Pressure is defined as the force applied perpendicularly over a specific unit of area. This measurement is a foundational concept in science, influencing everything from the performance of a jet engine to the circulation of blood in the human body. Because pressure is involved in almost every physical system, a diverse array of specialized tools has been developed to accurately quantify it. These instruments range from simple liquid-filled tubes to sophisticated electronic sensors.
Tools for Measuring Atmospheric Pressure
The most common tools designed to measure the pressure exerted by the Earth’s atmosphere are barometers, which are essential devices for meteorology. Historically, the mercury barometer, invented by Evangelista Torricelli in 1643, was the standard. This device functions by balancing the weight of a column of mercury inside a sealed glass tube against the external atmospheric pressure. The height of the mercury column directly reflects the current air pressure.
Due to the toxicity of mercury, modern applications largely rely on the aneroid barometer, which operates without liquid. The aneroid barometer contains a sealed, flexible metal capsule, often made from an alloy of beryllium and copper, from which most of the air has been removed.
Changes in the external atmospheric pressure cause the thin walls of this capsule to expand or contract. A system of mechanical levers and springs amplifies this movement and transfers it to a pointer, which displays the reading on a calibrated dial. This design makes the aneroid barometer safer, more portable, and suitable for use in aircraft as an altimeter, since atmospheric pressure decreases predictably with altitude.
Tools for Measuring Fluid and Differential Pressure
Measuring the pressure of liquids and gases within closed systems, such as pipes or tanks, requires instruments like manometers and mechanical gauges. Manometers are simple, highly accurate devices used for measuring low-range pressures or the difference between two pressures. The simplest form is the U-tube manometer, which is a U-shaped tube partially filled with a liquid like water or mercury.
When one side of the U-tube is exposed to the pressure source and the other to the atmosphere, the liquid level shifts. The height difference between the two columns is measured, providing a reading of the pressure relative to the surrounding atmosphere, known as “gauge pressure.” Differential manometers connect to two separate pressure points to measure the difference between them, which is useful for monitoring flow rates or filtration systems.
For the higher pressures common in industrial and mechanical applications, the Bourdon tube gauge is a ubiquitous tool. This mechanical gauge uses a C-shaped or coiled metal tube that is sealed at one end and open to the pressure source at the other. As the internal pressure increases, the elastic tube attempts to straighten out. This motion is translated through a linkage mechanism to rotate a pointer across a circular dial, providing a direct reading.
Tools for Specialized and Electronic Measurement
Specialized applications, particularly in medicine, rely on unique instruments like the sphygmomanometer, which measures blood pressure. This device consists of an inflatable cuff, a pump or electric mechanism, and a gauge or monitor. The cuff is inflated to temporarily stop blood flow in the artery, and the pressure is then slowly released while a clinician listens for the turbulent blood flow sounds using a stethoscope.
The pressure readings at the first sound (systolic) and the final disappearance of the sound (diastolic) are recorded using the attached gauge, which is often an aneroid type. For modern industrial and research needs, pressure transducers and sensors have become the standard. These electronic devices convert the physical force of pressure into a measurable electrical signal, such as a voltage or current.
Two common types are strain gauge and piezoelectric sensors, both utilizing a diaphragm that deforms under pressure. Strain gauge sensors work on the principle that the electrical resistance of a material changes when it is stretched or compressed. Strain gauges bonded to the diaphragm convert this deformation into a change in resistance, which is electronically measured and translated into a pressure reading.
Piezoelectric sensors use special materials that generate an electrical charge when subjected to mechanical stress. While strain gauges are used for stable, static pressure measurements, piezoelectric sensors excel at measuring rapid, dynamic pressure changes, such as those found in engine combustion analysis.
Understanding Pressure Measurement Units
Interpreting any pressure reading requires understanding the units used, which vary widely across different fields. The internationally recognized standard unit is the Pascal (Pa), defined as one Newton of force per square meter of area. Because the Pascal is a relatively small unit, the kilopascal (kPa), equal to 1,000 Pascals, is frequently used in engineering and for atmospheric pressure measurements.
The unit Pounds per Square Inch (psi) is prevalent in the United States, especially for mechanical applications like measuring tire pressure or hydraulic system pressure. The Bar is another common unit, equal to 100,000 Pascals, and is often used in meteorology and for gauges in scuba diving and industrial systems. Millimeters of Mercury (mmHg) is a historical unit derived directly from mercury barometers. This unit is still commonly used in medicine to express blood pressure readings, such as 120/80 mmHg.