Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-09-28
2001-11-20
Warden, Sr., Robert J. (Department: 1744)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S400000, C204S431000, C205S793000, C436S152000, C422S097000
Reexamination Certificate
active
06319375
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and apparatus for identifying a component of gas mixture.
2. Description of Related Art
Potentially dangerous gas mixtures may be found in many workplace environments, the dangers including the risk of fire or explosion from combustible gases, oxygen enrichment or deficiency and exposure to toxic gases. These dangers are well known and gas detection instruments are available to detect a wide range of gases. These instruments typically contain one or more gas sensors, which give a proportional electrical response dependent upon the concentration of the gas to be detected. If the concentration exceeds allowed concentration limits, then the instrument will provide an alarm to warn nearby personnel, or it may activate other remedial action, such as increasing, the ventilation. Gas detection instruments for safety applications are broadly divided into two groups, portable instruments, which are designed to be hand held or worn by the user and provide personal monitoring, and fixed instruments, typically wall mounted, which provide area monitoring.
Combustible gases are often characterized by their lower explosive limit, (LEL), which is the minimum concentration of that particular gas in air, which can support combustion. If the concentration is below the LEL, then the gas will not burn without the continued support of an external ignition source. If the concentration of the gas is greater than the LEL, then once ignited, the combustible gas-air mixture will burn, without the need for an external heat source. Indeed, many combustible gas-air mixtures will explode if ignited at concentrations greater than the LEL. At very high concentrations of the combustible gas, there may be insufficient oxygen to support the combustion, and the combustible gas-air mixture will no longer burn. This upper concentration limit for flammability is known as the upper explosive limit (UEL). The upper and lower explosive limits depend on the gas to be detected, as may be seen from the following data, taken from the CRC Handbook of Chemistry and Physics, 68
th
Edition, 1987-1988, CRC Press, Boca Raton, Fla. Limits of Flammability of Gases in Air
Gas
LEL (% vol.)
UEL (% vol.)
Acetylene
HCCH
2.50
80.00
Ammonia
NH3
15.5
27.0
Benzene
C
6
H
6
1.40
7.10
Hydrogen
H
2
4.00
74.24
Methane
CH
4
5.00
15.00
Pentane
C
5
H
12
1.40
7.80
Xylene
C
6
H
4
(CH
3
)
2
1.00
6.00
There are three main types of sensors used to detect combustible gases. For general leak detection, metal oxide, especially tin oxide sensors are used. The electrical conductivity of the metal oxide changes when exposed to the combustible gas at high temperatures. These sensors are rarely used for safety monitoring, since they commonly lack the precision necessary for this application.
Infrared sensors typically measure the absorption of the gas at 2940 cm
−1
(~3.4 &mgr;m), which corresponds to the carbon-hydrogen (C—H) bond stretching frequency. The absorption of the infrared light depends on the number of C-H bonds stretching in the molecule. One of the limitations of infrared detectors is that molecules such as carbon monoxide (CO) and hydrogen (H
2
) do not have an absorbance at or near 3.4 &mgr;m bond, since they do not have any C—H bonds. Even molecules such as acetylene (HCCH) and benzene (C
6
H
6
) which both have C-H bonds often have low sensitivity at 3.4 &mgr;m since the triple bond in acetylene and the aromatic ring in benzene shift the absorbance of the C—H stretch to higher frequency. These effects of molecular substitution on the C—H bond vibration frequency are well known, and can be found in standard texts such as D. H. Williams, I. Fleming, “Spectroscopic methods in Organic Chemistry”, third Edition, McGraw-Hill book Company, Ltd., London, 1980.
The other major type of sensor for combustible gas is the catalytic bead sensor, which measures heat of combustion. The detector bead of a catalytic bead sensor comprises a small platinum coil encased in a ceramic bead containing precious metal catalysts. The combustible gas enters the sensor and travels to the catalytic bead by natural diffusion. The gas is combusted at the bead surface, aided by the catalysts and the resulting release of heat raises the temperature of the bead. This rise in temperature results in an increase in resistance of the platinum coil, which is normally detected using a Wheatstone bridge. Within the sensor, there is usually a second bead, the reference or compensator bead, which is constructed similarly to the detector bead, without the catalyst. The compensator bead comprises one of the other arms of the Wheatstone bridge, and it is used to cancel out any other non-combustion related responses of the beads, such as changes in ambient humidity or thermal conductivity of the gas. The response of the catalytic bead depends primarily on the heat of combustion of the gas and the rate at which the gas can diffuse to the detector bead.
It is common practice to express the concentration of combustible gases as a percentage of the LEL, and thus 2.5% by volume of methane is 50% LEL. The response of catalytic bead sensors is approximately linear over their useful range (0 to 100% LEL), and setting the empirically determined proportionality constant between the output response and the concentration is called calibration. However, the sensitivity to gas varies with the type of gas. Compared to a relative response to 50% LEL of methane of 1.0, the response to 50% LEL pentane is only about 0.5. A more thorough discussion of catalytic bead sensors may be found in the review by J. G. Firth, “Measurement of Flammable Gases and Vapors” in C. F. Cullis, J. G. Firth (Eds.), “Detection and Measurement of Hazardous Gases”, Heinemann, London, 1981.
Many of the commonly encountered toxic gases are detected using amperometric electrochemical gas sensors. A typical electrochemical sensor is usually constructed with two or more electrodes in contact with an electrolyte. The electrode is usually separated from the outside environment by a gas porous membrane, and other diffusion barriers. The gas to be detected enters the sensor and passes through the membrane to the working electrode, where is it either oxidized or reduced; alternatively, the rate of oxidation or reduction of the electrode or another species in electrolyte may be limited by the availability of the toxic gas. The resulting electrical current is proportional to the rate at which the gas is being consumed by the electrode. The output current is therefore usually linearly proportional to the gas concentration, since the response is limited by the rate at which the gas to be detected can diffuse into the sensor.
The nature of the response of the sensor to a toxic gas depends on both the design of the sensor and the nature of the gas. Some gases such as carbon monoxide (CO) and hydrogen (H
2
) are oxidized at the electrode, whereas other gases such as chlorine and nitrogen dioxide are usually reduced at the sensor electrode. While the oxidation of carbon monoxide to carbon dioxide (CO
2
) is a two-electron process, the oxidation of hydrogen sulfide (H
2
S) to sulfuric acid (H
2
SO
4
) is an eight-electron process. Thus, a diffusion limited sensor which responds to both hydrogen sulfide and carbon monoxide will give a stronger response to the hydrogen sulfide, for a given concentration of gas.
The above examples of sensor technology are intended to illustrate that the signal obtained for a combustible or toxic gas depends on both the sensor technology employed, and on the properties of the individual gases. This fact poses a quandary for personnel who risk being exposed to a variety of different gases. If they use a broad band sensor, i.e. a sensor that is sensitive to a wide variety of gas types, then there is the Ahrisk that the alarm levels will not be appropriate for any given gas. However, if they instead decide to use sensors selective for a particular gas, then there is the risk that if an unanticipated hazardous gas is present, then
Dennison, Scheiner Schultz & Wakeman
Industrial Sceintific Corporation
Olsen Kaj K.
Warden, Sr. Robert J.
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