Fluid measuring apparatus
BACKGROUND OF THE INVENTION 
     This invention relates to sensitive, fast responding fluid measuring apparatus and, more particularly, to an improved hot wire anemometer. 
     There are two common types of temperature sensors employed in anemometers and chromatography to measure gas characteristics. One type is a thermistor bead having a negative temperature coefficient of resistance. Although high sensitivity can be achieved with a thermistor bead, its response to changes in the gas characteristics is slow due to the relatively large mass of the bead. The other type is an elongated thin electrically conductive wire, called a hot wire, which has a positive temperature coefficient of resistance. Although a hot wire responds quickly to changes in the gas characteristics, it does not generally permit as high a sensitivity as a thermistor bead. 
     In a hot wire anemometer, the hot wire is connected to serve as one branch of an electrical bridge circuit. Current passing through the hot wire heats the wire, thereby increasing its resistance. The hot wire is disposed in an elongated cavity through which the gas to be measured flows and cools the hot wire accordingly. If the type of gas passing through the cavity is known, the resistance change of the hot wire is a measure of the gas flow rate. If the flow rate of the gas passing through the cavity is unknown, the resistance change of the hot wire is a measure of the thermal conductivity of the gas and, hence, the gas type. 
     The longer the hot wire for a given volume of the hot wire cavity or the smaller the volume of the hot wire cavity for a given hot wire length, the greater is the cooling effect per unit of gas flow through the cavity. Therefore, to achieve high sensitivity, the objective is to provide a large ratio of hot wire length to cavity volume. The factors limiting this objective are the restrictions on the overall size of the apparatus, the ability to bore a long, straight cavity having a small cross section in a piece of material, and support for the hot wire so it remains out of contact with the sides of the cavity. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a hot wire is bent in half to extend along the length of an elongated cavity enclosed on its sides. The ends of the wire are supported at one end of the cavity, and the middle of the wire is supported at the other end of the cavity to maintain the two halves of the wire in spaced approximately parallel relationship from each other, and in spaced relationship from the sides of the cavity. Thus, for a cavity having a given length, the length of the hot wire can be doubled and corresponding increase in sensitivity can be achieved. 
     According to another aspect of the invention, the hot wire cavity is formed in a housing that comprises a block of material of high heat conductivity having a first surface and a plate of the material having a matching second surface removably attached in abutment with the first surface. The cavity is a groove formed in the first surface that has an open side enclosed by the second surface. It is possible to machine on a surface of the material a groove that is longer and smaller in cross section than a hole having no open sides bored into the material. Thus, higher sensitivity can be achieved. 
     In the preferred embodiment, the ends of the hot wire are anchored to a printed circuit board fitted in a recess at the end of the hot wire cavity. The middle of the wire is wrapped around the free end of a quartz rod disposed in a chamber at the other end of the hot wire cavity. The rod is deformed to exert tension upon the hot wire and thus absorb its thermal expansion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The features of a specific embodiment of the best mode contemplated of carrying out the invention are illustrated in the drawing, in which: 
     FIG. 1 is a top sectional view of fluid measuring apparatus incorporating the principles of the invention; 
     FIG. 2 is a front sectional view of the apparatus of FIG. 1; and 
     FIG. 3 is a side sectional view of a portion of the apparatus of FIG. 1 illustrating one of the hot wire cavities. 
    
    
     DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT 
     In the drawing, a block 10 and a plate 11 comprise a housing for fluid measuring apparatus. Block 10 and plate 11 are made of a material having high thermal conductivity, such as aluminum or steel to make the apparatus thermally stable. Block 10 has a surface 12 in which straight elongated grooves 13 and 14 each having an open side are formed by machining. A circular printed circuit board 15 is secured to the end of a cylindrical plug 16 that fits in an elongated cylindrical chamber 19 lying on an axis transverse to groove 13. Mutually isolated, electrically conductive L-shaped pads 17 and 18 are formed on the surface of circuit board 15. As illustrated in FIG. 1, pads 17 and 18 each have an arm parallel to the length of groove 13 and an arm perpendicular to the length of groove 13. The perpendicular arms of pads 17 and 18 are longitudinally offset from each other, aligned with groove 13, and spaced apart a distance slightly less than the width of groove 13. A circular printed circuit board 23 is secured to the end of a cylindrical plug like plug 16 in an elongated cylindrical chamber like chamber 19 lying on an axis transverse to groove 14. Mutually isolated, electrically conductive L-shaped pads 25 and 26 are formed on the surface of circuit board 23. Pads 25 and 26 each have an arm parallel to the length of groove 14 and an arm perpendicular to the length of groove 14. The perpendicular arms of pads 25 and 26 are longitudinally offset from each other, aligned with groove 14, and spaced apart a distance slightly less than the width of groove 14. 
     An elongated cylindrical chamber 27 is formed at the other end of groove 13 and an elongated cylindrical chamber 28 is formed at the other end of groove 14. Chambers 27 and 28 lie along axes that are transverse to the length of grooves 13 and 14. Cylindrical plugs 29 and 30 fit in chambers 27 and 28. One end of a quartz rod 31 in chamber 27 is anchored to plug 29 so the other, free end of rod 31 is aligned with groove 13. One end of a quartz rod 32 in chamber 28 is anchored to plug 30, so the other, free end of rod 32 is aligned with groove 14. 
     A thin elongated i.e., uncoiled electrically conductive hot wire 38 is bent in half to extend along the length of groove 13. This permits the length of the hot wire to be doubled without increasing the volume of the hot wire cavity, thereby improving sensitivity accordingly. Since block 10 and plate 11 are made of a good thermal conductor and good thermal conductors are also good electrical conductors, it is necessary to avoid short circuits that the two halves of wire 38 be precisely positioned in groove 13 without contacting each other or the sides of groove 13. The center of hot wire 38 is wrapped around the end of rod 31 and the ends of hot wire 38 are soldered to pads 17 and 18, respectively, such that the two halves of wire 38 are in spaced, approximately parallel relationship from each other and in spaced relationship from the sides of groove 13, as depicted in FIG. 3. The solder connection of the ends of hot wire 38 to pads 17 and 18, respectively, rigidly supports the ends of hot wire 38. Thus, the ends of hot wire 38 are held in fixed relationship longitudinally and laterally with respect to each other and electrically isolated from each other by circuit board 15, including pads 17 and 18 and the solder connections. The free end of rod 31 has a hook 37 (FIG. 2) that prevents the middle of hot wire 38 from slipping off rod 31, and maintains wire 38 in spaced relationship from the bottom of groove 13. Hook 37 permits longitudinal movement of the middle of hot wire 38, i.e., movement parallel to its longitudinal axis, and prevents lateral movement of the middle of hot wire 38, i.e., movement perpendicular to its longitudinal axis. The free end of rod 31 is deformed toward groove 13 sufficiently so hot wire 38 remains in tension over the full range of anticipated thermal expansion. As illustrated in the drawing, hot wires 38 and 39 and grooves 13 and 14 are horizontally oriented. It should be noted there is no problem of the horizontally oriented hot wires sagging, because the hot wires are held in tension by the rods around which their respective centers are wrapped. Thus, rod 31 absorbs all the thermal expansion of wire 38, which insures that the two halves of hot wire 38 do not sag when they expand, thereby contacting each other or the sides of groove 13 and causing a short circuit. Although the lateral position of the middle of hot wire 38 remains fixed, the two halves of hot wire 38 are free to move longitudinally to equalize the tension therebetween during thermal expansion. To place wire 38 in tension, the ends of hot wire 38 could be pulled, to the right as viewed in FIG. 1, thereby bending the free end of rod 31 toward groove 13, while soldering the two halves of wire 38 to pads 17 and 18; thereafter, the ends of hot wire 38 extending beyond pads 17 and 18 could be trimmed. 
     Plugs 16 and 29 can be rotated and translated in chambers 19 and 27, respectively, to make minor adjustments in the alignment of hot wire 38 in groove 13. Hook 37 is displaced from the center axis of plug 29. Thus, when plug 29 is rotated, the middle of hot wire 38 moves laterally relative to the sides of groove 13. When plug 16 is rotated, pads 17 and 18 also rotate to move the ends of hot wire 38 laterally relative to the sides of groove 13. When plugs 16 and 28 are translated, hot wire 38 moves laterally relative to the bottom of groove 13. After the adjustment is complete, set screws, not shown, are tightened to prevent further movement of plugs 16 and 29. 
     A thin elongated electrically conductive hot wire 39 is bent in half to extend along the length of groove 14. The center of hot wire 39 is wrapped around the end of rod 32, and the ends of hot wire 39 are soldered to pads 25 and 26, respectively, such that the two halves of wire 39 are in spaced approximately parallel relationship from each other and in spaced relationship from the sides of groove 14. The free end of rod 32 is deformed toward groove 14 sufficiently so hot wire 39 remains in tension over the full range of anticipated thermal expansion. 
     At one end electrically conductive leads 46, 47, 48, and 49, which would in general be much thicker than hot wires 38 and 39, are soldered to pads 17, 18, 25, and 26, respectively. At the other end, leads 46, 47, 48, and 49 are connected to measuring and recording apparatus 50. By way of example, apparatus 50 could comprise the bridge circuit shown in FIG. 2 of my U.S. Pat. No. 3,735,752, which issued May 29, 1973. The disclosure of this patent is incorporated herein by reference. In any case, apparatus 50 includes an electrical power source that serves to heat hot wires 38 and 39. 
     Plate 11 has a surface 40 that matches surface 12 of block 10. Surface 40 is held in abutment with surface 12 by screws 41, which pass through bores in plate 11 to engage threaded bores in block 10. Surface 40 covers the open sides of grooves 13 and 14, respectively, to form hot wire cavities enclosed on their sides. This construction permits higher sensitivity because a longer and narrower hot wire cavity can be formed by machining a groove along the surface of a piece of material than by boring a hole into the middle of the material. Further, a big advantage of this construction is ease of accessibility to the hot wire cavity to replace or reposition the hot wire in the event of a short circuit. Although surfaces 12 and 40 are preferably flat, to minimize the volume of the hot wire cavities and eliminate the need for hot wire guides in addition to the quartz rods, they could be curved if circumstances dictate. The hot wire cavities could also have a bend or curve if circumstances dictate. 
     Conduits 51, 52, and 53 in plate 11 couple chamber 27, the center of groove 13, and recess 16, respectively, to the exterior of the housing of the apparatus. 
     If the apparatus is employed as a spirometer to measure the flow rate of a patient&#39;s breath, conduit 52 could be connected to the throat of a venturi tube through which the patient&#39;s breath flows. With specific reference to FIG. 3 of my U.S. Pat. No. 3,735,752, conduit 52 could be connected to passage 80 therein. In this case, no conduits connect chamber 28, groove 14, or recess 24 to the exterior of the housing. Hot wire 38 serves to compensate for ambient temperature changes. 
     If the apparatus is employed in the field of chromatography, conduits analogous to conduits 51, 52, and 53 would couple chamber 28, groove 14, and recess 24 to the exterior of the housing. A gas to be detected would be applied to conduit 51 and flow therefrom through groove 13 to conduits 50 and 52 from which the gas would escape to the atmosphere. A known gas, which serves as a reference, would be supplied to the conduit coupled to the center of groove 14 and flow through groove 14 to the conduits coupled to chamber 28 and recess 24, respectively, from which the reference gas would escape to the atmosphere. 
     The described embodiment of the invention is only considered to be preferred and illustrative of the inventive concept; the scope of the invention is not to be restricted to such embodiment. Various and numerous other arrangements may be devised by one skilled in the art without departing from the spirit and scope of this invention. For example, the hot wire could be doubled back on itself a plurality of times to further increase the wire length to cavity volume ratio. This device can also be used as an air gage to measure small displacements.