Method and device for measuring oxygen partial pressure
O Unlted States Patent 1 1 1111 3,881,152  
 Tasaki Apr. 29, 1975 [5 METHOD AND DEVICE FOR MEASURING 2.744.234 5/1956 Munda&#39;y ct al. 324/36 OXYGEN PARTIAL PRESSURE 3.026.472 3/1962 Greene et al. 324/36 3.612.990 10/197] Dcl Duca 324/36 [76] Inventor: Aku-a Tas kl. 1-3-9 3.612.991 10 1971 Greene 324/36 Yamada-higashi. Suita-shi. Osaka-m&#34; Japan Primary E.\&#39;uminerStanley T. Krawczewicz [22] Filed: Sept. 19, 1973 Attorney. Agent, or Firm-Gardiner. Sixbey. Bradford and Carlson [2]] Appl. No.: 398,859  
  [57] ABSTRACT [52] U.S. Cl 324/36; 73/27 A 51 lm. Cl. G0lr 33/12 The Puma Pressure of Oxygen a gas [58] Field of Search 324/36; 73/27 A termined by measuring the torque acting loons exposed to a non-uniform magnetic field. One of [56] References cued the balloons is filled with a gas comprising a known UNITED STATES PATENTS proportion of oxygen while the other balloon contains the gas mixture. 2.416.344 2/1947 Pauling 324/36 2.666.893 1/1954 Munday 324/36 4 Claims, 2 Drawing Figures 4 4 2 Q 4 11 \g 2 Q 4 I METHOD AND DEVICE FOR MEASURTNG OXYGEN PARTIAL PRESSURE The present invention relates to the measurement of the partial pressure of oxygen in a gas mixture and, more particularly, to a method and device for continuously monitoring the partial pressure of oxygen in a gas mixture by magnetometric means.  
  The need for a simple and sensitive device for measuring and monitoring the oxygen partial pressure of air arises from the situation that in modern, enclosed, airconditioned buildings, the oxygen content of the air in the building can often fall to a dangerously low level because of an insufficient intake of external air. Similar situations arise in subterranean mining, submarines and scientific submersible devices, high altitude aircraft and manned space vehicles. It is known that the human brain function is impaired when the oxygen content in the breathing air falls to a level of 15 to 17 percent from the normal level of 21 percent.  
  The underlying principle of many of the known methods and devices for measuring the oxygen partial pressure of air and other gas mixtures containing oxygen is based on the fact that oxygen is paramagnetic while practically all other gases are weakly diamagnetic. It is not surprising, therefore, that the oxygen partial pressure of a gas mixture has been determined by measuring the magnetic susceptibility of the gas mixture. In most instances, however, such measurements have been very tedious requiring expensive instruments.  
  It is, therefore, the object of the present invention to provide a method and means for continuously and inexpensively measuring and monitoring the oxygen partial pressure of gas mixtures such as air.  
  The magnetic susceptibility of pure oxygen at a pressure of one atmosphere is (0.1434 i 0.0004) X 10 electromagnetic units per cubic centimeter at 20 C. This value changes inversely with the absolute temperature. The force which a non-uniform magnetic field exerts on a paramagnetic substance is given by the formula:  
 where X represents the magnetic susceptibility per gram of the material, m is the mass of the material, and H is the magnetic field strength. The force, F, in this equation is expressed in dynes. The gradient of the magnetic field produced by a simple permanent magnet is of the order of 10 oersted 2/cm. Therefore, the force which acts on the volume of oxygen of IO cubic centimeters (14.3 mg) is, according to the above formula, of the order of 0.1 dyne. This force corresponds approximately to the gravitational force acting on a mass of 0.1 mg. Since the torque or twisting moment measurable with a torsion balance under laboratory conditions is of the order of dyne/cm, the feasibility of utilizing these factors to construct a simple, reasonably accurate device to measure the partial pressure of oxygen in gas mixtures was studied.  
  Accordingly, the objectives of this invention have been achieved by measuring the torque acting on two balloons exposed to a non-uniform magnetic field, one of the balloons being filled with an oxygen-containing gas as reference and the other of the balloons with the gas mixture.  
  The device according to the present invention will be hereinafter described with reference to the accompanying drawings which illustrate, diagrammatically and by way of example only, a preferred embodiment of the present invention. In the drawings:  
  FlG. 1 shows a side elevational view of a device of this invention; and  
  FIG. 2 is a section on line 22 of FIG. 1 and shows the arrangement of the motive portions 10 of the device.  
  ln FIGS. 1 and 2, the motive portions of the invention, 10, are suspended from a support member 1] by means of a crossbar l2 and an adjustment bolt 13 fastened thereto. A tubular shaft 14 is connected to bolt 13 by means of torsion assembly 15. Attached to shaft 14 are arms 16 which horizontally support two balloons or spheres l7 and 19. The balloons are rigidly fixed to shaft 14 by means of arms 16 so that an axis passing through the centers of the balloons is in a plane that is perpendicular to the axis of rotation of torsion assembly l5.  
  Balloon 17 is a sealed reference or comparison balloon. In operation in an air conditioning system, balloon 17 would contain fresh air comprising 21 percent oxygen at a pressure of one atmosphere. In the measuring or monitoring of any other enclosed system comprising oxygen or some other paramagnetic gas, balloon 17 would be filled with the chosen reference gas at conditions of temperature and pressure called for by the particular system.  
  The other balloon, 19, is the analysis balloon and is filled with a gas the oxygen partial pressure of which is sought to be determined. The device shown in the drawings is designed to be used with an air conditioning system and, therefore, balloon 19 is perforated with a number of holes 18 so that its interior is constantly exposed to the ambient air of the particular enclosed system in which it is placed.  
  The two balloons are exposed to a non-uniform magnetic field by positioning the balloons close to the surface of permanent magnet 20. The surface of magnet 20 which faces the balloons may be specially configured, such as with a longitudinal groove or channel (as shown in FIG. 1), to allow optimum exposure to the magnetic field.  
  With reference again to the operation of the device shown in the drawings, as long as the oxygen content of the ambient air remains at the predetermined level of the reference gas, the magnetic forces acting on balloons l7 and 19 are balanced and the balloons will not be caused to rotate. However, when the oxygen partial pressure of the closed environment decreases to a level lower than that of the gas in reference balloon 17, balloon 19 is caused to move away from magnet 20 as a result of balloon 17 being attracted more strongly by the magnetic field. The respective to and fro movement of the balloons results in a small rotation of shaft 14. The actual rotation takes place in assembly 15 which comprises a rigid bar 21 and a fine ribbon or filament 22 which is capable of being twisted along its longitudinal axis. Bar 21 is firmly mounted to bolt 13 and shaft 14 depends from the lowermost end of filament 22. Filaments similar to that utilized in the practice of this in vention are used in the manufacture of torsion balances, torsion galvanometers, torsion electrometers and like scientific and technical instruments.  
  A casing 23 preferably encloses the device of this invention to reduce the chance of error or malfunction caused by even moderate air disturbance in the vicinity of balloons l7 and 19 and torsion assembly 15. The casing is especially required when the device is used in combination with an air conditioning system. However, to provide an accurate and continuous monitoring of the air quality of such a system, the casing must be provided with vents (not shown) to allow an influx and outflow of the ambient air. These vents may, in turn, be provided with baffles or other devices to restrict or eliminate air currents within casing 23 and to effect an even sampling of the ambient air.  
  The rotation of shaft 14 may be measured directly or by remote means. A preferred technique for measuring the rotation is illustrated in FIG. 1 and involves the use of a light beam emanating from a source remote from casing 23. The light beam passes through window 24 mounted in casing 23 and is reflected to photoresponsive device 25 which reads the rotational deviation from a zero point. Device 25 may also automatically actuate a valve, pump, or such other apparatus (not shown) which allows admission of more fresh air or stored oxygen to the closed environment. Reflection of the light beam in the device may be effected by mirror 26 which is directly mounted to shaft 14 so that it rotates simultaneously with it and balloons l7 and 19.  
  The oscillatory movement of shaft 14 is preferably damped, such as by plate 27 attached to the end of shaft 14 and immersed in a container of liquid paraffin 28 appropriately positioned on base plate 29. Any other suitable damping means may be substituted for that shown in the drawings.  
  The zero point of the device can easily be determined since the forces acting on the balloons approach zero as the magnet is moved away from the balloons. Thus, magnet is fixed to platform support 30 which is slidably mounted on base plate 29 so that the magnet may be moved toward and away from the balloons. Platform support 30 may be slidably actuated, for instance, by means of a screw actuator 31 mounted in journal 32. Screw actuator 31 is rotated by means of a worm gear 33 mounted on screw shaft 34 which projects downwardly through base plate 29 and is provided with knob 35 for manual operation.  
  It is to be noted that the sensitivity of the device of this invention can be varied in a wide range by slightly changing the position of magnet 20. By moving the permanent magnet, it is also possible to compensate for changes in the magnetic susceptibility of oxygen associated with temperature changes. However, the effect of temperature on the magnetic susceptibility is negligible, being approximately 3 percent for a change of 10 C.  
  A device according to the present invention has been constructed and installed for testing with the air conditioning unit of a modern, enclosed building. The balloons were made of celluloid and had diameters of 35 mm. Both balloons weighed 2.4 grams after one of them was punctured with about ten holes and the other was filled with fresh air at about one atmosphere. These balloons were affixed to a cupro-nickel tubular shaft at a center-to-center distance of mm. The torsion assembly comprised a copper bar and a phosphorus bronze filament measuring 12 mm long, 0.141 mm wide and 0.015 mm thick. A permanent magnet having the overall dimensions of 12 cm X 7 cm X 5 cm was used. The damping of oscillation was satisfied by using a 18 mm X 12 mm phosphorus bronze plate at the end of the cupro-nickel shaft. During the testing of the above device, the oxygen content of the air in the building was determined with an accuracy of better than 1 percent. It is to be expected, however, that the sensitivity of this device can be increased by a factor up to about 5 by reducing the weight of the balloons and by using a permanent magnet of special design and configuration.  
 What is claimed is:-  
  l. A method for measuring the partial pressure of oxygen in an ambient gas mixture comprising the steps of exposing two balloons to a non-uniform magnetic field, one of said balloons enclosing an oxygen-containing gas as reference, the other of said balloons having a series of perforations to provide a balloon filled with said ambient gas mixture, and said magnetic field acting upon said balloons from the same direction, and measuring the torque acting on said two balloons.  
  2. A method according to claim 1, wherein one of said balloons contains air comprising 21% oxygen and the other of said balloons is filled with the ambient air of an enclosed, air conditioning system.  
  3. A device for measuring the partial pressure of oxygen in an ambient gas mixture comprising a first balloon enclosing an oxygen-containing reference gas, a second balloon having a series of perforations, said perforations providing means for filling said balloon with said ambient gas mixture, torsion means to which said balloons are horizontally mounted, said mounted bal loons being rotatable in a plane perpendicular to the axis of torsion, means for establishing a non-uniform magnetic field through said balloons so that said balloons are acted upon by forces in the same direction, and means co-oper ating with said torsion means for measuring the torque acting on said balloons by reason of said non-uniform magnetic field.  
  4. A device according to claim 3, wherein said first balloon is filled with air comprising 21 percent oxygen and said second balloon is filled with ambient air of an enclosed, air conditioning system.