When designing a pressure sensor, even before checking the electrical characteristics of a device, two questions must be answered: What type of pressure must be measured and what is the unit of the measurement?
Types of Pressure
Three terms describe the type of pressure: absolute, gauge or differential.
- Absolute pressure is zero-referenced against a perfect vacuum
- Gauge pressure is zero-referenced against ambient air pressure.
- Differential pressure is the difference in pressure between two points.
Then, pressure can be described by many units and is confusing at first look: Pascal, psi (pound per square inch), bar, inHg (inch mercury), mm H2O (millimeter water), mmHg (millimeter mercury), atm (atmosphere) -- just to name the most common ones -- are regularly used to measure pressure and different pressure sensors (whether they are board mount pressure sensors or industrial pressure sensors) can be ordered depending on the requested unit of the application.
Various reasons explain these numbers of units: historical and legacy reasons as pressure is one of the oldest environment parameter that was measured, habits per country, and range of the pressure evaluated to have easily readable and convenient numbers.
Pascal in SI Units for Pressure
The SI unit for pressure is Pascal (Pa). One Pascal is defined as one Newton per square meter. As a reminder, one Newton is the force needed to accelerate one kilogram of mass at the rate of one meter per square second. Hence, the following formulas:
Pascal is the unit that must be used when performing calculations with other values following the international System of Units. But Pa is not the most convenient way to evaluate pressure in applications.
For instance, it’s very useful to reference pressure on earth versus the pressure at sea level. Hence, one atmosphere (atm) was defined as the pressure at sea level and the reference is calculated as the weight of the air above sea level at 45 Degrees North on a column of air of one square meter. This weight is typically around 10,333kg. With an acceleration of 9.80665m/s2 we end up with:
Pascal unit is based on the metric system that is different than the traditional imperial system that uses pound and inch instead of gram and meter. Hence, the pressure expressed in pound per square inch (PSI) is also defined. As one pound equal 0.45359237 kg and one inch equal to 0.0254 meter, we have the equality:
Mercury Manometer Pressure
Historically, the mercury manometers were the first accurate pressure gauges. Therefore, the units millimeters of mercury or inch of mercury were very common units and are still used for blood pressure for instance. Because the density of mercury depends on temperature and surface gravity, both of which vary with local conditions, specific standard values for these two parameters were adopted. This resulted in defining a millimeter of mercury as the pressure exerted at the base of a column of mercury one millimeter high with a precise density of 13,595.1 kg/m3 (density of mercury at 0oC) when the acceleration due to gravity is exactly 9.80665 m/s2. Based on these numbers:
The imperial version of the millimeter of Mercury (mmHg) is the inch of mercury (inHg). As one inch equal to 25.4 mm,
The inch of mercury is still used in the United States.
mmH20 to PA Conversion
Because pressure is commonly measured by its ability to displace a column of liquid in a manometer, pressures are not only expressed with mercury but also with water giving a new unit called millimeter water (mmH2O). The definition of mmH2O is the pressure exerted at the base of a column of water one millimeter high with a precise density of 1,000 kg/m3. By definition of Pa, we get immediately the conversion:
The mmH2O pressure unit is used for monitoring low air flow pressures used in building ventilation systems or very low water levels in rivers and streams. Historically low air pressures were measured using water column manometers but although these instruments have been mostly replaced by digital manometers which use electronic sensors, the mmH2O pressure unit remains in use. Lung pressure is measured is centimeters of water (cmH2O) that is just a ratio of 10 with mmH2O.
Standard Atmospheric Pressure
For atmospheric air pressure, the unit bar is still vastly used even if it’s not an SI unit. At sea level, the pressure is defined as 1,000mbar or 1bar that is equal to 100 kPa. Many meteorologists all around the world continue to report weather information in mbar as it’s a convenient value. In the same family of units, the decibar (dbar) that is equal to one tenth of bar, is usually used by oceanographers to measure underwater pressure because pressure in the ocean increases by approximately one decibar per meter depth.
When designing pressure sensors, all these units can be confusing and various conversions can be required. The table below summarizes all the conversions of the most common units.
We hope that shed a bit of light on the different units used to measure pressure and helps you decide which sensor to use in your next project!