MXPA99009406A - Method and device for determining o2 and n2o in mixtures of ga - Google Patents

Abstract

The present invention relates to a process for determining O2 and N2O in gas mixtures by quantitative electrochemical reaction, in which a mixture of gases is fed to a zinc-air cell, the gas cathode being in contact with the gas cathode. the gas mixture and the electrodes of the zinc-air cell are connected to a circuit consisting of diodes and resistors and are adjusted to at least two electrochemical operating ranges, being that the current flowing in the respective range of operation for the indication of the proportion of O2 and N2O in the gas mixture. The invention also relates to a device for carrying out the method, which is constituted by a box (2, 3) that houses the mixture (5) of gases and a cell (1) zinc-air and a circuit connected to the electrodes of the zinc-air cell (1) containing diodes (7, 8) and resistors (9, 10) as well as points (11, 12) of taking and arranging for the measurement and indication of the intensity of the current

Description

METHOD AND DEVICE FOR DETERMINING 02 AND N20 IN GAS MIXTURES Description of the invention The invention relates to a method and a device for determining oxygen (O2) and laughing gas (N20) in gas mixtures by means of a quantitative electrochemical reaction. An electrochemical method for determining poisonous gases in the air is known from US-Re 31,914. In addition, US-A 4,495,051, US-A 4,894,138 and US-A 4,914,424 describe galvanic cells that are used as oxygen sensors. In particular WO 97/35186 discloses a method and a device for determining the proportion of oxygen in the breathing air that is fed to medical breathing apparatus. For this purpose, an alkaline zinc-air cell is used as the sensor. Zinc-air alkaline cells are produced globally in large quantities as button cells, and are used as primary cells in particular in hearing aids. If the supply of oxygen is limited by obstructing the flow and / or diffusion, then the maximum supply current becomes the limit of the current and is then already only a function of the oxygen concentration of the medium which abuts the cathode of 02. In this way, the zinc-air cell is converted into a sensor cell for the measurement of the oxygen concentration or for the measurement of the amount of oxygen when it is used in an enclosed volume. When monitoring the breathing air of patients in anesthesia by laughing gas it was found that it is not enough to only measure the oxygen content, but also it is necessary to control the concentration of the laughing gas. The invention proposed the task of indicating a method and a device for determining oxygen (02) and laughing gas (N20) in gas mixtures, with which oxygen and laughing gas can be simultaneously detrimental in the air of respiration. According to the invention, the task is solved by the method of the type under consideration indicated in claim 1, with the features set forth therein. Further improvements to the method, the device according to the invention and advantageous improvements to the device are indicated in the dependent claims. The adjustment of the electrochemical ranges of operation is carried out by adjusting the voltage ranges. In this, a lower voltage range of 0.4 to 0.8 V and a higher voltage range of 0.8 to 1.5 V are set. The almost simultaneous determination of gas and oxygen is preferably carried out in such a way that to adjust the voltage range lower a diode is temporarily bypassed. The device according to the invention for carrying out the method is constituted by a box that houses the gas mixture and a zinc-air cell, a circuit connected to the electrodes of the zinc-air cell comprising diodes and resistors as well as points of a and arrangements for the measurement and indication of the intensity of the current. Preferably, the circuit comprises a periodically operable bridge switch that is connected in parallel with at least one diode. The circuit is perfected by providing zener diodes as diodes. Extensive experimental investigations have shown that the laughing gas behaves inertly in the oxygen electrode of a zinc-air cell with activated carbon as a catalyst. Only in electrodes of silver or with activated carbon with silver, as well as in the electrodes with platinum as a catalyst, a cathodic reduction of the laughing gas in which nitrogen appears as a product of the reaction is found in the voltage range between 0.45 V and 0.9 V . However, oxygen also reacts in this range. For the solution of the task of detrining the concentration of oxygen and laughing gas in a gas mixture with a cell, the following method was developed. It is determined by the voltage range of a cell with a zinc anode and a gas cathode in alkaline solution. Below a voltage of 0.4 V, an electric current with cathodic development of hydrogen in the gas electrode flows in an inert atmosphere. In the case of the presence of laughing gas, this is reduced cathodically, the current flow ends in approximately 0.9 V. At higher voltages the laughing gas is inert. In the case of the presence of oxygen, it is reduced throughout the voltage range to approximately 1.45 V. From 1.6 V, oxygen is also inert. According to the invention, the simultaneous determination of oxygen and laughing gas in the breathing air supplied to a cell is carried out in such a way that two current measurements are carried out in different voltage windows in succession. A lower voltage window covers the voltage range from 0.4 V to 0.8 V, and a higher voltage window the voltage range from 0.8 to 1.5 V. In FIG. 1 a measurement device according to the invention is schematically represented with a circuit with which two tension windows are produced. According to the figure, a commercial zinc-air cell 1 is located in a fixing receptor support 2. The fixing receptor support 2 is inserted in the gas test chamber 3 only partially represented so that the gas inlet opening 4 of the zinc-air cell 1 is oriented towards the gas test chamber 3. Due to this, the gas mixture 5 which substantially comprises nitrogen, oxygen, laughing gas and carbon dioxide is in contact with the gas cathode 6 of the zinc-air cell 1. The circuit connected to the zinc-air cell 1 comprises diodes 7, 8, preferably zener diodes, and resistors 9, 10. The diodes 7, 8 determine a window of lower tension, which is evaluated in the points 11 of taking . Additionally, a bridging switch 13 is provided. With the bridging switch 13, the diode 8 can be bridged, so that the diode 7 only determines a higher voltage window for the gas cell. As soon as there is presence of oxygen it is possible to read a current in the lower voltage window at the intake points 11 and in the upper tension window at the intake points 12, which in the absence of the laughing gas is the same magnitude in both voltage windows if the condition of the current limit is satisfied. If only the presence of laughing gas exists, then the current only flows in the upper voltage window, the lower voltage window remains without current. If hilar gas and oxygen are present, then different currents flow in both voltage windows, from which by differentiation - for example weighted - the concentrations of oxygen and laughing gas in the gas mixture can be determined. The actuation of the switch 13 allows the change from one voltage window to the other. The measurement arrangement requires a certain time to adjust the stationary current, which is a function of the time constants of the mechanical and electrochemical processes and consequently of the constructive circumstances. If the switch 13 is successively activated and deactivated as a flip-flop with a certain frequency, then a current signal with a direct current amplitude and an alternating current amplitude is produced. The latter is the measure for the proportion of laughing gas.

Claims (1)

1. CLAIMS Method for determining 02 and N20 in gas mixtures by quantitative electrochemical reaction, characterized in that a mixture of gases is fed to a zinc-air cell, being that the electrodes of the zinc-air cell are connected to a circuit consisting of diodes and resistors, which conforms to at least two electrochemical operating ranges, using the current flowing in the respective operating range for the indication of the ratio of 02 and N20 in the gas mixture. Method according to claim 1, characterized in that the adjustment of the electrochemical operating ranges is carried out by adjusting the voltage ranges. Method according to claim 1 and / or claim 2, characterized in that a lower voltage range of 0.4 to 0.8 V and a higher voltage range of 0.8 to 1.5 V are adjusted. Method according to the claim 3, characterized in that a zener diode is temporarily bypassed to adjust the upper voltage range. Device for carrying out the method according to one or more of claims 1 to 4 comprising a box housing the gas mixture and a zinc-air cell, a circuit connected to the electrodes of the zinc-air cell which it contains diodes and resistors as well as points of take and arrangements for the measurement and indication of the intensity of the current. Device for carrying out the method according to claim 6, characterized in that at least one of the diodes is connected in parallel to a bridging switch priodicly operable. Device for carrying out the method according to one or more of claims 4 to 6, characterized in that the diodes are zener diodes.