WO1992007256A1

WO1992007256A1 - Sensor for measuring the concentration of oxygen in a gas

Info

Publication number: WO1992007256A1
Application number: PCT/FR1991/000815
Authority: WO
Inventors: Michel Billon; Eric Martin; Michel Corazza
Original assignee: Universite De Rennes 1
Priority date: 1990-10-18
Filing date: 1991-10-17
Publication date: 1992-04-30
Also published as: FR2668264B1; FR2668264A1

Abstract

The invention relates to sensors for measuring the concentration of oxygen in a gas. More precisely, it concerns a transducer for measuring the concentration of oxygen by using the latter's paramagnetic properties. According to the invention, the sensor is of the type which uses the paramagnetic properties of oxygen to measure the pressure caused by the attraction of oxygen in a measurement volume (30) subjected to a constant magnetic field, the sensor comprising a pressure-measuring element (39) incorporating a piezoelectric cell (34).

Description

Sensor for measuring the concentration of oxygen in a gas.

The field of the invention is that of sensors for measuring the concentration of oxygen in a gas.

More specifically, the present invention relates to an oxygen concentration measuring transducer exploiting the paramagnetic properties of oxygen.

In particular, oxygen concentration measurements in gases are carried out in hospitals for the dosing and regulation of anesthetic mixtures, for the breathing of infants, or in industry for the control of industrial processes.

The sensors for measuring the gaseous oxygen content are grouped into two main families, depending on whether their operation is based on physical or chemical principles.

In the family of sensors for the physical measurement of the concentration of oxygen in a gas, there are known sensors using oxygen paramagnetism.

Oxygen is the most magnetic of all known gases. A soap bubble filled with oxygen is attracted by a magnet, while filled with a non-magnetic gas like nitrogen, or less magnetic than oxygen, the soap bubble is on the contrary repelled by oxygen of the environment that surrounds it.

The paramagnetic properties of oxygen are for example exploited in a sensor developed by the company KLOGOR (protected name) described in an unpublished French patent application. This request relates to a device for physically measuring the concentration of oxygen in a gas mixture. The device is composed of a movable assembly consisting of a swing ended by two hollow balls filled with a non-paramagnetic gas, the swing being mounted free in rotation between two pole pieces of soft iron, forming the air gap of a magnetic field generator. When the area of the air gap is placed in the gas mixture whose oxygen concentration is to be known, an induction field is generated between the pole pieces. The paramagnetic character oxygen produces a concentration of dO ₂ molecules in the air gap, driving out the balls from the moving assembly. More specifically, the swing is rotated under the effect of the torque resulting from the repulsive action exerted on the balls, caused by the increase in local pressure. In this sensor, the measurement is carried out using the principle of galvanometers with a moving frame, that is to say by placing a mirror on the connecting rod of the moving element and by measuring the angle deviation of an incident beam. directed at the mirror.

This known device works satisfactorily, but nevertheless poses a certain number of functional problems:

- The moving part has a certain inertia, so that it is necessary to wait a certain time before obtaining a measurement of the oxygen concentration. Not having an indication of this concentration quickly can have serious consequences, especially if the device is used to analyze a gas mixture breathed by a person under anesthesia or by an infant.

The same applies if this type of device is to be used to control an alarm signal, indicating for example a rapid drop in oxygen concentration in a mixture breathed by a patient. - The device has a low sensitivity, because the volume of the balls is limited.

- it must be frequently calibrated and tared and has a certain fragility. This fragility does not, for example, make it possible to embark on board ambulances, where the risks of shock are numerous. In addition, their assembly is delicate and involves watchmaking technology. This technology leads to fragility, high manufacturing cost and unstable settings.

- it is ill-suited to cases where the pressure differences are very large.

There are also chemical type sensors (reaction, electrochemical or catalytic combustion) for measuring oxygen concentration. Electrochemical sensors, in particular with lead electrodes, have the disadvantage of degrading rapidly in the presence of a high oxygen content, and their response time is very long (of the order of 10 to 30 seconds for a time d establishment of the measurement at 90%). Catalytic combustion sensors at high temperature (450 ° C) have the disadvantage of not being linear in the case of high oxygen contents. In addition, certain constituents of the mixture to be analyzed can decompose at operating temperature and pollute the sensor.

The present invention aims in particular to overcome these various drawbacks.

More specifically, one of the objectives of the present invention is to provide a new type of physical sensor for the concentration of oxygen in a gas mixture which is at the same time rapid, precise and stable.

Another objective of the present invention is to provide such a sensor which has a cost price much lower than existing physical sensors.

An additional objective is to provide such a sensor whose sensitivity is high and independent of the oxygen concentration.

These objectives, as well as others which will appear subsequently, are achieved by means of an oxygen concentration sensor in a gas, of the type exploiting the paramagnetic properties of oxygen by measuring a pressure caused by the attraction. oxygen in a measurement volume subjected to a constant magnetic field, said sensor comprising a pressure measuring element comprising a piezoelectric cell.

Advantageously, said measuring element consists of a pressure piece returned to the air gap of a magnetic field generator and cooperating with said piezoelectric cell.

Preferably, said pressure piece covers in section a large part of the air gap of said magnetic field generator.

In this way, the sensitivity of the device is increased, by generating on the pressure piece a force all the greater as the surface on which is exerted. local pressure is important.

Preferably, said piezoelectric cell consists of a bar resting on two support points, said pressure piece transmitting the pressure stresses on said bar between said two support points. This type of mounting allows the sensor to operate for a wide range of pressures.

Advantageously, said bar is constituted by a bimetallic strip formed by two piezoelectric layers bonded in opposition.

Advantageously, said constant magnetic field is generated by two poles of soft iron or other suitable material, forming an air gap.

According to an advantageous embodiment of the present invention, said magnetic poles are asymmetrical with respect to the direction of the magnetic field lines generated by said poles.

Preferably, an anti-magnetic screen is arranged between said piezoelectric cell and said poles.

Preferably, the measurement of said concentration of oxygen in said gas is carried out by measuring the difference in frequencies with respect to the resonance frequency of said piezoelectric cell, or by direct measurement of the charge induced at the terminals of said piezoelectric cell. However, if the oxygen pressure around the pressure piece is constant or very slightly variable, the voltage across the terminals of the piezoelectric cell decreases and tends towards 0 by an electrical depolarization phenomenon.

This is why, according to another embodiment, the piezoelectric cell consists of a bar resting on a single fulcrum and carries at its end the non-magnetic pressure piece. By vibrating the piezoelectric cell around its resonant frequency, the pressure piece sees its position vary with respect to the induction curve, in rhythm with the vibration frequency.

By placing the pressure piece in the center of the air gap and exciting it using a "carrier" frequency signal, equal to the resonance frequency of the piezoelectric cell, a quasi-instantaneous differential pressure p is obtained on the two faces of the pressure piece, of the form: p = k. Bo. Δ B. c where Bo is the average magnetic induction, ΔB is the variation of magnetic induction between the two extreme positions of the pressure piece, and c is the oxygen concentration.

If we consider that x = X.sin ώr.t is the law of displacement of the pressure piece around its equilibrium position where X is the amplitude of the displacement, ώr is the resonance pulsation, then ΔB = f (x) and p becomes p = k'j_.c + non-linearity terms The signal at the terminals of the piezoelectric cell is therefore the sum of two signals: - the signal due to the carrier frequency the useful signal The measurement oxygen concentration is then effected by amplitude demodulation of the signal collected.

Preferably, this demodulation is carried out digitally. Other characteristics and advantages of the present invention will appear on reading the following description of a preferred embodiment of the present invention, given by way of explanation and without limitation, and of the appended drawings, in which:

- Figure 1 shows the principle of paramagnetism of oxygen used in the present invention;

- Figure 2 shows the principle of direct piezoelectricity used in the present invention;

- Figure 3 shows a bimetallic strip on which is exerted a pressure force; - Figure 4 shows a section of a preferred embodiment an oxygen concentration sensor according to the present invention;

- Figure 5 shows a section of a second embodiment of an oxygen concentration sensor according to the present invention.

Figure 1 shows the principle of oxygen paramagnetism used in the present invention.

A magnetic field is created by two poles of magnets N and S located along an axis Z. The poles N and S are distinct and separated by a gas mixture containing oxygen. The intensity of magnetic induction | B | is maximum on the Oz axis, between the poles.

As oxygen is paramagnetic, B and the magnetic field vector H are collinear, and verify the relation: B = μ.H = μ ₀ (1 + X) .H where: - μ ₀ is the magnetic permeability of the vacuum

- μ is the magnetic permeability of the medium

- X is the magnetic susceptibility (positive for paramagnetic bodies).

Perpendicular to Oz, in a direction Ox, we isolate a volume 1 of length dl and surface ds representing an element of volume V = ds.dl. If M is the magnetic moment of the element of volume 1, the force projected according to Ox and applying to volume 1 is equal to: F = M | δB / δx | = XHV | SB / δx \ = X.5 (B ² ) .V / 2.μ ₀ .δx

A pressure PI on the left and a pressure P2 on the right are therefore applied to the volume element 1. We therefore have:

| F | = (Pl - P2) .ds

- _* • →

The elementary work of the force F in displacement dl is given by: dw = F.dl = (PI - P2) .ds.dl for dx = dl

(PI - P2) .ds.dl = X.5 (B ² ) .V.dx / 2.μ ₀ .δx Or under certain conditions: (PI - P2) = Xd (B ² ) /2.μ ₀ If the volume element 1 moves from 0 to infinity, we have: PI = P and P2 = 0, hence

P = XB ² /2.μo, with X = 2.10 " ⁵ , μ ₀ = Î ^ .IO ^{" 6} and P = 8.B ²

If we take for example B ≈ 6500 Gauss = 0.65 Wb / m ² , we obtain P = 3.4 Pa. Thus, if we take a magnetic induction of 0.65 Wb / m ² , the cell being filled with pure oxygen, a local pressure difference of 3.4 Pa is obtained. In order for the oxygen molecules to be attracted to the pole region, the magnetic induction must not be uniform.

FIG. 2 represents the principle of direct piezoelectricity used in the present invention.

The direct piezoelectric effect is the phenomenon by which electric charges appear on the faces of certain crystals when they are subjected to pressures or mechanical tensions, as shown in FIG. 2.

Two parameters characterize the material 10 in the direction of application of a force F:

- compliance s, inverse of the dΥoung y module,

- the charge constant d representing the deformation obtained at a given electric field. Clues specifying the direction of the excitations and responses are not reported, knowing that they are responses projected in the direction of the excitations.

By neglecting the transverse effects, one can write that the relative compression e due to the force F is equal to: e = σ / y with σ = F / π.a ² where: - a is the radius of the disc 10 - 1 is the thickness of the disc 10 - σ is the longitudinal stress due to F

- U is the potential difference between the two equipotciiiic sides of the disc. Now, according to the formula of l pman, we have: e = dU / 1 We thus obtain: σ.s ≈ Fs / jr.a ² = dU / 1 i.e.: U = (Fl / jr.a ² ) .s / d

To calculate the induced charge, it is necessary to know the static capacity Cg of the sample 10. If e ₀ is the peπnittivity of the vacuum (= 8.85.10 ^"12 F / m) and e _r the relative peπnittivity of the material, we have:

The charge Q = C _Q .U is equal to:

Q = Fe ₀ .e _r .s / d (1) Under certain boundary conditions, the piezoelectric voltage constant g depends on d and e according to the relation: g = à / o / e.

By reporting g in relation (1), we therefore obtain:

Q = F.s / g According to this relation, the charge Q observed does not depend on the shape of the sample 10, but only on the parameters s and g of the material and the excitation force F.

The following table includes some characteristics of certain materials that can be used as piezoelectric elements:

The numerical values above locate the limit of this assembly, the induced value being very weak since Q = Fs / g. The calculations were made for low frequencies far from the resonant frequency of the ceramic.

The pressure can be measured directly across the piezoelectric element, but it is of course also possible to detect a pressure by working at a frequency close to the resonant frequency of this element.

It is also possible to envisage using a stack of two glued strips, as shown in FIG. 3.

FIG. 3 represents a bimetallic strip on which a pressure force is exerted. The piezoelectric element 21 consists of two plates glued so that the polarizations are reversed. The element 21 is held fixed by one of its ends in a frame 20. An excitation force F applied to the free end of the bimetallic strip induces a charge Q on the electrodes, proportional to the force F. For a material of the type P x E - 5, for example, the value of the load is given by the relation:

Q = 4.10- ¹⁰ .l ² .h ² C / N and that of the output voltage by: U - = 2.10- ² .l ² hl _t w VN where: - 1 is the free length

- h is the total thickness of the bimetallic strip

- l _t is the total length of the bimetallic strip

- w is the width of the bimetal strip The value of the capacity is deduced by the relation Q - = C.U.

The following numerical example will make it possible to better understand the operation of the assembly described.

The characteristics of the bimetallic strip are as follows: - 1 = 30 mm - h = 0.6 mm

- w - = 1.6 mm

- l _t = 35 mm For an applied force F = 0.58.10 N, we obtain:

- Q = 0.58 pC

- U = 310 μV

If two elements are used in series, the potential difference is equal to 620 μV.

FIG. 4 represents a section of a preferred embodiment of an oxygen concentration sensor according to the present invention.

This sensor uses the paramagnetism of oxygen and the piezoelectricity phenomenon previously described. The sensor shown has a volume 30 for measuring the concentration of oxygen in a gas mixture. Two magnet poles 31 and 32 fixed on a magnetic circuit 33 generate a constant magnetic field in the measurement volume 30. A piezoelectric element 34 fixed between two anti-vibration supports 35 and 36 is placed parallel to the field lines of the measurement volume 30. A two-part anti-magnetic screen 37, 38 separates the magnetic circuit 33 and the magnets 31 and 32 from the piezoelectric element 34. A non-paramagnetic interface part 39 is fixed to one of the faces of the piezoelectric element 34 and centered on the axis of symmetry 40 of the magnets 31 and 32. The sensor shown is mounted on a support or frame 41,42. The presence of a flow of gas containing oxygen in the measurement volume 30 where a high magnetic field prevails, causes, due to the parama¬ gnétisme of oxygen, a displacement of the interface part 39 not pa ¬ ramagnetic in the direction of the piezoelectric element 34. This displacement generates a pressure force on the piezoelectric element 34 and electric charges appear on this element. As the element 34 is characterized by a certain capacity, it follows that a potential difference corresponding to the pressure exerted is measurable across the terminals of the piezoelectric element 34.

Using the previous numerical example, if for 100% oxygen the potential difference is 1 volt full scale, the transfer coefficient of the bimetal strip is: 1 / (2 x 0.58) = 0.86 V / pC.

It is therefore quite possible to use a bimetallic strip as described, since there are amplifiers whose sensitivity is greater than 10 V / pC.

The piezoelectric element 34 is preferably constituted by a bimetallic strip fixed at its two ends, that is to say having two support points.

Of course, it is also possible to use a type of piezoelectric element using another configuration of structural deformation, for example by means of a bar embedded at only one of its ends.

Thus, according to Figure 5, the oxygen concentration sensor according to the invention has a structure similar to the embodiment shown in Figure 4 except as regards the mounting of the piezoelectric cell and the pressure element. According to FIG. 5, the sensor has a piezoelectric cell 43 fixed on a single anti-vibration support 44. A non-paramagnetic interface piece 39 is fixed to the end of the piezoelectric cell 43 between the magnets 31 and 32.

This embodiment is more particularly suitable for detecting small variations in pressure and therefore in oxygen concentration by vibrating the pressure piece 39 at a carrier frequency equal to the resonance frequency of the piezoelectric cell 43. The signal collected at the terminals of the piezoelectric cell consists of the sum of two signals:

- the signal due to the carrier frequency;

- the signal due to the useful frequency.

The measurement of the oxygen concentration is then carried out by amplitude demodulation of the signal collected. This demodulation step is preferably carried out digitally.

The magnet poles 31 and 32 are preferably made of soft iron. One can also consider generating the magnetic field in the measurement volume using electromagnets, or any suitable electronic device. According to a preferred embodiment of the present invention, the magnetic field generated will be as high as possible. The interface piece 39 is preferably non-paramagnetic and can for example be diamagnetic. This part preferably covers a large part of the surface of the piezoelectric element, in order to allow a very precise measurement of the concentration of oxygen in the gas analyzed. The anti-magnetic screen has the function of avoiding a disturbance of the pressure measurement of the interface piece 39 on the piezoelectric element 34. It is advantageously made of Permalloy, but other materials ensuring such shielding are suitable also.

Figures 4 and 5 show poles of magnets 31 and 32 asymmetrical with respect to the direction of the magnetic field lines generated by the poles

31 and 32, due to the asymmetry of the assembly shown. It is of course possible to carry out the present invention using two symmetrical poles, for example in the form of cones, and two piezoelectric elements each located on either side of the poles. The sensor according to the present invention is particularly intended for measuring the oxygen content of a gas mixture circulating in a pipe. This is why the measurement volume comprises means of connection (not shown) to such a pipe.

As already specified, the voltage across the terminals of the piezoelectric element is proportional to the concentration of oxygen in the gas analyzed. The rapidity of the response to a change in concentration dO ₂ is only limited by the duration of the flow of the gas in the measurement volume and the response time of the piezoelectric element. The device according to the invention has a response time of the order of a few tens of milliseconds for an establishment time of 90% (for a nominal flow rate of 90 ml / min). It also has a sensitivity of

0.1% in the whole range of possible variation of oxygen concentration (0 to 100%).

The response time of the bimetallic strip can be estimated by knowing its resonant frequency. In the case of the bimetallic strip described with reference to FIG. 3, its resonant frequency is given by: t. = 400.h / l ² = ^400.0,6.10 ^3/900 = 266 Hz.

We thus arrive at a response time of the order of 50 ms.

It should be noted that the passage of gas through the measurement volume results in a reduction in the flow rate due to the pressure drop. There is then a variation in local pressure and thereby disturbance of the oxygen concentration measurement. To overcome this drawback, it is therefore necessary to work at a constant flow rate. However, since the assembly according to FIG. 4 does not have a moving part, the pressure drop can be considered constant within a certain range, and it is easy to implement a correction of the measured voltage value. The present invention has the particular advantage of not requiring any moving part, such as for example the movable frame in the prior art described. This characteristic makes it very fragile and therefore very easily movable. In addition, the sensor has a long service life and its construction is simplified. The very low consumption of the sensor, of the order of a few mA, and its compactness recommend it particularly for applications with limited space.

Claims

1. Oxygen concentration sensor in a gas, of the type exploiting the paramagnetic properties of oxygen by measuring a pressure caused by the attraction of oxygen in a measurement volume (30) subjected to a magnetic field. that constant, characterized in that it comprises a pressure measuring element (39) comprising a piezoelectric cell (34).

2. Sensor according to claim 1 characterized in that said measuring element consists of a pressure piece (39) recalled in the air gap (31,32) of a magnetic field generator and cooperating with said piezoelectric cell stick (34).

3. Sensor according to claim 2 characterized in that said pressure piece (39) covers in section a large part of the air gap (31,32) of said magnetic field generator.

4. Sensor according to any one of claims 1 to 3 characterized in that said piezoelectric cell consists of a bar (34) resting on two support points (35,36), said pressure piece (39) transmitting the pressure stresses on said bar (34) between said two support points

(35.36).

5. Sensor according to claim 4 characterized in that said bar (34) is constituted by a bimetallic strip formed by two piezoelectric layers bonded in opposition.

6. Sensor according to claim 1 characterized in that said constant magnetic field is generated by two poles (31,32) forming an air gap.

7. Sensor according to claim 6 characterized in that said poles

(31,32) are asymmetrical with respect to the direction of the magnetic field lines generated by said poles (31,32).

8. Sensor according to any one of claims 6 and 7 characterized in that an anti-magnetic screen (37,38) is arranged between said piezoelectric cell (34) and said poles (31,32).

9. Sensor according to any one of claims 1 to 8 characterized in that the measurement of said oxygen concentration in said gas is carried out by direct measurement of the charge induced at the terminals of said piezoelectric cell (34).

10. Sensor according to any one of claims 1 to 8 characterized in that the measurement of said oxygen concentration in said gas is carried out by measurement of frequency difference with respect to the resonant frequency of said piezoelectric cell (34) .

11. Sensor according to one of claims 1 to 3 characterized in that said piezoelectric cell is constituted by a bar resting on a single bearing point and carries at one of its ends said pressure piece (39) .

12. Sensor according to claim 11 characterized in that the measurement of said oxygen concentration is carried out by vibrating said pressure piece (39) at a carrier frequency equal to the resonance frequency of the piezoelectric cell and by treating the signal obtained by demodulation of amplitu¬.

13. Sensor according to claim 12 characterized in that the demodulation - amplitude is carried out digitally.