US 3712325 A
A gas mixer useful for production of test gases for blood gas analyzers. The mixer comprises a pair of high-precision pressure regulators, each adapted for receiving a different gas from a supply source. A separate pair of orifice plates are provided downstream from each regulator. The plates of each pair are arranged in parallel. The outlets of one orifice plate of each pair are connected into a pipe to provide a source of mixed gas. The outlets of the other plate of each pair are connected into another pipe to provide a separate source of mixed gas.
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Description (OCR text may contain errors)
United States Patent mi Harnoncourt 1 1 Jan. 23, 1973 GAS MIXER Primary Examiner-Henry T. Klinksiek  Inventor: Karl Harnoncourt, Graz, Austria Atmmey watsn Cole Grmdle Watson  Assignee: AVL AG, Schaffhausen, Switzerland 57 ABSTRACT Filedl y 21, 1971 A gas mixer useful for production of test gases for  APPL No: 145,765 blood gas analyzers. The mixer comprises a pair of A w r high-precision pressure regulators, each adapted for Foreign Application Priority Data receiving a different gas from a supply source. A separate pair of orifice plates are provided May 22, 1970 Austria ..4647/7O downstream from each regulaton The plates of each  U S Cl I 137/597 pair are arranged in parallel. The outlets of one orifice  11/00 p a of ac pa are ne e nto a p pe to provide  Field of Search 597 606 a source of mixed gas. The outlets of the other plate of each pair are connected into another pipe to provide a  Reierences Cited separate source of mixed gas.
UNITED STATES PATENTS 3,650.296 3 1972 Johnson et al ..l37/597 x 4 3 D'awmg F'gures PATENTEDJAN 23 mm FIG./
INVENTOR KARL HARNONCOURT WATSON COLE GRINDLE & WATSON ATTORNEYS GAS MIXER The invention relates to a gas mixer for the production of streams of gases mixed at a predetermined ratio at a constant rate, in particular of test gases for blood gas analyzers.
Conventional gas mixers are operated on the basis of Hagen-Poiseulles law of flow, according to which the rate of flow of a gas passing through a pipe in a laminar stream is proportional to the viscosity of the gas and the effective length of the pipe. Accordingly, in order to keep the flow of gas at a constant level, a laminar current must prevail, and furthermore, both the input and the output pressures must be kept constant, and finally, temperature variations are not admissible since viscosity is to a considerable extent dependent on temperature. Naturally, it is rather difficult to fully meet these exacting requirements. However, for streams of mixed gas in conventional gas mixers used in connection with blood gas analyzers, constancy of the resulting mixture is a must if dependable results are to be obtained from the tests, since these results are known to be greatly influenced by even minor variations of the sampling pressure and of the composition of the test gases.
It is the purpose of the present invention to eliminate the drawbacks of conventional gas mixers. For that purpose, for each component of the gas mixture a system of flow comprising a set of orifice plates preceded by a high-precision pressure regulator and adapted to be impinged upon by the said component of the gas mixtures is provided in a manner known per se, the set of orifice plates of each flow system consisting of two orifice plates arranged in parallel relation to each other at the input end, the outlets of each orifice plate of the existing flow systems terminating in a sampling pipe for the supply ofa current of mixed gases.
In the field of gas chromatography a flow stabilizer for carrier gas is used, by means of which constant gas flows are achieved with the aid of an adjustable throttle followed by a set of four selectively insertable orifice plates. The efficiency of this known system is due to the fact that its operation cannot be impaired by variations of the output pressure of gases passing through the orifice plate at sound velocity, since obstructions are propagated at sound velocity only.
Along similar lines, the present invention provides for the gas flow of each system to be directed in two branch streams through a orifice plate, whereupon one branch stream each of different flow systems are combined to produce a single stream of mixed gases. Consequently, at least two different streams of gases mixed at definite ratios are made available at the outlet of the sampling pipe, constancy of such mixed streams being assured by the maintenance of a constant pressure prevailing in front of the orifice plates alone. Therefore, the gas mixer according to the invention is a particularly useful supplement to blood gas analyzers, the smooth operation of which is contingent upon the presence of two gas streams composed of carbon dioxide and air or carbon dioxide and oxygen, or of two other gaseous constituents mixed at different ratios.
According to a further embodiment of the invention, the orifice plates of all flow systems are mounted on a thermostatized metal block. This makes it possible for variations of ambient temperature, liable to upset the stability of the gas streams, to be eliminated. Where the gas mixer is used as a supplement to a blood gas analyzer, it may be advisable to use the attached thermostatizer for keeping the temperature of the metal block at a constant level.
Furthermore, according to the invention, each orifice plate is preceded by a sintering filter, the pores of which are of about the same size as the holes of the orifice plates. This serves to protect the extremely sensitive orifice plates whose holes measure only a few thousands of a millimeter, positively against impurities.
According to another feature of the invention, the mixing ratio of the gaseous components are adjustable with a high degree of accuracy by means of a highprecision pressure regulator. In this manner, such deviations of the gas composition from the predetermined mixing ratio as may arise owing to inevitable measurement tolerances of the orifice plates, can be compensated by simple means.
Further details of the invention will become apparent from the following description of embodiments of the invention with reference to the accompanying drawings in which FIG. 1 is a schematic drawing illustrating the basic idea of a gas mixer of known design,
FIG. 2 shows the principle of gas flow upon which the present invention is based, and
FIG. 3 is a schematic view of a gas mixer according to the invention.
FIG. 1 is a longitudinal sectional view of a capillary tube 1, wherein a laminar gas flow in the direction indicated by the arrow 2 prevails. The length of the pipe is designated by reference symbol 1. The gas enters the device under a pressure p and leaves the same under a pressure p According to l-Iagen-Poiseulles law, for the flow the relation prevails, wherein v is the flow and 1; the viscosity of the gas.
If the flow v through the capillary tube 1 is to be maintained at a constant level, both the input pressure p and the output pressure p, and also the viscosity 1; of the gas must be kept constant. This is, however, very difficult to achieve. The gas mixer of know design comprises two pairs of capillary tubes, one tube of each pair being impinged upon by pure oxygen and the other tube by pure carbon dioxide. Thus two mixed gas currents of a different composition will be available at the output of the unit, both currents being subject to the aforesaid temperature and pressure variations.
On the other hand, the gas mixer according to the invention achieves a constant gas flow along the lines illustrated in FIG. 2. Reference number 1' designates a pipe wherein an orifice plate 3 is located. When a gas flows through the orifice plate 3 at sound velocity, any influence of the orifice plate output pressure p upon the gas flow is eliminated. In order for the gas inside the orifice plate to attain sound velocity, it will suffice for the condition to be met. The flow speed v is then obtained by means of the simple relation v p -F,
wherein F is the hole size of the orifice plate.
The first relation necessary for the attainment of sound velocity, will always obtain at least in those instances where the emerging gas streams have only atmospheric pressure to overcome. This is the case when, for example, a gas mixer is used for the supply of test gases to blood gas analyzers.
The gas mixer according to the invention shown in FIG. 3 applies the flow principle illustrated in FIG. 2. The device comprises two flow systems 4 and 4, of which the first, designated by reference number 4, is impinged upon by means of compressed air from a gas bottle 5 and the second, designated by reference number 4, by carbon dioxide from a gas bottle 5'. The gas bottles are provided with pressure-reducing valves 6 and 6' respectively, in a manner known per se, followed by filters 7 and 7 respectively.,By means of the pressure-reducing valves 6 and 6' the pressure of the gas emerging from the bottles 5 and 5' is reduced to approximately 6 kgs per square centimeter. Thereupon, the gases are subjected to preliminary cleaning in the filters 7, 7 designed as sintering filters with a silica-gel insert, the latter serving as a means for dessicating the gas so as to avoid corrosion in the attached pipings. In one of the following sintering filters 8, 8 any carryover traces of silica-gel are retained. The gases thus purified and pressure-reduced are then delivered directly to two high-precision pressure-regulators 9 and 9, respectively.
The high-precision pressure regulators 9, 9' designed as proportional regulators and impinged upon under the same input pressure of 6 kilograms per square centimeter maintain the pressure p of both flow systems 4, 4' in front of the orifice plates at a constant level, namely at the pressure set during calibration. The effect of the ambient temperature upon pressure amounts to a mere 0.3 per cent for every 10 degrees Centigrade, and is thus negligible.
The pressure regulator 9 is followed by two orifice plates 10, 11, and the pressure regulator 9 by two orifice plates 12, 13 arranged in parallel relation to each other. The ratio of the hole sizes of these orifice plates 10 through 13 is 17 l7 2 1, resulting in identical flow ratios, provided the pressures prevailing in front of the orifice plates are the same. The orifice plates 10 through 13 are thin metal lamina perforated by means of lazer rays so as to produce holes of approximately 0.001 millimeter. The orifice plates are mounted on a common thermostatized metal block 14 in order to compensate ambient temperature fluctuations.
The outlets of one orifice plate of each of the two flow systems 4, 4' terminate by pairs in a common sampling pipe. The outlets of the orifice plates 11 and 12 and the outlets of the orifice plates 10 and 13 are combined to produce a common sampling pipe 16. Thus a gas stream comprising two component parts and having a predetermined composition is available at the outlets of each of the sampling pipes 15 and 16. For example, assuming that the afore-mentioned ratio of hole sizes for the orifice plates 10 through 13 prevails, the sampling pipe 15 will deliver a test gas containing 5.55 per cent carbon dioxide, and the sampling pipe 16 another test gas containing 10.53 per cent carbon dioxide.
1. A gas mixer for the production of constant streams of gases mixed at a predetermined ratio from different aseous components, particularly for the production of est gases for blood gas analyzers, comprising a flow system for each gaseous component, consisting of a high-precision pressure regulator followed by a set of orifice plates, each set comprising two orifice plates having an inlet and outlet, the inlets of the said two orifice plates being arranged in parallel relation to each other, and each of the said flow system being impinged upon by one of the said gaseous components, the outlets of each orifice plate of the existing flow systems being interconnected, a sampling pipe communicating with each of the said interconnected outlets of the said orifice plates and delivering a stream of gases mixed at a predetermined ratio.
2. A gas mixer according to claim 1, comprising a thermostatized metal block whereupon the orifice plates of all of the aforesaid flow systems are mounted.
3. A gas mixer according to claim 1, wherein a sintering filter precedes each of the said orifice plates, the pores of the said sintering filter being approximately of the same size as the diameters of the holes of the said orifice plates.
4. A gas mixer according to claim 1, wherein the mixing ratio of the afore-mentioned gaseous components is accurately regulated by means of the said high-precision pressure regulator.