Abstract:
A magnetic analyzer is provided for detecting the concentration of a paramagnetic gas, such as oxygen, in a gas being tested. This detection is based on detecting the surface-pressure generated between a gas being tested and a comparison gas. The surface-pressure is sensed by a condenser-microphone which is coupled to comparison gas passageways in two pairs of magnetic pole pieces which are inserted in a measuring chamber filled with the gas being tested and which alternately generate a magnetic field.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an improvement on a magnetic analyzer of oxygen, and, in particular, to a magnetic analyzer which can determine the concentration of oxygen in a gaseous sample from the variation of surface-pressure owing to the difference in magnetizing coefficients. 
     2. Description of the Prior Art 
     Magnetic analyzers of oxygen are classified roughly into those of the heat ray type and those which make use of variations in surface-pressure. 
     The former type is an analyzer which makes use of the cooling of heated rays and applies the principle that a magnetizing coefficient of oxygen, which is a paramagnetic gas, is reduced with a rise of temperature in accordance with Curie&#39;s law. That is to say, if heat rays arising from Joule heat or the like are located in an unequal magnetic field formed by a strong magnet in a measuring chamber made of non-magnetic materials, magnetic force is generated with a strength that is in proportion to the content of oxygen. Consequently, heat rays located in a magnetic field are cooled excessively compared with those not located in the magnetic field by virtue of such magnetic forces. On the basis of the above-mentioned principle, the concentration of oxygen in a gaseous sample can be determined by detecting the difference of temperature between these heat rays. 
     Accordingly, in the analyzers of this heat ray type, the measurement is remarkably influenced by the variation of thermal properties of coexistent gases. For example, the thermal conductivity and specific heat when hydrogen and carbon dioxide are present are remarkably different from oxygen alone. Such coexistent gases can be present in great quantities, and their contents are varied at times. Moreover, the analyzers of this type have a defect in that some special safety countermeasures are required against explosion in a case where the concentration of oxygen in explosive gases is measured because the temperature of heat rays is comparatively high. 
     The latter type of analyzer (i.e. the type which makes use of the variation in surface-pressure) is superior to the heat ray type in the above-mentioned points, and a high practical value can be expected for it. 
     The analyzers of the surface-pressure type apply the following principle. As shown in FIG. 1, a minute quantity of non-magnetic gas (for example, pure nitrogen), or a mixture of paramagnetic gases and non-magnetic gases (for example, air), is introduced into a passageway &#34;a&#34; of gases to be measured as a suitable comparison gas through a minute hole 6. This hole 6 is provided on one magnetic pole N in the portion where the magnetic poles N and S are arranged in said passageway &#34;a&#34; of gases to come closest each other so that the strongest magnetic field is formed. 
     In such a case, it has been known that kinetic pressure P, expressed by the following Quinke&#39;s equation, is generated in a vertical direction relative to boundary surfaces &#34;b&#34; and &#34;c&#34; between the comparison gas and the gas to be measured if the gases at said boundary surfaces &#34;b&#34; and &#34;c&#34; have magnetizing coefficients of x 1  and x 2 , which respectively exist in said magnetic field having the strength of H: 
     
         P=K(x.sub.1 -x.sub.2)H.sup.2                               ( 1) 
    
     where K is a constant including elements such as temperature; x 2  is the magnetizing coefficient of the comparison gas; and x 1  is the magnetizing coefficient of the gas to be measured. In a case when the condition of x 1  &gt;x 2  is satisfied, the surface-pressure is generated on the boundary surface between x 1  and x 2  in the direction from x 1  to x 2 . 
     Accordingly, if the strength of said magnetic field H and any one of said magnetizing coefficients x 1  and x 2  are held constant, the magnetizing coefficient of another gas can be determined from the variation of said surface-pressure P. Then the content of a paramagnetic gas, such as oxygen, in the gas to be measured can be determined from the magnetizing coefficient. 
     However, all of the conventional analyzers of the surface-pressure type consist of one pair of magnetic pole pieces which face each other in the measuring chamber at a minute distance and one pair of false pole pieces made of non-magnetic materials having the same shape and size as said magnetic pole pieces. In such analyzers the variation of surface-pressure owing to the difference in magnetizing coefficients has been detected from the pressure inside a passageway of the comparison gas provided on one side of said magnetic pole pieces and the pressure inside a passageway of the comparison gas provided on one side of said false pole pieces which corresponds to said magnetic pole piece mentioned above. Consequently, the variation of surface-pressure owing to the difference in magnetizing coefficients is remarkably small, for example only 2 to 3×10 -1  microbar per 1% O 2 . As a result, the measurement by means of the conventional techniques is quite difficult, and often led to error. 
     This is particularly the case in an analyzer in which the variation of surface-pressure is detected by a condenser-microphone. In such a case, the S/N ratio of the signal introduced in the oxygen content indicating portion is large because the variation of surface-pressure is minute. Also, although the voltage to be loaded on said condenser-microphone should be high in order to transform this minute variation of surface-pressure directly into an electrical output with high accuracy and high speed, it is known that the maximum possible voltage which can be loaded on said condenser-microphone is dependent upon the distance between a fixed pole and a condenser-film which serves as a movable pole, and upon the tension of said condenser-film. If a voltage higher than said maximum allowable voltage is loaded, the condenser-film is bent. Consequently, the distance between poles is shortened to start an electric discharge. In other words, Coulomb force hinders the possibility of increasing the voltage which can be loaded. 
     SUMMARY OF THE INVENTION 
     It is, therefore, the object of the present invention to eliminate the above-mentioned defects of conventional magnetic analyzers of oxygen of the surface-pressure measurement type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the principle of measurement by a magnetic analyzer of oxygen of the surface-pressure measurement type. 
     FIG. 2 shows the construction of one of the preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 2, one of the preferred embodiments of the present invention will be described below. 
     In FIG. 2, two pairs of magnetic pole pieces 2-2&#39;, 3-3&#39; face each other at a distance of a minute gap in a measuring chamber 1 surrounded by the wall 1&#39; made of non-magnetic materials. A strong magnetic field is generated between said magnetic pole pieces 2 and 2&#39; as well as between said magnetic pole pieces 3 and 3&#39; through a connective iron 5, 5&#39; by alternatively passing an electric current through electromagnetic coils 4, 4&#39;. Holes 6, 7 for discharging a comparison gas are provided on the same side magnetic pole pieces 2, 3. 
     A comparison gas introduced from an inlet 8 through a flow-rate adjusting device 9 is divided into said magnetic pole pieces 2, 3 and then introduced into said measuring chamber 1 through said minute gap provided between said magnetic pole pieces 2 and 2&#39; as well as between said magnetic pole pieces 3 and 3&#39;. 
     The gas to be measured is introduced into said measuring chamber 1 through an inlet 10. An outlet is shown by 11. 
     The magnetic pole pieces 2, 3 are provided with pressure introducing passageways 12, 13 opening into said holes 6, 7 for discharging a comparison gas. These pressure introducing passageways 12, 13 open into chambers C a  and C b  which are provided on both sides of a condenser-film 14 in a condenser-microphone 16 consisting of said condenser-film 14 and two fixed poles 15, 15&#39; which face each other on both sides of said condenser-film 14. Said condenser-microphone 16 should have an electrostatic capacity of about 50 PF and the sensibility expressed by the variation of electrostatic capacity for a minute difference in pressures inside of both sides of said condenser-film 14 of ΔC=0.0001.PF. Besides, the fixed poles 15, 15&#39; face each other on both sides of said condenser-film 14, and, consequently, Coulomb forces act in reverse directions to each other between said pole 15 and said condenser-film 14 as well as between said pole 15&#39; and said condenser-film 14. As a result, contrary to the conventional condenser-microphone with a single fixed pole, Coulomb force which hinders the possibility that the voltage to be loaded on said condenser-microphone is heightened can be eliminated. Therefore, a high voltage can be loaded. 
     After the signals taken out from said fixed poles 15, 15&#39; of said condenser-microphone 16 are amplified by the amplifier 17 and 17&#39;, respectively, they are added by a differential amplifier 18 and then are introduced into an oxygen content indicating portion not shown in the figure. 
     The case where pure nitrogen is used as the comparison gas will now be explained as an example. If the gas to be measured is pure nitrogen, the magnetic surface-pressure is not generated. Instead, only the back pressure acts, and the difference between the surface-pressure in the space of said magnetic pole pieces 2-2&#39; and that in the space of said magnetic pole pieces 3-3&#39; is zero because both the comparison gas and the gas to be measured have the same magnetizing coefficient. The minute gaps between said magnetic pole pieces 2 and 2&#39; as well as between said magnetic pole pieces 3 and 3&#39; are adjusted so that the variation of an electrostatic capacity of said condenser-microphone 16 may be zero under the state that said difference of surface-pressures is zero. 
     Then, if the gaseous mixture containing oxygen is introduced into said measuring chamber 1 as the gas to be measured, an electric current is passed through said electromagnetic coil 4 and 4&#39; alternatively. Consequently, strong magnetic fields are alternatively generated in the space between said magnetic pole pieces 2 and 2&#39; and between said magnetic pole pieces 3 and 3&#39;. As a result, surface-pressure is generated in proportion to the difference between the magnetizing coefficient of the gas to be measured and that of the comparison gas alternatively in a minute gap between said magnetic pole pieces 2 and 2&#39; and a minute gap between said magnetic pole pieces 3 and 3&#39;. Accordingly, such alternatively generated surface pressures are transmitted to said condenser-microphone 16 through said pressure introducing passageways 12, 13 to push said condenser-film 14 in the form of a differential pressure between the space between said magnetic pole pieces 2 and 2&#39; and that between said magnetic pole pieces 3 and 3&#39;. 
     Such a surface-pressure is expressed as follows on the basis of the above-mentioned equation (1): 
     
         P.sub.2-2&#39; =K.sub.1 (x.sub.1 -x.sub.2)H.sub.1.sup.2        (2) 
    
     
         P.sub.3-3&#39; =K.sub.2 (x.sub.1 -x.sub.2)H.sub.2.sup.2        (3) 
    
     where 
     P 2-2&#39;   and P 3-3&#39;   are surface-pressures generated between said magnetic pole pieces 2 and 2&#39; and between said magnetic pole pieces 3 and 3&#39;, respectively; 
     K 1  and K 2  are constants including elements such as temperature; 
     x 1  and x 2  are magnetizing coefficients of the gas to be measured and the comparison gas, respectively; and 
     H 1  and H 2  are the strengths of the magnetic fields generated between said magnetic pole pieces 2 and 2&#39; and between said magnetic pole pieces 3 and 3&#39;, respectively. 
     The dislocation of said condenser-film 14 in said condenser-microphone 16 is put out from each fixed pole 15, 15&#39; in the form of the variation of electrostatic capacity between said condenser-film 14 and said fixed poles 15, 15&#39;, respectively. Therefore, if the output signals on the basis of P 2-2&#39;   and P 3-3&#39;   are added by said differential amplifier 18, in a case when K 1  =K 2  =K, H 1  =H 2  =H, x 2  (pure nitrogen)=O the pressure P can be expressed as follows: ##EQU1## where C is the concentration of oxygen in the gas to be measured and X 1  is in proportion to C. K&#39; and   are constants. 
     As understood from the equations (4) and (5), the quantity of the output signal can be doubled by adding by means of said differential amplifier and the concentration of oxygen is indicated on the basis of this doubled signal. 
     Namely, in the present invention, not only a high voltage can be loaded on said condenser-microphone 16, but also the quantity of the output signal can be doubled by adding by means of said differential amplifier. As a result, the noise introduced from the outside as well as the noise due to an alternative current included in the power source can be lowered. Also, the S/N ratio can be remarkably improved. Thus, the concentration of oxygen can be determined with high accuracy on the basis of a minute variation of surface-pressure. 
     As shown above, according to the present invention, the measurement is not affected by the variation of thermal properties of coexistent gases and its stability is high. Besides the abovementioned effects, the following effects are produced: the quantity of the signal accompanied with a minute variation of surface-pressure can be doubled; the noise can be reduced; and the S/N ratio can be remarkably improved. Therefore, the concentration of oxygen can be detected with a high sensibility and with high speed. 
     All of the above-mentioned effects arise from the fact that said surface-pressure as determined on the basis of the difference of magnetizing coefficients is detected by a condenser-microphone consisting of one condenser-film and two fixed poles which are located on both sides of said condenser-film. This determination is of the differential pressure between two pairs of magnetic pole pieces. The thus detected output signals are added by the differential amplifier and then the concentration of oxygen is determined by measuring the quantity of said output signal. 
     It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of this invention. Numerous other arrangements may be readily devised by those skilled in the art which embody the principles of the invention and fall within its spirit and scope.