US 3009097 A
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Unite Stes Patent 3,009,097 METHOD OF OXYGEN DETECTION John P. Strange, Murrysville, Pa., assigner to Mine Safety Appliances Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 19, 1958, Ser. No. 716,165 8 Claims. (Cl. 324-33) This invention relates to, and has for its primary object the provision of, a method for detecting the presence and measuring the concentration of a given gas or vapor as a contaminant in a pure gas or mixture of pure gases, by the use of an ionization chamber, where the contaminant is electronegative and the background gas is nonelectronegative, as those terms are deiined below.
Molecular gases can be classiied as belonging either l) to a type called electronegative, in which the molecules exhibit electron ainity or an ability to pick up free electrons and form negative ions analogous to similar tendencies of certain well known atomic species, eg., O, C1, etc.; or (2) to a type called nonelectronegative, in which the molecules do not exhibit electron ainity and do not form negative ions. This ai'nity for electrons is not dependent on ionization of the gas and is not to be confused with the aiiinity of positive ions of an ionized molecule for electrons. Thus, O2, HC1, SO2, C12 may be classified as examples of electronegative molecules, since they readily attach electrons to form negative molecular ions. On the other hand, He, Ne, A, N2 and H2, if pure, are examples of gases that do not pick up free electrons and are non-electronegative.
It is known that the measured ionization current of a pure non-electronegative gas (such as H2, He2, N2, etc.) consists mostly of positive ions and free electrons. in an ionization chamber containing a static sample of such a gas and a fixed radiation source, the ionization current produced by an applied electrical iield is initially (i.e. immediately after the electrical field is turned on) a function of the number of ions formed, the rate at which they recombine, and the mobility of the charged carriers. Because the electrons are free and because their mobility is high, the initial ionization current is high. Also, because of the high mobility `of the free electrons, they are collected by the positive electrode much more rapidly than the positive ions are collected by the negative electrode. However, in a static ionization chamber, the equilibrium current that is established a short time after the electrical lield is applied is considerably lower than the initial current and is a function, not only of the factors enumerated above, but also of the space charge formed by the low mobility positive ions.
The present invention is predicated on the discovery that providing a continuous ilow of a pure, non-electronegative gas or gases through an ionization chamber will sweep the uncollected positive ions out of the chamber and eliminate Vthe space charge effect produced by such ions, thereby permitting the ionization current in the chamber to be maintained at a high level, approaching the initial ionization current obtained in a static chamber. If a small amount of an electronegative contaminant `gas or ygases is added to the stream of sample gas owing through such a dynamic chamber, the ionization current will be markedly reduced, because the molecules of the contaminant, which are characterized by their electron ainity, will remove or reduce the number of free electrons in the mixture to form low mobility negative ions.
While the present invention is applicable to the detection of minute quantities of any electronegative gas or gases in a non-electronegative carrier gas or gases, it will be described herein, for convenience, in connection with the detection of oxygen in otherwise pure hydrogen.
Suitable apparatus for practicing the method of this invention is shown in the accompanying drawings, in which:
FIG. l shows diagrammatically an ionization chamber and an electrical circuit for measuring changes in the ionization current in accordance with this invention;
FIG. 2 shows part of a detector system using a second ionization chamber as a compensator chamber, with the gas sample arranged for parallel flow through both chambers; and
FIG. 3 is similar to FIG. 2 except that the two chambers are arranged for seri-es ow of the sample gas.
Referring to FIG. l, the detector ionization chamber, generally designated by the numeral 1, includes an outer electrode 2 in the form of a cylindrical tube of electrically conductive material. To the ends of the electrode are tted closure members 3, one of which is provided with an inlet passage 4, for introducing a gas sample, and the other with an outlet passage 5. A pump (not shown) can be used to draw the gas sample through the chamber. An insulated well 6 is mounted on and communicates with the chamber and supports an inner electrode 7, which has an active portion 8 coaxial with the outer electrode. A suitable source of constant alpha radiation,
f such as a small amount of radium, is distributed within the ionization chamber, for example, in the form of a deposit 9 on the inner wall of the outer electrode.
The detector ionization chamber is connected in an electrical circuit that includes the chamber and a fixed resistor 15 of substantially equivalent resistance as series arms of a Wheatstone bridge. A potentiometer rheostat 16 provides the other two arms and permits zero adjustment o-f the bridge. A suitable voltage is applied to the bridge by a battery 17, which is connected through a switch 18 to bridge terminals 19 and 20. Bridge balance or unbalance is measured by a conventional cathode follower electrometer circuit, generally designated by the symbol V, having a very high impedance; and this circuit is connected across the bridge, between bridge terminal 21 and slider 22 of the potentiometer rheostat. The bridge is initially balanced with pure hydrogen, for example, passing through the chamber. If a small amount of oxygen is now added thereto, there will be a sharp decrease in the ionization current in the chamber, which is translated into a voltage diterence in the bridge. The resulting bridge unbalance can be measured directly by the meter M, or by the required adjustment of the potentiometer rheostat slider 22 to bring the bridge in balance. The entire system is very sensitive and gives a measurable response to even slight traces of oxygen.
In order to neutralize background materials in the gas sample, as well as changes in gas pressure, temperature, rate of How, and other ambient conditions, it is frequently desira'ble to provide a second ionization chamber that will act as -a compensator. Such a chamber 25, arranged for parallel ilow of sample gas through the compensator and detector chambers is shown in FIG. 2, in which the electrical circuit is identical with that of FIG. l (and accordingly not shown in full), except that the compensator chamber 25 is substituted for the fixed resistor 15. The compensator chamber is designed to be dimensionally and electrically equivalent to the detector chamber 1. Since the compensator chamber should respond only to changes in the background material and ambient conditions of the sample gas, the oxygen must be removed from the sample before it enters that chamber. This may be done by introducing the sample through an inlet 26, where it is equally divided between conduits 2.7 and 28, the former leading to the detector chamber 1 and the latter leading to an oxygen removal element 29, which is in turn connected to the inlet 30 of the compensator chamber. The oxygen removal element may take a number of forms well known to the art, including chemical means for removing the oxygen, means for physically adsorbing the oxygen, or magnetic means for separating the oxygen by taking advantage of its paramagnetic qualities as compared to those of hydrogen. With the bridge balanced, the addition of oxygen to the sample gas will be reflected by a reduction of the ionization current across the detector chamber, as compared with that across the compensator chamber. Similarly, an increase in the oxygen concentration will be refiected by a greater difference between the ionization currents of the two chamers.
In FIG. 3, the compensator and detector chambers are connected in the electrical circuit as in FlG. 2, but the sample gas is arranged to flow first through the detector chamber and then through the compensator chamber, with the oxygen removal element between the two chambers. Such a series fiow arrangement permits the use of a smaller gas sample then does the parallel flow arrangement, without any reduction in the rate of flow of the sample.
It is among the advantages of this method of gas detection that it is simple to apply, that it is extremely sensitive and accurate in its results, and that it involves the use of relatively inexpensive materials. lt has been used successfully to detect the presence of oxygen in hydrogen, ethylene and methane, where the oxygen ccncentration in the carrier gas was as low as l parts per million. While the invention has been described herein, for convenience, with respect to the detection of oxygen in otherwise pure hydrogen, it will be understood that the same method is equally applicable to the detection and measurement of other electronegative contaminants in an otherwise pure Sample of a non-electronegative gas or gases. For example, the procedure described above for the detection of oxygen in otherwise pure hydrogen is equally applicable to the detection of oxygen in a number of otherwise pure non-electronegative gases, such as helium, nitrogen, argon, neon, ethylene, acetylene, paraffin hydrocarbons, etc. Likewise, the presence of other electronegative gases, such as hydrogen chloride, sulfur dioxide, chlorine, carbon monoxide, etc., may be similarly detected as a contaminant in any of the non-electronegative gases mentioned above. This invention is also applicable to the detection of a contaminant mixture consisting of any two or more electronegative gases in a non-electronegative carrier gas; and the carrier gas may also consist of a mixture of any two or more nonelectronegative gases. In all cases in which a compensator chamber is used, it will be understood that the electronegative contaminant gas or gases to be detected are removed by any suitable means from the gas stream before it enters the compensator chamber, so that only the nonelectronegative gas or gases enter that chamber.
While it is preferable for ionization to take place in the detector and compensator chambers in order that the electrical field across those chambers will be immediately operative to inhibit the recombination of ions and electrons, thereby increasing sensitivity, it will be understood that this invention can also be practiced successfully if the gases are ionized just before entering the electrical field.
The terms electronegative gas, non-electronegative carrier gas, and carrier gas are used in the appended claims to designate a single gas or mixture of two or more gases of the types referred to.
According to the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.
1. The method of detecting minute quantities of an electronegative gas in a mixture of said gas and a nonelectronegative carrier gas that includes the following steps: exposing substantially all of the mixture to a source of ionization to produce positive ions and free electrons, exposing the ionized mixture to an electrical field between two oppositely charged electrodes for collecting electrons at one of those electrodes to produce a measurable electron current while passing the mixture through the electrical field for continuously removing from the field positive ions formed by ionization of the mixture and negative ions formed by the attachment of free electrons to molecules of the electronegative gas, and measuring the decrease in the resulting electron current from that produced under the same external conditions by a moving stream of the carrier gas alone.
2. The method according to claim 1, in which the mixture is exposed to the source of ionization while it is passing through the electrical field.
3. The method according to claim 1, in which the gas to be detected is at least one member selected from a group of electronegative gases consisting of oxygen, hydrogen chloride, sulfur dioxide, carbon monoxide, and chlorine; and in which the carrier gas is at least one member selected from the group of non-electronegative gases consisting of hydrogen, helium, neon, argon, nitrogen, ethylene, acetylene, and parafiine hydrocarbons.
4. The method of detecting the presence and measuring the concentration of an electronegative gas in a mixture of said gas and a non-electronegative carrier gas that includes the following steps: separating the mixture into two moving mixture streams, removing from the first mixture stream the constituent to be detected so as to leave a residual stream portion consisting of the carrier gas alone, exposing separately the residual stream portion and the second mixture stream to equal ionizing sources for ionizing the gases therein to produce positive ions and free electrons, exposing the residual stream portion and the second of the mixture streams to separate electrical fields of substantially equal intensityr defined by pairs of oppositely charged electrodes for collecting electrons on one electrode of each pair of electrodes to produce measurable electron currents across each pair of electrodes while passing the residual stream portion and the second mixture stream through said electrical fields for removing from each electrical field positive ions formed by ionization and negative ions formed by the attachment of free electrons to molecules of the electronegative gas, and measuring the difference in the electron current produced across each pair of electrodes.
5. The method according to claim 4, in which the residual stream portion and the second mixture stream are each exposed to the ionizing source while passing through the electrical field.
6. The method according to claim 4, in which the gas to be detected is at least one member selected from a group of electronegative gases consisting of oxygen, hydrogen chloride, sulfur dioxide, carbon monoxide, and chlorine; and in which the carrier gas is at least one member selected from the group of non-electronegative gases consisting of hydrogen, helium, neon, argon, nitrogen, ethylene, acetylene, and paraffine hydrocarbons.
7. The method of detecting the presence and measuring the concentration of an electronegative gas in a mixture of said gas and a non-electronegative carrier gas that includes the following steps: ionizing the mixture to produce positive ions and free electrons, exposing the ionized mixture to an electrical field between a first pair of oppositely charged electrodes for collecting electrons on one of those electrodes to produce a measurable electron current while passing the mixture through said electrical field for removing from between the electrodes positive ions formed by ionization and negative ions formed by attachment of electrons to molecules of the electronegative gas, then removing from the mixture the constituent that is to be detected so as to leave a residual portion consisting of the carrier `gas alone, ionizing the residual portion of the mixture to produce positive ions and free electrons, exposing the ionized residual portion to an electrical ield between a second pair of oppositely charged electrodes ,producing an electrical reld of the same intensity as that produced by the rirst pair of electrodes for collecting electrons on one of the second pair of electrodes to produce a second measurable electron current while passing said residual portionl through said electrical Ifield for removing `from bet-Ween the second pair of electrodes positive ions formed by ionization, and measuring the difference in the electron currents between the two pairs of electrodes.
8. The method according to claim 7, in which the gas to be detected is at least one member selected from a group of electronegative gases consisting of oxygen, hy-
drogen chloride, sulfur dioxide, carbon monoxide, and chlorine; and in-which the carrier gas is at least one member selected from the group of non-electronegative gases consisting of hydrogen, helium, neon, argon, nitrogen, ethylene, acetylene, and paralline hydrocarbons.
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