenced to different elevations.

The weatherman will always report barometric pressure referenced to sea level, so that is why your electronic barometer is different, because it is showing the true pressure at 5,500ft elevation.

You may find that the electronic barometer has a setting somewhere for entering your elevation above sea level. If you set this correctly you should find the reading will change and read closer to what the analogue barometer displays.

This table shows how air pressure changes with elevation based on the US standard atmosphere model.

Using weather station to subtract ambient air pressure

We want to measure the difference in pressure inside a cask compared to outside barometric pressure. We would like to use a barometric range to measure the ambient air pressure inside the cask and use a local weather station reading for the outside pressure. Is there anything else we should consider with using this method?

You will need to know what type of barometric pressure the weather station is providing. Typically public weather stations give out the pressure reading adjusted to sea level, so it is likely that it will need to be adjusted for altitude at your location.

If the weather station belongs to you, then check whether it has been adjusted for altitude above sea level. If it has not been adjusted then it will be reading actual atmospheric pressure at your location. If the altitude has been entered, then you will need to adjust the readings manually to actual atmospheric pressure. Some weather station may allow you to display both sea level and local barometric pressure, so you do not have to adjust the pressure manually.

Another factor to consider is whether the weather station is positioned at the same height as the cask. If the height difference is significant, then this will also need to be compensated, either manually or adjusting the altitude setting for the weather station by the amount of height difference.

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