Measuring air pressure

[edit] Introducing air pressures

Air pressure refers to the force of the weight of air molecules. Atmospheric pressure is the force of these molecules on surfaces that results from the gravitational pull of the earth. Changes in atmospheric pressure or pressure differentials cause air flows and as such can influence weather patterns.

Within buildings airflow is also caused by pressure differentials between different areas, inside and outside and channels of flow. Air flows originate from areas of high pressure, and move to an area of lower pressure. Heating molecules increases the spacing between them, thus reducing density and causing it to rise.

Buildings can be subject to atmospheric pressure, localised wind pressure, pressure differences from height, temperature, mechanical intervention, infiltration, exfiltration, ventilation and so on.

There are a number of devices used to measure air pressures in buildings, some of which are listed below.

Images from BSRIA instrument sales and hire

[edit] Manometer

A manometer is a device used to measure pressure differences, this could be pressure relative to atmospheric pressure (a barometer), within a vessel or chamber, a gas or liquid, to calculate flow rates through a device such as a duct or blood pressure in a person.

They are commonly used in the construction industry in building services to measure system air pressure in heating, ventilation, and air conditioning systems, airflows, positive and negative pressures in ducts, or pressure differentials across filters and coils.

There are effectively four types of manometers, the first two relying on a fluid, in a closed or open tube; the third, aneroid manometer, without a fluid; and finally a digital aneroid manometer.

[edit] Barometer

A barometer is a type of manometer that measures pressure relative to a prevailing atmospheric pressure. Air in the atmosphere has a weight which presses against all surfaces on the earth as gravity pulls it down, barometers measure this pressure. There are barometers containing a fluid (mostly mercury) in a closed or open tube design, those without a fluid, using mechanical parts (aneroid barometers) and contemporary or digital barometers.

Barometers measure atmospheric pressure, the units were originally referred to as 'atmospheres', describing a unit of air pressure at sea level at a temperature of 15 degrees C. Later this was defined as 1.01325 x 105 pascals. Today this is often referred to as 'bar' (100 kilo-pascals). So one atmosphere (atm) is almost the same as one bar, but not quite. These devices are often found outside windows in homes, used to measure the outside air pressure to assist in weather predictions, as rising air pressure tends to indicate a good weather forecast whilst falling pressure might suggest rain or bad weather.

[edit] Anemometer

An anemometer is an instrument that is used to measure wind speed or wind pressure, wind flow and wind direction. They are a common part of weather stations and are sometimes seen on rooftops. The simplest type of anemometer has a spinning wheel, which spins faster as the wind blows more strongly. The number and speed of rotations is used to calculate wind speed.

There are many different types of anemometer, including cup anemometers, vane anemometers, hot-wire anemometers, ping-pong ball anemometers as well as laser doppler, ultrasonic and acoustic resonance anemometers.

[edit] Balometer

A balometer is a type of airflow meter that resembles an air collection hood and is sometimes referred to as an air flow hood. It is used to measure the flow rate of air leaving or entering the ventilation outlet of an airflow system. Some, balometers also measure the atmospheric pressure of a space as well as the temperature and relative humidity of the flowing air.

Balometers contain a grid with many holes called, a flow grid, and a micro-manometer. This measures the flow and pressure throughout the grid and compares it to the atmospheric pressure in the space.

[edit] Related articles on Designing Buildings

• Adaptive comfort.
• Adverse weather.
• Air change rates
• Air conditioning.
• Air infiltration.
• Air permeability testing.
• Air quality.
• Air tightness in buildings.
• Balometer.
• Barometer.
• Climate.
• Cold stress.
• Computational fluid dynamics.
• Cross ventilation.
• Cultivating Cleaner Air with BSRIA.
• Displacement ventilation.
• Domestic ventilation systems performance
• Draughts in buildings.
• Effective ventilation in buildings.
• Extreme weather.
• Extreme weather event.
• Heat recovery ventilation.
• Indoor air quality.
• Mechanical ventilation.
• Natural ventilation.
• Passive building design.
• Stale air.
• Thermal comfort.
• UV disinfection of building air to remove harmful bacteria and viruses.
• Ventilation.
• Weather.
• Wind.