Barometry

Summary

Barometry, the science and engineering of pressure measurement in gases, takes its practitioners far beyond its original realm of weather prediction. The sensors used in barometry range from those for the near vacuum of space and the small amplitudes of soft music to those for ocean depths and the shock waves of nuclear-fusion explosions.

Definition and Basic Principles

Barometry is the science of measuring the pressure of the atmosphere. Derived from the Greek words for "heavy" or "weight" (baros) and "measure" (metron), it refers generally to the measurement of gas pressure. In gases, pressure is fundamentally ascribed to the momentum flowing across a given surface per unit time, per unit area of the surface. Pressure is expressed in units of force per unit area. Although pressure is a scalar quantity, the direction of the force due to pressure exerted on a surface is taken to be perpendicular and directed onto the surface. Therefore, methods to measure pressure often measure the force acting per unit area of a sensor or the effects of that force. Pressure is expressed in newtons per square meter (pascals), in pounds per square foot (psf), or in pounds per square inch (psi). The pressure of the atmosphere at standard sea level at a temperature of 288.15 kelvin (K) is 101,325 pascals or 14.7 psi. This is called 1 atmosphere. Mercury and water barometers have become such familiar devices that pressure is also expressed in inches of water, inches of mercury, or in torrs (1 torr equals about 133.3 pascals).

The initial weather-forecasting barometer, the Torricelli barometer, measured the height of a liquid column that the pressure of air would support with a vacuum at the closed top end of a vertical tube. This barometer is an absolute pressure instrument. Atmospheric pressure, or barometric pressure, is obtained as the product of the height, the density of the barometric liquid, and the acceleration because of gravity at the Earth's surface. The aneroid barometer uses a partially evacuated box with spring-loaded sides that expand or contract depending on the atmospheric pressure, driving a clocklike mechanism to show the pressure on a circular dial. This portable instrument was convenient for mountaineering, ballooning, and mining expeditions to measure altitude by the change in atmospheric pressure. A barograph is an aneroid barometer mechanism adapted to plot a graph of the variation of pressure with time, using a stylus moving on a continuous roll of paper. The rate of change of pressure helps weather forecasters to predict the strength of approaching storms.

The term "manometer" derives from the Greek word manos, meaning "sparse," and denotes an instrument used to measure the pressure relative to a known pressure. A U-tube manometer measures the pressure difference from a reference pressure by the difference between the height of a liquid column in the leg of the U-tube connected to a known pressure source and the height of the liquid in the other leg exposed to the pressure of interest. Manometers of various types have been used extensively in aerospace engineering experimental test facilities, such as wind tunnels. The pitot-static tubes used to measure flow velocity in wind tunnels were initially connected to water or mercury manometers. Later, electronic equivalents were developed. Inclined tube manometers were used to increase the instrument's sensitivity in measuring small pressure differences amounting to fractions of an inch of water.

Background and History

In 1643, Italian physicist and mathematician Evangelista Torricelli proved that atmospheric pressure would support the weight of a thirty-five-foot water, column leaving a vacuum above that in a closed tube, and that this height would change with the weather. This is considered the first measurement of atmospheric pressure and the basis for all later barometry. Later Torricelli barometers used liquid mercury to reduce the size of the column and make such instruments more practical.

The technology of pressure measurement has evolved gradually since then, with the aneroid barometer demonstrating the reliability of deflecting a diaphragm. This led to the use of electrical means to measure the amount of deflection. The most obvious method was to place strain gauges on the diaphragm and directly measure the strain. Later electrical methods used the change in capacitance caused by the changing gap between two charged plates. Such sensors dominated the market until the 1990s at the low end of the measurement range. Piezoresistive materials expanded the ability of miniaturized strain gauge sensors to measure high pressures changing at high frequency. Microelectromechanical system (MEMS) technology enabled miniaturized solid-state sensors to challenge the market dominance of diaphragm sensors. In the twenty-first century, pressure-sensitive paints allowed increasingly sensitive and faster-responding measurements of varying pressure with very fine spatial resolution.

How It Works

Barometry measures a broad variety of pressures using an equally broad variety of measurement techniques, including liquid column methods, elastic element methods, and electrical sensors. Electrical sensors include resistance strain gauges, capacitances, piezoresistive instruments, and piezoelectric devices. The technologies range from those developed by French mathematician Blaise Pascal, Greek mathematician Archimedes, and Torricelli to early twenty-first century MEMS sensors and those used to conduct nanoscale materials science.

Pressures can be measured in environments from the near vacuum of space to more than 1,400 megapascals (MPa) and from steady state to frequencies greater than 100,000 cycles per second. Sensors that measure with respect to zero pressure or absolute vacuum are called absolute pressure sensors. In contrast, those that measure with respect to some other reference pressures are called gauge pressure sensors. Vented gauge sensors have the reference side open to the atmosphere so that the pressure reading is with respect to atmospheric pressure. Sealed gauges report pressure with respect to a constant reference pressure.

Where rapid changes in pressure must be measured, errors due to the variation of sensitivity with the rate of change must be considered. A good sensor is one whose frequency response is constant over the entire range of frequency of fluctuations that might occur. Condenser microphones with electromechanical diaphragms have long been used to measure acoustic pressure in demanding applications, such as music recording, with flat frequency response from 0.1 cycles per second (hertz) to well over 20,000 hertz, covering the range of human hearing. Pressure-sensitive paint in certain special formulations has been shown to achieve excellent frequency response to more than 1,600 hertz, but only when the fluctuation amplitude is quite large, near the upper limit of human tolerance. Using digital signal processing, inexpensive sensors can be corrected to produce signals with frequency response quality approaching that of much more expensive sensors.

In the 1970s, devices based on the aneroid barometer principle were developed. In this principle, the deflection of a diaphragm causes changes in electrical capacitance that are then indicated as voltage changes in a circuit. In the 1980s, piezoelectric materials were developed, enabling electrical voltages to be created from changes in pressure. Microdevices based on these largely replaced the more expensive but accurate diaphragm-based electromechanical sensors. Digital signal processing enabled engineers to use the new small, inexpensive devices to recover most of the accuracy possessed by the more expensive devices.

Applications and Products

Barometry has ubiquitous applications, measured by a wide variety of sensors. It is key to weather prediction, measuring the altitude of aircraft, and measuring blood pressure to monitor health. Pressure-sensitive paints enable measurement of surface pressure as it changes in space and time. The accuracy of measuring and controlling gas pressure is fundamental to manufacturing processes.

Weather Forecasting. Scientists learned to relate the rate of change of atmospheric pressure to the possibility of strong winds, usually bringing rain or snow. For example, if the pressure drops by more than three millibars per hour, winds of up to fifty kilometers per hour are likely to follow. Powerful storms may be preceded by drops of more than twenty-four millibars in twenty-four hours. If the pressure starts rising, clear, calm weather is expected. However, these rules change with regional conditions. For instance, in the Great Lakes region of the United States, rising pressure may indicate an Arctic cold front moving in, causing heavy snowfall. In other regions, a sharply dropping pressure indicates a cold front moving in, followed by a quick rise in pressure as the colder weather is established. As a warm front approaches, the pressure may level out and rise slowly after the front passes.

Modern forecasters construct detailed maps showing contours of pressure from sensors distributed over the countryside and use these to predict weather patterns. Aircraft pilots use such maps to identify safe routes and areas to avoid. Using Doppler radar wind measurements, infrared temperature maps and cloud images from satellites, and computational fluid dynamics, modern weather bureaus can issue warnings about severe weather several hours in advance for smaller local weather fronts and storms and several days ahead for major storms moving across continents or oceans. However, the number of weather-monitoring pressure sensors available to forecasters is quite inadequate to issue accurate predictions for minor weather changes, particularly when predicting rain or snow.

Electrical Gauges. Gauges operating on the electrical changes induced by the deflection of a diaphragm are used in industrial process monitoring and control where computer interfacing is required. Unsteady pressure transducers come in many ranges of amplitude and frequency. Piezoresistive sensors are integrated into an electrical-resistance bridge and constructed as miniature self-contained, button-like sensors. These are suitable for high amplitudes and frequencies, such as those encountered in shock waves and explosions and transonic or supersonic wind tunnel tests. Condenser microphones are used in acoustic measurements. As computerized data-acquisition systems became common, but pressure sensors remained expensive, pressure switches enabled dozens of pressure-sensing lines connected through the switches to each sensor to be measured one at a time. This required a long time to collect data from all the sensors, spending enough time at each to capture all the fluctuations and construct stable averages, making it unsuitable for rapidly changing conditions. Inexpensive, miniaturized, and highly sensitive solid-state piezoelectric sensors and fast, multichannel analog-digital converters have made it possible to connect each pressure port to an individual sensor, vastly reducing the time between individual measurements at each sensor.

Aircraft Testing. Water and mercury manometers were used extensively in aerospace test facilities, such as wind tunnels, where banks of manometers indicated the distribution of pressure around the surfaces of models from pressure-sensing holes in the models. Pressure switches connecting numerous pressure-sensing ports to a single sensor became common in the 1970s. In the 1990s, inexpensive sensors based on microelectromechanical systems technology enabled numerous independent sensing channels to be monitored simultaneously.

Sphygmomanometers for Blood Pressure. Other than weather forecasting, the major common application of pressure measurement is in measuring blood pressure. The device used is called a sphygmomanometer. The high and low points of pressure reached in the heartbeat cycle are noted on a mercury manometer tube synchronized with the heartbeat sounds detected through a stethoscope.

Bourdon Tubes for Household Barometry. The Bourdon tube is a pressure-measuring device in which a coiled tube stretches and uncoils depending on the difference between pressures inside and outside the tube and drives a levered mechanism connected to an indicator dial. Diaphragm-type pressure gauges and Bourdon-tube gauges are still used in most household and urban plumbing. These instruments are highly reliable and robust, but they operate over fairly narrow ranges of pressure.

Pressure-Sensitive Paints to Map Pressure Over Surfaces. So-called pressure-sensitive paints (PSPs) offer an indirect technique to measure pressure variations over an entire surface, using the fact that the amount of oxygen felt at a surface is proportional to the density and, thus, to the pressure if the temperature does not change. These paints are luminescent dyes dispersed in an oxygen-permeable binder. When illuminated at certain ultraviolet wavelengths, the dye molecules absorb the light and move up to higher energy levels. The molecules then release energy in the infrared wavelengths as they relax to equilibrium. If the molecule collides with an oxygen molecule, the energy gets transferred without the emission of radiation. Therefore, the emission from a surface becomes less intense if the number of oxygen molecules being encountered increases. This occurs when the pressure of air increases. The observed intensity from a painted surface is inversely proportional to the pressure of oxygen-containing air. Light-intensity values at individual picture element (pixel) are converted to numbers, compared with values at some known reference pressure, and presented graphically as colors. Typically, an accurate pressure sensor using either piezoelectric or other technology is used for reference. By the 2010s, pressure-sensitive paints had reached the sensitivity required to quantify pressure distributions over passenger automobiles at moderate highway speeds, given expert signal processing and averaging a large number of images.

Smart Pressure Transmitters for Automatic Control Systems. Wireless pressure sensors are used in remote applications, such as weather sensing. Modern automobiles incorporate tire pressure transmitters. Manifold pressure sensors send instantaneous readings of the pressure inside automobile engine manifolds so that a control computer can calculate the best rate of fuel flow to achieve the most efficient combustion. Smart pressure transmitters incorporating capacitance-type diaphragm pressure sensors and microprocessors can adjust their settings remotely, perform automatic temperature compensation of data, and transmit pressure data and self-diagnosis data in digital streams.

Nuclear Explosion Sensors. Piezoresistive transducers have been developed to report the extremely high overpressure, as high as sixty-nine megapascals, of an air blast of a nuclear weapon, with the microsecond rise time required to measure the blast wave accurately. One design uses a silicon disk with integral diffused strain-sensitive regions and thermal barriers. Another design uses the principle of Fabry-Perot interferometry, in which laser light reflecting in a cavity changes intensity depending on the shape of the cavity when the diaphragm bounding the cavity flexes because of pressure changes. This sensor has the response speed and ruggedness required to operate in a hostile environment where there may be very large electromagnetic pulses. In such environments, a capacitance-based sensor or piezoelectric sensor may not survive.

Extreme Applications of Barometry. The basic origins of pressure can be used to explain the pressure due to radiation as the momentum flux of photons. At Earth's orbit around the Sun, the solar intensity of 1.38 kilowatts per square meter causes a radiation pressure of roughly 4.56 micropascals. Solar sails have been proposed for long-duration missions in space, driven by this pressure. Close to the center of the Earth, the pressure reaches 3.2 to 3.4 million bars. Inside the Sun, pressure as high as 250 billion bars is expected, while the explosion of a nuclear-fusion weapon may produce a quarter of that. Metallic solid hydrogen is projected to form at pressures of 250,000 to 500,000 bars.

Careers and Course Work

Because barometry is so important to so many industries and branches of scientific research, most students who are planning on a career in engineering, other technological jobs, or the sciences must understand it.

Modern pressure-measurement technology integrates ideas from many branches of science and engineering derived from physics and chemistry. The pressure-measurement industry includes experts in weather forecasting, plumbing, atmospheric sciences, aerospace wind-tunnel experimentation, automobile engine development, the chemical industry, chemists developing paint formulations, electrical and electronics engineers developing microelectromechanical sensors, software engineers developing smart sensor logic, and the medical community interested in using barometry to monitor patients' health and vital signs.

Pressure measurement is, therefore, a subject in courses offered in schools of mechanical, chemical, civil, and aerospace engineering. Numerous other related issues are also addressed in specialized courses in materials science, electronics, atmospheric sciences, and computer science.

Social Context and Future Prospects

Instruments measuring pressure, normal and shear stresses, and flow rate from numerous sensors are becoming integrated into computerized measurement systems. In many applications, such sensors are mass-produced using facilities similar to those for making computer chips. Very few ideas exist for directly measuring pressure, as it changes rapidly at a point in a flowing fluid without intrusive probes. If nonintrusive measurements become possible, they could help to understand the nature of turbulence and assist in a breakthrough in fluid dynamics.

Pressure measurement is difficult inside flame environments, where density and temperature fluctuate rapidly. Better methods of measuring pressure in biological systems, such as inside blood vessels, would significantly benefit diagnosing heart disease and improving health.

In the mid-2020s, pressure-measurement systems were still too expensive to allow sufficient numbers to be deployed to report pressure with enough spatial and time resolution to develop a real-time three-dimensional representation. Research in this area could improve the resolution and response and reduce costs. With more pressure sensors distributed worldwide, weather prediction would be more accurate and reliable. Digital barometers have become an essential tool in barometry and are now found in many smartphones. These barometers use atmospheric pressure data that can help make the smartphone's navigation systems more accurate. Scientists also use digital barometers in smartphones to make more accurate weather forecasts by crowdsourcing data from phones that have downloaded apps to help collect this information. With the development of the Internet of Things, smart devices increasingly integrate barometers, creating a network of weather and environmental data. Machine learning algorithms and data analytics interpret this data, discovering patterns and creating predictive models. 

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