Pressure
Pressure is a physical force exerted over a surface. Specifically, it is defined as force (F) divided by unit area (A). When expressed relative to a pure vacuum, this quotient is referred to as absolute pressure. When referred to relative to atmospheric pressure it is called gauge pressure. Pressure is an important concept in many chemical and physical processes. For example, many types of reactor vessels and boilers must be monitored to ensure they do not exceed specific pressure limits, lest they rupture. In the study of atmospheric science, an understanding of air pressure, also known as barometric pressure, is critical for making weather predictions.
Pressure is expressed in various units depending on the application. For example, air pressure is usually in terms of atmospheres, torr, or millimeters of mercury. Industrially, pressure is described in terms of pounds per square inch (psi) or kilograms per square meter. In the International System of Units, pressure is given as pascals (Pa) which are defined as 1.0 Newtons per square meter. The centimeter-gram-second system quantifies pressure in dynes per square centimeter.
Historically pressure has been measured with gauges that monitor the displacement of a mechanical element. One of the earliest measuring tools was a liquid-filled tube known as a manometer. Manometers were commonly used by scientists in the 1700s and 1800s and are still used today to calibrate other pressure gauges. A manometer consists of a cylindrical glass U-shaped tube that is partially filled with liquid. One end of the tube is exposed to the process that is generating pressure; the other end may be sealed or left open depending on the design of the instrument. The pressure exerts a force on the surface of the liquid and causes it to move a specific distance which is proportional to the magnitude of the force. The pressure differential can be measured by comparing the height of the liquid to calibrated marks on the side of the tube. Another type of manometer is the inverted bell-type that consists of two inverted U-shaped tubes, or bells, mounted on a balance beam. Each of these bells is located over a pressure inlet tube in such a way that the bell that is subjected to the highest pressure will rise. This type of apparatus is used to measure very small pressure changes such s those found in drying appliances like conveyor dryers and kilns.
Another type of pressure-sensing element is the Bourdon tube, which is a flexible tube made from steel, beryllium copper, and other special alloys. The tube, which is flattened slightly and sealed on one end, is formed into a coil. When the open end is exposed to pressure, the tube uncoils to a degree that corresponds to the magnitude of the pressure. Positive pressure causes the tube to expand and a vacuum (negative pressure as compared to air pressure) causes it to contract. As the coil moves, it displaces a needle or other indicator that points along a pre-calibrated scale to give the pressure measurement. Bourdon tubes have the advantage of being low cost, simply designed, and useful for measuring both high and low pressure. They also have several disadvantages: they are not extremely accurate at low pressures, the mechanical linkage to the indicator needle may require amplification to give a meaningful reading, and process materials may accumulate inside the tube since one end is sealed.
Other important mechanical pressure sensors include the bellows and diaphragm type elements. The bellows employs a coiled spring instead of a coiled tube as the responsive element. When exposed to pressure the spring stretches and moves a needle on a measuring gauge. The diaphragm is a flexible disk made of sheet metal with ridges carved into it. When the disk is exposed to pressure it deforms to one side or the other depending on which side is exposed to the higher pressure. These disks are very sensitive to small changes in pressure and therefore are useful in low pressure measurements as low as 13 Pa (0.1 torr).
Today, mechanical pressuremeasuring devices have been supplemented with electrical sensors and transmitters. Certain devices, like piezoelectric crystals, may directly register pressure changes. These crystals operate on the principle that certain quartz compounds can be cut and aligned in such a way that they will trigger a small electric current when pressure is applied to them. Other electronic sensing devices depend on a mechanical sensor like a bourdon tube or a diaphragm to register a change in pressure, and then they use electronic components to amplify and transmit the signal. The mechanical element transmits the pressure change to the magnetic coil of a transformer which in turn translates the pressure to an electronic signal that which can be relayed to a variety of output devices. Examples of this type of gauge include strain gauges, which measure the change in electrical resistance of a metal wire exposed to pressure via a mechanical sensor such a bourdon tube or a bellows, and capacitive pressure transducers which register changes in capacitance when an elastic element is displaced by pressure.
This is the complete article, containing 838 words
(approx. 3 pages at 300 words per page).
