Gas analyser

Abstract: The invention relates to a gas analyser (2), comprising a test chamber (8) for a gas sample, an inlet (6) for the gas sample and a gas sensor (16) sensitive to gas movement, for determining the concentration of a constituent gas in the gas sample. The inlet (6) is adapted to be connected to an inspiration line (22) of a respirator (18). When a shielding means (12) is arranged so it covers the inlet (6) of the test chamber (8), the gas sample remains relatively still inside the test chamber (8) during the analysis, regardless of the flow of the gas to be analysed. The gas analyser (2) can accordingly be devised for direct connection to the gas to be analysed.

Description: The present invention relates to a gas analyser according to the preamble to patent claim 1. A plurality of different kinds of gas analysers are available for determining the concentration of a constituent gas in a gas sample, e.g. optical, electrochemical and paramagnetic gas analysers. The different analysers have different advantages and disadvantages, depending on the physical principle they employ. Thus, paramagnetic analysers have the disadvantage of being sensitive to gas movements inside the test chamber. This means that the analyser must be located at a distance from the gas line, when measurements are to be made of flowing gas, e.g. as in respirators, and a gas sample must be extracted through a test line with a pump. So every sampling and analysis is encumbered by a time delay. On the other hand, paramagnetic analysers have the advantage of not affecting the gas being analysed. Their performance is also superior, and they are more economical. US 5,285,677 discloses a method and apparatus for selective detection of gases and vapours using paramagnetic and electric dipole moments. It would therefore be advantageous if analyses with paramagnetic analysers, and other analysers sensitive to gas movement, could be performed closer to the flow of gas from which the gas sample is to be taken. One object of the invention is to achieve a gas analyser that resolves the aforementioned problems. Another object is to achieve a gas analyser suitable for use with respirators. The objectives are achieved according to the invention when the gas analyser is devised as is evident from the characterising part of claim 1. Advantageous refinements and designs are evident from the dependent claims to claim 1. Through the use of a shielding means, the gas sample in the test chamber can be kept still, even though the test chamber is directly connected to the flowing gas in the inspiration line. A porous wall, permeable to gas, is sufficient to achieve a gas that is sufficiently still, but a valve providing complete shielding is also possible. Since the gas analyser is adapted be connected to the flowing gas in such a way, a new gas sample can be taken by utilisation of the flow and/or pressure of the flowing gas in the inspiration line. The test chamber's outlet can be connected to the expiratory line. When a valve is used, the gas can be allowed to flow alternately through or remain still inside the test chamber. Embodiments of the gas analyser according to the invention will be described in greater detail below, referring to the figures in which FIG. 1 shows one embodiment of the gas analyser and FIG. 2 shows the gas analyser in an application with a respirator. FIG. 1 shows a gas analyser 2, according to the invention, connected to a gas line 4. A gas, from which a gas sample is extracted for analysis of its concentration of a constituent gas, flows through the line. The gas analyser 2 comprises an inlet 6, a test chamber 8 and an outlet 10. A porous wall 12, permeable to gas, is arranged in the inlet 6 to reduce or prevent gas movements, caused by the gas flowing in the gas line 4, inside the test chamber 8. A valve 14 for retaining the gas sample inside the test chamber 8 is arranged at the outlet 10. After gas analysis the valve 14 is opened to discharge the gas sample through the outlet 10 while a new gas sample is admitted through the inlet 6 at the same time. A sensor 16 for the gas analysis is symbolically designated in the gas analyser 2. The sensor 16 advantageously consists of a paramagnetic sensor or some other sensor sensitive to gas movements inside the test chamber 8. FIG. 2 shows an application for the gas analyser 2 in a respirator 18. The respirator 18 has a gas connector 20 for admitting the gases that make up the breathing gas for a patient. The gases are mixed and carried to the patient through an inspiration line 22. The gas is subsequently carried from the patient through an expiratory line 24, which can contain a flow meter 26 and a valve 28, to atmosphere or an evacuation unit. Respirators of this kind are well known and do not require detailed description in this context. The oxygen content of the breathing gas is important in respirator care, and the gas analyser 2 is directly connected to the inspiration line 22 to extract a sample from it in order to measure that content. The extraction can be passive, based on pressure difference between the inlet 6 and the outlet 10. After the analysis, gas samples are sent to the expiratory line 24. The purpose of this is to avoid influencing volume determinations in the respirator 18. When the test chamber 8 has a small measurement volume, e.g. 2 cm 3 , the gas can be sent directly to atmosphere, or part of the expiratory line 24 after the flow meter 26 or after the valve 28, without any significant impact on volume determinations. However, the outlet 10 can also be connected directly to atmosphere. Pressure gradients in the system during different phases of the respiratory cycle can be utilised to make gas samples flow from the inspiration line 22 in the gas analyser 2 and then (after analysis) from the gas analyser 2 to the expiratory line 24. Alternative embodiments are possible. For example, the wall 12 can be replaced with a valve or a pump, preferably a pneumatic sponge valve, a micro-mechanical valve or a pump. A porous wall, permeable to gas, can be arranged at the outlet. A pump, preferably a micro-mechanical pump, can be arranged at the outlet to pump gas samples out of the test chamber. The most salient feature of the invention is the provision of movement-sensitive sensors in the gas analyser. With a means to shield the gas sample in the test chamber from the influence of flowing gas in the inspiration line, thereby enabling the gas analyser to be located as close as possible to the flowing gas.

Claims: A gas analyser (2), comprising a test chamber (8) for a gas sample, an inlet (6) for the gas sample, a shielding means (12) arranged to cover the inlet (6), a gas sensor (16), sensitive to gas movements, for determining the concentration of a constituent gas in the gas sample and an outlet (10) for discharging the gas sample after analysis, characterised in that the inlet (6) is adapted to be directly connected to an inspiration line (22) in a respirator (18), the outlet (10) is connectable to surroundings at a lower pressure than the pressure of the gas, a valve (14) is arranged at the outlet (10) and the analyser is adapted to passively change the gas sample, wherewith a new gas sample is admitted into the test chamber (8) when the valve (14) is opened. The gas analyser according to claim 1, characterised in that the analyser is adapted to passively change the gas sample once every respiratory cycle. The gas analyser according to claim 1 or 2, characterised in that the outlet (10) is connectable to an expiratory line (24) in the respirator (18). The gas analyser according to any of the above claims, characterised in that the shielding means consists of a porous wall, permeable to gas (12). The gas analyser according to any of the above claims, characterised in that the gas sensor (16) consists of a paramagnetic gas sensor. The gas analyser according to any of the above claims, characterised in that an additional, porous wall, permeable to gas, is arranged so that it covers the outlet (10)

Inventors: CASTOR ROLF, CASTOR, ROLF
Applicants: SIEMENS ELEMA AB, SIEMENS-ELEMA AB
Application year: 1998
Main classification: G01N33
IPC codes: A61M16, G01N27, G01N1, G01N33