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Measurement and Sensing

Measurement and sensing


A fundamental premise of all scientific research is that scientists can make measurements, expressed in precise mathematical terms, of conditions and events. Indeed, some argue that a field of study can only be called a science to the extent that the data being collected can be mathematically measured. These criteria apply to environmental science as they do to other physical and biological sciences.

Measurements serve a number of functions in environmental studies. One is to provide baseline information about certain aspects of the environment . A source of concern to scientists and non-scientists alike is the change that appears to be taking place in the earth's ozone layer. Mounting evidence appears to suggest that, as a result of human activities, the concentration of ozone in the stratosphere seems to be decreasing. That finding is significant because the ozone layer acts as a shield against potentially harmful ultraviolet radiation from the sun.

But how do scientists know that changes are taking place in the ozone layer? Such a conclusion can be drawn only if information exists regarding the "normal" concentrations of ozone in the stratosphere. One purpose of measurements, therefore, is to accumulate a huge volume of information regarding the "normal" condition of the environment.

A second purpose of measuring is to determine how some aspect of the environment may be changing, often as a result of human activities. Our concern about the ozone layer, for example, arises out of readings taken by high-flying aircraft over a period of more than a decade. Those readings show consistent changes (decreases) from the level that is regarded as natural or normal. These levels are not fluctuating, that is, decreasing and regaining normalcy. Rather, there is a trend toward decreased ozone levels.

Dozens of techniques are available for measuring environmental characteristics. Many of these techniques are not unique to environmental science. For example, the use of thermometers to record temperatures is vitally important in many environmental studies. But temperature-taking is not a specialized procedure used by environmental scientists. The measurement of radioactivity is another example. Geiger counters are used to determine the level of radiation in environmental studies just as they are in scientific research, medical applications, industrial processes and other situations.

Over the years, a number of techniques have been developed for measuring specific environmental characteristics. Many of these techniques can be classified according to their use in the measurement of air quality or water quality .

An example of the former is the high-volume sampler (HVS) used in measuring particulates in the atmosphere . An HVS is essentially a modified vacuum cleaner that sucks air into a hose and forces it through a filter paper. The difference in the weight of the paper before and after it has trapped the particulate matter is used to determine the weight of the particulates. Since the total volume of air passing through the HVS can be easily measured, the systems provides of measure of particulates in mass per volume.

Techniques for measuring various components of a gas are also becoming more sophisticated. At one time, for example, the determination of ozone levels was made simply by suspending a piece of natural rubber of known size and weight in the air. Since ozone causes rubber to crack, the speed at which cracking occurred was taken as a measure of ozone levels in the air.

As with most other gases, ozone is now measured by chemical means. A sample of gas is passed through some device, often a "bubbler," that contains a compound that will react with the gas being tested. The concentration of that gas can be measured, then, by determining the amount of chemical change that takes place in the measuring device.

For example, a reaction with which many high school students are familiar can be used to determine the level of sulfur dioxide in the air. Sulfur dioxide reacts with lead dioxide (PbO2) to form a characteristic black precipitate of lead sulfate.

If lead dioxide is added to a bubbler through which a sample of gas is passed, the amount of darkening in the device (due to the formation of lead sulfate) is a measure of the concentration of sulfur dioxide in the gas.

Many environmental measurements now make use of sophisticated chemical techniques. One example is infrared spectrometry. The term spectrometry refers to the measurement of energy absorbed or emitted by various compounds. All molecules are held together by electrons that vibrate with characteristic frequencies. If energy is added to those molecules, they will absorb frequencies of energy that match their characteristic frequencies, but no others. Each kind of molecule can be identified, therefore, by a "map" of the frequencies that it does and does not absorb.

Infrared spectrometry is the most common form of the technique used because most molecules vibrate with frequencies in the infrared region. Techniques such as infrared spectrometry are valuable because they can detect concentrations of a material at much lower levels than can most chemical systems.

The increasing sophistication of measuring techniques does not mean that all simple procedures have been abandoned. For example, one method for measuring the level of air pollution requires no more than good eyesight and a reference card. The reference card contains the Ringelmann scale, a set of six squares that range from pure white to pure black. Each square contains an amount of hatching that corresponds to completely pure air (pure white, no hatching) to badly polluted air (totally black). The four intermediary squares contain increasingly more hatching, constituting equivalents of 20 percent, 40 percent, 60 percent and 80 percent "blackness."

For comparison, opacity (darkening) of air can also be determined by electronic means more precisely. A photometer, for example, is a device that records the amount of light transmitted by a sample of air. The light is converted to an electric current that can be read on a meter. In some industrial and power plants , a photometer is attached to the smokestack to obtain a continuous record of the opacity of the gases being emitted.

The two most common types of air quality measurements are those for ambient air quality and emissions. Ambient air refers to the outdoor air that surrounds us. Ambient air quality measurements provide information on the possible accumulation of harmful compounds such as carbon monoxide , sulfur dioxide, nitrogen oxides , and hydrocarbons .

Emission measurements reveal the level of such compounds being released from a power plant, a factory, or some other source. In many cases, emissions can be studied by simply drilling a hole in a smokestack, extracting a small sample of exhaust gases, and studying their composition by methods described above.

Many biological, chemical, and physical methods are available for measuring water quality. The presence of pathogens in a sample of water can be determined, for example, by standard bacteriological techniques in which a water sample is allowed to incubate for some period of time and the number of bacteria produced counted visually or electronically.

At one time, most water tests were fairly straightforward chemical tests. The concentration of nitrogen in a water sample, for example, can be determined by a standard procedure known as the Kjeldahl test and the amount of chlorination by precipitation with a silver salt. Today, most water tests can be conducted by more sophisticated instrumental techniques. A photometer can be used to compare an unknown water sample with a known standard to determine the concentration of nitrogen, phosphorus , chlorine , or some other component.

One of the most basic measurements of water quality is that of oxygen demand. The more polluted a water sample is, the more organic matter it is likely to contain. The more organic matter, the greater the amount of oxygen the sample will require to decompose the organic material.

Traditionally, the method for determining this characteristic was biochemical oxygen demand (BOD), a test in which a sample of water is studied over a five-day period. To overcome the lengthy time required to conduct this test, modifications such as total organic carbon (TOC) have been developed.

Specialized measuring techniques are sometimes required for particular types of environmental studies. Determining the amount of noise pollution in an area, for example, requires the use of a sound level meter, a device consisting of a microphone, amplifier, frequency-measuring circuit, and read-out screen.

One consequence of the improved technology now available for making measurements is that smaller and smaller concentrations of a substance can be detected. Chemical means can routinely detect the presence of a substance to the level of one part in a thousand or one part per million (ppm). The most advanced technologies today have stretched that sensitivity to levels of one part per billion (ppb) and even one part per trillion (ppt).

The efficiency of these measuring devices poses some new issues for environmentalists. What does it mean, for example, to learn that a toxic substance exists in the soil at a level of one ppb if we know the material is harmful only in much higher concentrations? Does any level of exposure to the substance pose a hazard, or can we safely ignore such a minuscule quantity of the substance?

A field of measurement of increasing importance in environmental studies is remote sensing. The term refers to any method by which an object or an area is studied at some great distance. Most commonly today, the term is used to describe surveys of the earth's surface by satellites orbiting around it.

Remote sensing procedures depend on the fact that various types of materials absorb and reflect solar energy in different ways. Instruments in satellites that can measure these differences can, therefore, detect variations in land and ocean surfaces.

Remote sensing uses three parts of the electromagnetic spectrum: the visible, infrared, and microwave regions. Perhaps the most common form of remote sensing is that which uses photography. Photographic equipment has now been developed to the point where objects of no more than a few yards apart can be distinguished from outer space.

Since some objects and features emit radiation in electromagnetic regions other than the visual, infrared, and microwave techniques are also used. The images obtained from any one of these methods can be further improved by computer enhancement of the original photographs.

Remote sensing has now been used for a number of environmental applications. Some examples include the locating of possible mineral reserves, the tracing of water drainage patterns, the determination of soil moisture, the tracing of plant diseases, the calculation of snow and ice masses, and the measurement of biological productivity in the oceans.

See also Drinking-water supply; Emission standards; Greenhouse gases; National Ambient Air Quality Standards; Ozone layer depletion; Radiocarbon dating; Water pollution

[David E. Newton ]


RESOURCES

BOOKS

McGraw-Hill Encyclopedia of Science & Technology. 7th ed. New York: McGraw-Hill, 1992.

Vesilind, P. A., J. J. Peirce, and R. Weiner. Environmental Engineering. 2nd ed. Boston: Butterworths, 1988.

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