6.8 CONTINUOUS AIR-QUALITY MONITORS
6.8.8 C ALIBRATION OF C ONTINUOUS M ONITORS
6.8.8.1 Specifications for Continuous Air-Quality Monitors
It is unfortunate that terms used in specifying and calibrating air-monitoring instru- mentation are sometimes ambiguous. Thus, we find the term dynamic dilution being used to mean diluting a flowing gas stream under carefully controlled constant flow conditions to produce an accurately and precisely known concentration. A preferred term would be steady-state dilution, which can be used to describe gas in flow but has none of the connotation of transient, which the term dynamic might imply. To be most useful, specifications for instrumentation where flowing fluids are involved should provide information about the instrument, which would impart to the user a knowledge about both its steady-state and transient performance.
Steady-state calibrations should be made at several levels of concentration over the expected range of concentrations that will be found. Comparison to the accepted standard method, absorption in a bubbler with West and Gaeke analysis for sulfur dioxide, for example, should be made as well. Replicate runs at the several levels of concentration can be compared through statistical analysis using factors such as the multiple correlation coefficient. In addition, the drift of the zero and span or range of the instrument should be low, so that very little error will be introduced into the accuracy and precision of the instrument.
Dynamic response testing can be used to determine transient and frequency response characteristics of continuous monitors. The most complete dynamic description of a system is its transfer function, which is the ratio of the Laplace transformed output signal to the Laplace transformed input signal. Knowing the transfer function means that the output signal can be determined from any given input signal. Frequency response curves are essentially a graphical record of the transfer function and are thus useful to depict the dynamic response of a system.
Transient response curves such as might be provided from a step test are valid only for the particular input function, which produced the curves and are, thus, of lim- ited usefulness. Even more limited in specifying instruments is the rise time and fall time, which provides no information about the shape of the transient curve. The bad news is the difficulty to which the experimenter is put in order to determine the transient characteristics or the transfer function. The good news is that the need for the detailed transient information provided by the transfer function is not usually required.
Additional instrument specifications are concerned with the effect of the ambient conditions imposed on the instrument including interfering substances. A common glossary of terms follows. These terms have been classified according to definition, steady-state specification, dynamic specification, and ambient specification.
6.8.8.2 Steady-State Calibrations
One of the most difficult problems encountered in calibrating air pollution instru- mentation is the production of low-level concentrations, which are in the range
encountered in the atmosphere. There are several suppliers of gas cylinders with prepared and certified low-level gas concentrations.
Most usually, these gases will have to be diluted before use, which poses a prob- lem in maintaining strictly controlled temperatures, pressures, and flow rates. This writer discovered a paradox when using a cylinder of carbon monoxide prepared to a specified low concentration. A small but significant difference between two low lev- els of concentration was found when calibrating a nondispersive infrared analyzer.
The companies supplying the gases were contacted. One company reported that they were sure of their analysis because they used the best instrument for low-level car- bon monoxide concentration determination—a nondispersive infrared instrument.
The net result was that the company was comparing nondispersive infrared analyzer instruments and not conducting a calibration with a true standard. From the author’s viewpoint, this situation is to be avoided.
The permeation tube is a device that can provide low-level concentrations. These tubes are made of a material such as Teflon and contain a compound whose con- centration is desired, sealed into the tube as a liquid. The vapor will then perme- ate through the walls of the tube at a constant rate dependent on the surface area exposed and the temperatures to which the tube is subjected. The basic determina- tion of permeation rate can be made gravimetrically; thus, a permeation tube can become a true standard. Table 6.3 lists some of the gases for which permeation tubes are available.
Tubes of proper length can be used to calibrate gas analyzers at a steady-state flow condition. An apparatus must be prepared to hold the tube in a pure-air or nitrogen stream of known flow rate and at a constant temperature. A tube holder may be made from a standard vacuum trap with the tube in the center well and the outer section filled with glass beads to promote heat exchange. The tube holder is placed in a con- stant temperature bath. Concentration can be varied through a manipulation of pure carrier gas flow rate, temperature, or tube length.
The length of the tube required to produce the desired gas concentration can be estimated from the formula given below:
L CF
= KP
t
(6.2)
TABLE 6.3
Available Permeation Tubes
Sulfur dioxide Propane Nitrogen dioxide Propylene Hydrogen sulfide Butane
Chlorine Butene-1
Hydrogen fluoride Cis-butene-2 Dimethyl sulfide Trans-butene-2 Dimethyl disulfide Ethylene oxide Methyl mercaptan Others
L is the length in centimeters
C is the concentration in parts per million (volume) F is the flowrate of carrier gas in cm3/min
K is the constant dependent upon gas at 1 atm, 25°C—0.382 for SO2; 0.532 for NO2; and 0.556 for propane
Pt is the permeation rate for stated operating temperature
Note that concentration can be varied by changing tube temperature or length or carrier gas flowrate.
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7 Cost Estimating
Cost is the basic and crucial decision-making factor in the selection of air-pollution control equipment. Evaluating costs can be straightforward, as long as there is an understanding as to the objective of the analysis and an appreciation of the time value of money. The classic cost trade-off is between capital and operating costs (see Figure 9.1). This chapter is designed to assist the practicing engineer with cost engineering techniques for estimating capital cost of equipment and for determining the annualized cost of operating air-pollution control systems, such as those required for the cost analysis portion of a best available control technology analysis. It is not intended to cover detailed cost accounting methods that include different types of depreciation and taxes.