Measurement and Control Basics 3rd Edition (complete book)

electrochemical signal that is proportional to the concentration of CO gas in the ambient air. The resulting electrical signal is temperature compensated. The signal is also amplified by an electronic circuit to drive a frontpanel meter on the instrument for the purpose of indicating the percentage of CO.

Sulfur Dioxide Analyzers

Several types of general-purpose analyzers are available for measuring sulfur dioxide. Most utilize some form of spectrophotometry. Ultraviolet spectrophotometers provide high accuracy and sensitivity. Ranges as narrow as 0 to 100 ppm are encountered, but instruments can also detect concentrations up to 100 percent by volume. Ultraviolet analyzers are capable of fast response, that is, 1 s or less.

Infrared analyzers can also be used to measure sulfur dioxide. The instruments lack the sensitivity and response of ultraviolet devices, but are more versatile and less costly.

Fluorescence analyzers are used for sulfur dioxide monitoring in ranges from 0 to 0.25 ppm and 0 to 5,000 ppm. Such analyzers emit light when exposed to ultraviolet radiation, with an intensity that varies with the concentration of sulfur dioxide.

Nitrogen Oxide Analyzers

Nitrogen oxides are measured with spectral or electrochemical analyzers. Which instrument you select often depends on whether you desire data that show nitric oxide, nitrogen dioxide, or total oxides of nitrogen.

Chemiluminescence instruments are accurate and sensitive. These analyzers respond directly to nitric oxide; nitrogen dioxide must be reduced for detection to be possible. Chemiluminescence occurs when the samples react with ozone. Intensities, which are measured with photomultipliers, are correlated with nitric oxide concentration. Detection ranges vary from 0 to 0.1 ppm and 0 to 10,000 ppm. Instruments can be specified for concentrations as low as 0.5 ppm.

Ultraviolet analyzers are capable of monitoring oxide as well as dioxide. The lower detection limits are only 10 ppm for nitric oxide; therefore, converting the nitric oxide to the dioxide usually raises sensitivity. You should take measurements before and after the oxidation to compensate for initial nitrogen dioxide concentrations.

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Infrared analyzers are sensitive to nitric oxide but not to dioxide. Units are available for measurements from 0 to 1,000 ppm and 0 to 10,000 ppm. Ranges can also be specified for 0 to 1 percent and 0 to 10 percent.

Hydrogen Sulfide Analyzers

Hydrogen sulfide is difficult to monitor accurately and often must be conditioned chemically. Some analyzers expose sample gases to chemically treated paper tape. Hydrogen sulfide reacts with the tape, and the resulting color change is used to infer hydrogen sulfide concentration.

Conventional fluorescence analyzers are also used to monitor hydrogen sulfide, but the hydrogen sulfide is first converted to sulfur dioxide. Automatic titrators are also employed to determine hydrogen sulfide concentrations.

Ultraviolet analyzers respond to hydrogen sulfide, but their sensitivities are low. Polarographic instruments can also be used, but filters must be used to remove unsaturated hydrocarbons.

Analyzer Measurement Applications

We will close the discussion of analyzers with a measurement system application that uses a gas analyzer. A typical SO2 stack analyzer instrument loop is shown in Figure 8-19. The system consists of an SO2 analyzer, a temperature element, and a flow measurement system. The temperature and flow signals are used in this system to obtain the amount of SO2 in pounds per hour. The computation is made in the control unit (AIT). Note that electrical heat tracing has been used on the analyzer sample line to keep the SO2 in the gaseous state.

The analyzer’s operation is based on the absorption of light by the sample gas. Rigidly defined, light is only that narrow band of electromagnetic radiation visible to the naked eye, as discussed earlier. However, in this discussion the term light is used to refer to electromagnetic radiation over the specific wavelengths covered by the analyzer. Wavelengths used for SO2 analysis are in the 280to 313-nm range for the measuring channel and 578 nm for the reference channel.

Figure 8-20 shows a block diagram of a typical SO2 analyzer. The optical system operates as follows: radiation from the light source (A) passes through the sample (B) by flowing through a sample cell. Some light of the measuring wavelength is adsorbed by SO2 in the sample. Light transmitted through the sample is divided by a semitransparent mirror (C) into two beams (D and H). Each beam then passes through its own optical filter

FT 100

Pitot Tube

Figure 8-19. SO2 stack gas analyzer

(E or I). Each filter permits only a particular wavelength to reach its associated phototube (G or K).

Optical filters in one beam permit only radiation at the measuring wavelength (J) to pass through. The measuring wavelength is chosen so that light intensity reaching the photomultiplier tube or phototube (K) varies greatly when SO2 concentration changes.

The optical filter in the second beam permits only light at the reference wavelength (F) to pass through. The reference wavelength is chosen so that light intensity reaching the reference phototube (G) varies little or not at all when SO2 concentration changes. Each phototube sends a current to its logarithmic amplifier (log amp) that is proportional to the intensity of the light striking the phototube. The signal output of the analyzer circuit is the voltage difference produced by the log amps.

If SO2 concentration increases, light arriving at the measuring phototube decreases, as does the measuring phototube current. The reference circuit is unaffected. Since voltage generated in the measuring circuit increases with the drops in phototube current, the output voltage (measuring voltage minus reference voltage) rises with a concentration increase.

This analyzer’s design also provides inherent compensation for changes in overall light intensity. Factors such as light source variations or dirt on the

238 Measurement and Control Basics

Figure 8-20. SO2 analyzer block diagram

cell windows, which affect equally the intensities of both the measuring and reference wavelengths, will change the output voltages to an equal extent. Therefore, these variations have minimal net effect on the difference or the final output voltage.

EXERCISES

8.1Given the following resistance values: (a) 50,000 Ω , (b) 200,000 Ω ,

(c) 250,000 Ω for a given solution, calculate the conductance of each solution.

8.2The [OH– ] ion concentration of an aqueous solution is 10–11. What is the value of the H+ ion concentration and pH? Is the solution basic or acidic?

8.3The specific gravity of a lead-acid cell in a 12-v (six-cell) battery is 1.24. Calculate the no-load voltage of the battery.

8.4Calculate the span in inches of a differential-pressure density instrument if the minimum specific gravity is 1.0, the maximum

specific gravity is 1.25, and the difference in liquid elevation is 50 in.

8.5If the air temperature is 80°F and the atmospheric pressure is 29.92 inHg, what is the maximum moisture content of the air?

8.6What is the frequency of an electromagnetic radiation source that has a wavelength of 100 meters?

8.7Find the photon energy and calculate the number of photons in an EM pulse that has an energy of 1 joule and a frequency of 1 x 109Hz.

8.8What is the intensity of a 1000-w point light source at 10 meters and 20 meters?

8.9What is the maximum wavelength for a resistance change by photon absorption for a CdS semiconductor?

8.10A photovoltaic cell generates 0.3 volts open-circuit when it is exposed to 10 w/m2 of radiation intensity. What is the open-circuit voltage of the cell at 20 w/m2?

BIBLIOGRAPHY

1.Considine, D. M. (ed.). Process Instruments and Controls Handbook, 3d ed., New York: McGraw-Hill, 1985.

2.Foxboro Company. Conductivity Cells. Technical Information, Foxboro, MA: The Foxboro Company, 1962.

3.Foxboro Company. Ion-Selective Measuring Electrodes. Technical Information, Foxboro, MA: The Foxboro Company, 1965.

4.Foxboro Company. Fundamentals of Ion-Selective Measurements. Technical Information, Foxboro, MA: The Foxboro Company, 1972.

5.Foxboro Company. pH Electrodes and Holders. Technical Information, Foxboro, MA: The Foxboro Company, 1979.

6.Foxboro Company. Theory and Application of Electrolytic Conductivity Measurement. Technical Information, Foxboro, MA: The Foxboro Company, 1982.

7.Kirk, F. W., and N. F. Rimboi. Instrumentation, 3d ed. Homewood, IL: American Technical Publishers, 1975.

8.Moore, R. L. Basic Instrumentation Lecture Notes and Study Guide. Volume 2, Process Analyzers and Recorders, 3d ed., Research Triangle Park, NC: ISA, 1982.

9.Quagliano, J. V. Chemistry, 2d ed., Englewood Cliffs, NJ: Prentice-Hall, 1963.

F that is directed along the line. Newton’s second law gives the acceleration of a body as follows:

Suppose the speed increases from v1 to v2 while the body undergoes a displacement d. From standard analysis of motion, we know that

The product Fd is the work (W) done by the force (F) over the distance d. The quantity 1/2mv2—that is, one-half the product of the mass of the body and the square of its velocity—is called its kinetic energy (KE).

The first term on the right-hand side of Equation 9-10, which contains the final velocity v2, is the final kinetic energy of the body, KE2, and the second term is the initial kinetic energy, KE1. The difference between these terms is the change in kinetic energy. This leads to the important result that the work of the external force on a body is equal to the change in the kinetic energy of the body, or

Kinetic energy, like work, is a scalar quantity. The kinetic energy of a moving body, such as fluid flowing, depends only on its speed, not on the direction in which it is moving. The change in kinetic energy depends only on the work (W = Fd) and not on the individual values of F and d. This fact has important consequences in the flow of fluid.

For example, consider the flow of water over a dam with height, h. Any object that falls through a height h under the influence of gravity is said to

244 Measurement and Control Basics

gain kinetic energy at the expense of its potential energy. Let's assume that water with mass m falls through the distance h, converting all its potential energy (mgh) into kinetic energy. Since energy must be conserved, the kinetic energy must equal the potential energy. Therefore,

This equation can be solved for velocity v to obtain the following:

Equation 9-13 shows that the velocity of water at the base of the dam depends on the height (h) of the dam and on gravity (g). Since gravity is constant at about 32 ft/sec 2 or 9.8 m/sec2 on the earth’s surface, the velocity depends only on the height h and not on the mass of the flowing fluid. This is an important property in the study of fluid flow. The following example will illustrate this property.

EXAMPLE 9-1

Problem: A valve is opened on the bottom of a storage tank filled to a height of 4 feet with water. Find the discharge velocity of the water just after the outlet valve is opened.

Solution: The velocity can be found from Equation 9-13 as follows:

v = 2gh

v = 2(32 ft / sec2 )(4 ft) =16 ft / sec

Flow in a Process Pipe

Another example of the relationship between energy and fluid velocity is the flow of fluid in a process pipe of uniform and fixed cross section (A), as shown in Figure 9-1. The differential pressure (∆ P) between the inlet and the outlet causes the fluid to flow in the pipe.

The flow of fluid is maintained by the energy difference between the inlet and the outlet. Let’s find the fluid velocity (v) in terms of the inlet pressure P1 and the outlet pressure P2, assuming no energy loss in the pipe. Since the pipe has a uniform area A, the pressure at the inlet is P1 and the pres-