Method for estimating a mole fraction in a gas mixture

Abstract

A method for estimating the mole fraction of two components (gas 1 and gas 2) in a gas mixture, components which are neutral gases, the gas mixture being a gas mixture with N constituents (N>2), the method comprising the implementation of the following measures: the mole fraction of constituents 3 to N in the mixture is evaluated by the use of sensors specific to each of these gases implementing commercially available methods on the gas analysis market; the pressure and temperature of the gas mixture are measured; the speed of sound in the gas mixture is measured; and the mole fractions of the two neutral gas components (gas 1 and gas 2) in the mixture are deduced from the previous measurements.

Description

Title of the invention: Method for estimating a molar fraction in a gas mixture

[0001] The present invention relates to the field of neutral gases having medical applications and the administration of which is carried out by inhalation in ventilation.

[0002] A ventilator that administers breathable air or oxygen may be required to operate with a different gas source. This may, for example, be a gas mixture containing a molar fraction of oxygen that allows the patient to remain alive, as well as another gas that completely or partially replaces the nitrogen normally contained in the ambient air.

[0003] The mixture at the source can be well controlled at the production stage, but its delivery to the patient's lungs can experience hazards.

[0004] Thus, by way of illustration, when ventilation is carried out using a face mask, there may be leaks from the mask to the outside or inlets from the outside to the inside of the mask. If air enters the mask, the entry of air consisting of approximately 21% oxygen and 79% nitrogen may modify the composition of the medical gas mixture administered.

[0005] In order to ensure the composition of the mixture inspired by the patient, the analysis of a sample in the mixture of gases expired by the patient gives an indication of what he has inspired. Indeed, the portion of neutral gas once this gas is absorbed by the body is expired in an outgoing mass flow equivalent to that entering. A part of the oxygen is absorbed and replaced by carbon dioxide at the outlet.

[0006] Similarly, water is present in the exhaled gas due to the humidity in the lungs.

[0007] If we disregard water in the following evaluation, we can then be in presence of a quaternary mixture (O2, CO2, N2, neutral gas) if the initial source is a binary gas mixture (O2, neutral gas) and an air inlet into the mask is effective.

[0008] It is known that certain neutral gases can have anesthetic properties such as Xenon, while other gases can act on the viscosity of the mixture such as helium and allow better transport of the breathed gas into the lungs despite abnormally high pathological hydraulic resistance of the bronchi in certain conditions.

[0009] Other neutral gases such as argon, neon, krypton can be of therapeutic interest if their asphyxiating effect is cancelled out by a proportion of oxygen added to the mixture.

[0010] However, neutral gases present a difficulty linked to the fact that it is difficult to measure their molar fraction in a mixture. Indeed, their chemical neutrality makes their detection by a chemical sensor complex. Methods such as gas chromatography or mass spectrometry allow this but are expensive.

[0011] It is therefore understood that it is almost necessary to attempt to obtain an indirect measurement of this neutral constituent by other means.

[0012] Thus when the exhaled gas mixture comprises four constituents of type (O2, CO2, N2, and neutral gas), the molar fraction of oxygen can be measured, the molar fraction of carbon dioxide can also be measured, it then remains to determine a molar fraction of nitrogen and a fraction of the other neutral gas. The sum of the two can be deduced by deduction of the other measurements, but their individual proportion must be deduced by means of an additional measurement.

[0013] The present invention proposes to provide a solution to the problem developed above, by using the speed of sound in the quaternary mixture, to deduce this proportion.

[0014] As will be seen in more detail below, the present invention makes it possible to estimate the molar fraction of two components (gas 1 and gas 2) in a gas mixture with N constituents (N>2), from the measurement of the molar fractions of constituents 3 to N by (N-2) sensors specific to each of these gases (gases which can be described as “measurable” by methods available on the gas analysis market) and from the measurement of the speed of sound, the pressure and the temperature in the gas mixture.

[0015] The installation may, optionally, advantageously include a gas dryer to remove moisture from the gas.

[0016] Thus, thanks to the present invention, when the gaseous components 1 and 2 are gases whose concentration measurement is difficult or even “impossible”, for example because they are neutral gases and there are no chemical reactors allowing them to react with them, then the speed of sound makes it possible to distinguish the proportion of one and the other.

[0017] It is known that the measurement of sound in a gas mixture is simple to carry out and can be carried out by a suitable sensor, for example a probe excited by a piezoelectric element whose frequency varies according to the speed of sound. It is also possible to place two sound probes opposite each other at a known distance and measure the transit time of the sound between the two sensors after emission of a sound signal from one probe to the other.

[0018] As an illustration, consider the case of a gas mixture emitted by a ventilated patient who is treated with a mixture of oxygen and argon, a mixture which may contain oxygen, CO2 emitted by the patient, water, nitrogen if his mask leaks and argon. As explained above, the oxygen concentration can easily be measured by various commercially available sensors (electrochemical sensor, paramagnetic sensor, zirconia probe sensor, etc.), while the CO2 concentration can easily be measured by an infrared sensor. The water portion of the gas mixture can be measured by hygrometry, but the water can also be simply removed from the gas sample to be analyzed by standard drying means.

[0019] The molar fractions of nitrogen and argon, on the other hand, cannot be measured directly and simply.

[0020] The pressure can be measured by a pressure sensor, the temperature can be measured by a temperature probe and the speed of sound in the gas mixture can be measured by a sound speed sensor.

[0021] It is possible to theoretically estimate the speed of sound in a gas mixture from the knowledge of its constituents and their molar fractions and by knowing the pressure and temperature of the mixture. The following formula can be used:

[0022] IV .p'; L ? y -,

[0023] where c is the speed of sound of the mixture, Cp the heat capacity at constant pressure of the mixture, Cv the heat capacity at constant volume of the mixture, P the pressure of the mixture, p the density of the mixture.

[0024] Cp, Cv, and p can easily be obtained from the characteristics and the molar fractions of each constituent as soon as we have a measurement of the pressure and the temperature. It is then sufficient to numerically search for the molar fractions of nitrogen and argon which will make it possible to obtain the speed of sound which is measured by the sensor and we thus obtain an estimate of the desired concentrations.

[0025] The present invention then relates to a method for estimating the molar fraction of two components (gas 1 and gas 2) in a gas mixture, components which are neutral gases, the gas mixture being a gas mixture with N constituents (N>2), the method comprising the implementation of the following measurements:

[0026] - the molar fraction of constituents 3 to N in the mixture is evaluated by using sensors specific to each of these gases implementing methods commercially available on the gas analysis market;

[0027] - measurements of the pressure and temperature of the gas mixture are available;

[0028] - a measurement of the speed of sound in the gas mixture is carried out;

[0029] - we deduce, from the previous measurements, the molar fractions of the two neutral gaseous components in the mixture.

Claims

Claims

1. Method for estimating the molar fraction of two components (gas 1 and gas 2) in a gas mixture, components which are neutral gases, the gas mixture being a gas mixture with N constituents (N>2), the method comprising the implementation of the following measurements: - the molar fraction of the constituents 3 to N in the mixture is evaluated by the use of sensors specific to each of these gases implementing methods commercially available on the gas analysis market; - measurements of the pressure and temperature of the gas mixture are available; - a measurement of the speed of sound in the gas mixture is carried out; and - the molar fractions of the two neutral gas components (gas 1 and gas 2) in the mixture are deduced from the preceding measurements.

2. A method according to claim 1, characterized in that one of said two neutral gases is nitrogen, while the other of said two neutral gases is a gas included in the group consisting of xenon, helium, argon, neon, and krypton.