Pressure is a fundamental physical concept that dictates the behavior of fluids and gases in every setting, from the atmosphere to industrial piping. Defined as the force exerted perpendicularly over a unit area, its measurement is a powerful tool for safety, efficiency, and process control in countless applications. Accurately quantifying this force allows professionals to maintain optimal conditions in systems ranging from a car’s braking mechanism to a human heart. Understanding how pressure is measured, what units are used, and how instruments function is essential for appreciating its impact on technology and daily life.
Defining Pressure and Its Basic Units
The conceptual foundation of pressure is the simple relationship where force is divided by the area over which it is applied, often expressed as $P=F/A$. Several distinct units are still in common use today due to varied historical and industrial contexts. The international standard unit is the Pascal (Pa), equivalent to one Newton of force distributed over one square meter. For higher-pressure applications, the Bar is a more convenient unit, representing $100,000$ Pascals, which is slightly less than the average atmospheric pressure at sea level. In North America, the Pound per Square Inch (PSI) is widely used, and the millimeter of mercury (mmHg), or Torr, remains the standard for medical measurements like blood pressure.
Distinguishing Types of Pressure
The way a pressure value is referenced determines its type, a distinction important for accurate measurement. Absolute Pressure ($P_{abs}$) uses a perfect vacuum—a state of zero pressure—as its reference point. Because it is referenced against an unchanging, fixed zero, an absolute pressure reading is not affected by changes in atmospheric pressure caused by weather or altitude. This measurement is necessary in applications requiring high precision or in systems operating under vacuum conditions.
Gauge Pressure ($P_{gauge}$) is the most common measurement type and is referenced against the local atmospheric pressure. When a tire gauge reads zero, it means the pressure inside the tire is equal to the pressure outside in the surrounding air. Gauge pressure is the difference between the absolute pressure of the system and the ambient atmospheric pressure. This makes it a practical measurement for closed systems like compressed air lines or hydraulic fluid.
A third category, Differential Pressure, measures the precise difference between two specific points within a system. Unlike absolute or gauge pressure, it is not concerned with atmospheric pressure or a vacuum. Instead, it measures the relative pressure drop or gain between two points in a pipe or vessel. This measurement is frequently used to monitor flow rates or to detect blockages by registering the pressure drop across a filter or an obstruction.
Tools for Sensing and Reading Pressure
Instruments for measuring pressure are broadly categorized into types based on their operating principle, starting with simple mechanical devices.
Manometers
Manometers rely on the physical displacement of a fluid column to determine pressure differences, with the U-tube manometer being the classic example. The pressure applied to one side causes the liquid to rise in the other side, and the height difference directly correlates to the measured pressure.
Bourdon Tube and Diaphragm Gauges
The most ubiquitous mechanical device is the Bourdon Tube Gauge, found in many industrial and everyday settings. This device uses a C-shaped tube, sealed at one end and connected to the pressure source at the other. As pressure increases, the curved tube attempts to straighten out, translating this deformation through a linkage to move a pointer across a calibrated dial. Another mechanical type, the diaphragm gauge, uses a flexible membrane that deflects under pressure, making it suitable for low-pressure measurements.
Electronic Transducers
For modern process control and digital data acquisition, Electronic Transducers are the standard, as they convert pressure into a proportional electrical signal. These devices typically employ a pressure-sensitive diaphragm bonded to a strain gauge. When pressure deflects the diaphragm, the strain gauge stretches or compresses, which changes its electrical resistance. This change in resistance is measured and converted into a voltage or current signal for digital display or computer reading.
Everyday Applications of Pressure Measurement
Pressure measurement is integrated into nearly every facet of modern life, ensuring safety and optimizing performance.
In automotive safety, measuring tire pressure is standard, as under-inflated tires lead to poor handling and increased risk of failure. The reading displayed by a standard tire gauge is a gauge pressure measurement, indicating how much the air inside the tire exceeds the ambient atmospheric pressure. Medical diagnostics also rely heavily on precise pressure measurement, most notably in determining blood pressure. The traditional sphygmomanometer measures the peak (systolic) and minimum (diastolic) pressure in the arteries in millimeters of mercury (mmHg), providing crucial information about cardiovascular health.
Another common application is the measurement of atmospheric pressure for weather forecasting and altimetry. A barometer measures the absolute pressure of the air, and changes in this reading can signal approaching weather systems. For pilots and hikers, the correlation between atmospheric pressure and altitude allows instruments to function as altimeters, using the lower pressure at higher elevations to determine height above sea level.