FR2658288A1 - Apparatus for measuring small variations in quantities of gas - Google Patents

Abstract

The invention relates to an apparatus for measuring small variations in quantities of gas generated by a sample. It is characterised in that it comprises at least one measurement unit, including: - a capillary tube (100); - a tank (200), situated in the region of the upper end of the capillary tube and filled with liquid (L); - closure means (210) for selectively connecting the upper end of the capillary tube with the tank or for isolating it from the said tank; - a measurement chamber (300) receiving the sample; - means (600, 608) for manipulating the measurement chamber in order successively to apply it against the lower end of the capillary tube and remove it from the measurement unit; and - scanning electrooptical detection means (400) for producing, at least periodically, electrical data representative of the height of a liquid/gas interface in the capillary tube, the said data making it possible to deduce the gas absorption or production activity of the sample.

Description

 The present invention relates generally to the field of measuring small variations in the quantities of gas, for example by release or absorption originating from living organisms, and more specifically relates to a microrespirometer of the type with variable pressure and volume.

The principle of a microrespirometer of this type is already known in the prior art, in particular by the article "The respirometer with variable pressure and volume; a simple and sensitive technique for the ecophysiological study of soil animals", Bruno VERDIER, report of the Viléme International Colloquium of the
Zoologie du Sol, Louvain-la-Neuve, Belgium, August 30-September 2, 1982.

 This document describes a measurement unit comprising a capillary tube which is in communication at its upper end with a reservoir of liquid and at its lower end with a chamber in which the sample to be observed is placed. The liquid descends into the capillary to an extent determined by the volume and pressure of the gaseous environment exposed to the sample. When the temperature is kept strictly constant, the variations in height of the gas-liquid interface in the capillary tube is entirely representative of the quantity of gas produced or absorbed by the sample, according to the laws given in the aforementioned document.

 However, this document only describes a prototype of a laboratory measuring device, which cannot be used industrially. More specifically, this known device comprises, in an isobaric and isothermal closed enclosure, a bath of heat-sealed liquid in which a set of measurement units are immersed as described above. A first drawback resides in the fact that the loading of each measurement unit with a sample of the medium to be studied each time involves opening the enclosure and then closing it, with consequently the need to re-establish in said enclosure the conditions for balance in temperature and pressure before the actual measurements can be meaningful.

 Furthermore, in this known device, the variations in height of the liquid-gas interface in each capillary tube are read visually by bringing into the enclosure a bottom coated with graph paper or the like. This type of measurement on the one hand is tedious and imprecise, and on the other hand prohibits any possibility of connecting the device to a device for automatic processing and interpretation of the measurement data collected.

 Finally all the operations necessary to operate this device its manual and also require a special qualification to comply with the protocols of use, a necessary condition for obtaining correct measurements.

 The present invention aims to overcome these limitations of the prior art and to propose a microrespirometry device which is precise and reliable, easy to use and which can be coupled to means for processing the measurements collected.

To this end, it relates to an apparatus for measuring small variations in the quantities of gas generated by a sample, characterized in that it comprises at least one measurement unit comprising:
- a capillary tube;
- a reservoir located in the region of the upper end of the capillary tube and filled with liquid;
shutter means for selectively placing the upper end of the capillary tube in communication with the reservoir or for isolating it from said reservoir,
- a measurement chamber receiving the sample;
- Means for manipulating the measurement chamber to selectively apply the latter against the lower end of the capillary tube and extract it from the measurement unit; and
- scanning electro-optical detection means for producing at least periodically electrical information representative of the height of a liquid / gas interface in the capillary tube, said information making it possible to deduce the activity of absorption or production of gas of the sample.

Preferred, but not limiting, aspects of the apparatus according to the present invention are set out below:
- the liquid is octane.

 - the internal diameter of the capillary tube is between approximately 0.1 and 1 mm.

 - the shutter means comprise a shutter head controlled to come to be applied in a sealed manner against the upper end of the capillary tube, which opens into a liquid reservoir common to all the measurement units.

 - The closure head is crossed by a passage which can be selectively connected to a vacuum source or to a gas source.

 - the shutter means comprise solenoid valve means capable of selectively isolating a load compartment specific to the measurement unit, and into which opens the upper end of the capillary tube, relative to a common reservoir.

 - It also includes level control means to keep the liquid level in the common tank constant.

 the level regulation means comprise an additional capillary tube, the lower end of which is immersed in the liquid of the common reservoir and the upper end of which is in the open air, an optical detector, the position of which on the additional capillary tube is adjustable, and a solenoid valve means for selectively filling or emptying said reservoir as a function of information delivered by the optical detector.

 the scanning electro-optical detection means comprise a set of individual optical detectors mounted on a common support, means for moving the support parallel to the capillary tubes of each measurement unit, and means for measuring the displacements of said support.

 - The means for moving the common support comprise a geared motor and a toothed belt.

 - the optical detectors are infrared radiation transmitters / receivers operating by reflection.

 - The displacement measurement means comprise a magnetic rule comprising a cursor secured to the common support and a magnetic measurement part arranged parallel to the capillary tubes and along which said cursor moves.

 - The measurement chamber comprises a cylindrical part with a vertical axis having in its upper face a recess, and a part for receiving the sample housed in said recess.

 the means for handling the measurement chamber comprise a drawer in which is provided at least one housing capable of receiving the measurement chamber closely and capable of assuming an open position, in which the housing can be loaded and unloaded from the outside of the device, and a closed position in which the housing is located directly above the lower end of the capillary tube, and in which a displacement means can push the measuring chamber upwards to apply it from tightly against the lower end of the capillary tube.

 - The displacement means comprises a pneumatic cylinder.

 - The drawer comprises two housings arranged side by side and means for selectively bringing one or the other of the two housings directly above the lower end of the capillary tube when the drawer is in the closed position.

 - the handling means are included in a massive thermostatically controlled packaging block.

 - The conditioning block is made of a material with good thermal conductivity and is provided in the block for the circulation channels of a thermostatically controlled fluid.

 - one of the measurement units is a reference unit comprising an electrooptical detection means capable of being fixed at an adjustable height on the capillary tube and delivering information from which a means with solenoid valves is controlled to maintain constant the gas pressure above the liquid in the tank.

 - Means are provided for magnetic stirring of the contents of the measurement chamber.

Other aspects, aims and advantages of the present invention will appear better on reading the following detailed description of a preferred embodiment thereof, given by way of non-limiting example and made with reference to the accompanying drawings, on which ones
FIG. 1 is a schematic elevation view of a measurement unit included in a microrespirometry device according to the invention,
FIG. 2 is a schematic front elevation view of an apparatus according to the present invention,
FIG. 3 is a schematic side elevation view of the apparatus of FIG. 2,
FIG. 4 is a cross-sectional view of part of the apparatus of FIGS. 2 and 3,
FIG. 5 is a cross-sectional view of another part of the apparatus of FIGS. 2 and 3,
FIG. 6 is an elevation view of a detail of the apparatus of FIGS. 2 and 3,
FIGS. 7a and 7b show in cross section an alternative embodiment of part of the apparatus visible in FIG. 5, and
FIGS. 8 and 9 are two cross-section views at two different depths of a variant of a part of the apparatus shown diagrammatically in FIG. 4.

 It will be noted as a preliminary point that, from one figure to another, identical or similar elements or parts are designated by the same reference numbers.

Referring firstly to FIGS. 1 to 3, the apparatus according to 1 invention comprises for example a plurality of measurement modules each comprising, schematically:
- A column 100 of calibrated glass, or capillary tube, whose internal diameter is uniform and of the order of 0.1 to 1 mm, depending on the application;
- An assembly 200 forming a reservoir and load compartments into which opens the upper end of the tube 100 and equipped with a shutter 210, for example of the electromechanical type, making it possible to establish or break the communication between the reservoir and the tube; the reservoir contains a liquid L with clearly defined physical characteristics, as will be seen below, authorized or not to enter the tube depending on the state of the shutter;
a measurement cell 300 into which the lower end of the tube 100 penetrates in a sealed manner, and which is capable of receiving substrates whose gas releases or absorptions are to be studied;
- A scanning optical detector 400 which is able to scan the capillary tube 100 and generate a different signal depending on whether the detector is opposite the aforementioned liquid or the gas located below and in communication with the measuring cell;
- A device 500 of the magnetic rule type, capable of associating with the position of the optical detector an absolute length value, with great precision (for example of the order of 10 micrometers);
- A conditioning module 600 comprising a drawer capable of receiving the measurement cell.

 a computer, not shown, receiving via an appropriate interface the information coming from the optical detector and from the rule 500 and comprising display and printing means, and arranged moreover to control measurement procedures as will be seen in detail later.

 We will now describe in detail the most important aspects of a preferred embodiment of the device.

 In this example, seven capillary tubes 100 are provided, each having a length of approximately 350 mm, an external diameter of approximately 6 mm and an internal diameter of 0.2 mm.

 Referring more particularly to FIG. 4, the reservoir 200 is produced for example from polymethyl methacrylate (PMMA) and comprises at its bottom as many circular openings 202 as there are tubes 100, opening into an interior space 200a common to all the units of measurement. The upper end of each tube bears against the lower face of an O-ring 204 sealing. The upper face of each seal is capable of receiving a head 212 of the shutter 210 in the closed position (rest position) thereof. A cable gland 206 ensures the tight fitting of the capillary on the base of the reservoir 200.

 The liquid used is preferably octane, because of its advantageous characteristics of low viscosity and low vapor pressure.

 Each of the shutters 210, here of the electromechanical type, is made of brass or the like. It comprises a rod 211 movable in a bore of a body 213 and carrying at its lower end the closure head 212. The rod is integral at its upper end with a pneumatic cylinder 214 which, according to the order, holds the head 212 in sealing abutment against the O-ring 204, thereby breaking the communication between the tube 100 and the reservoir 200, or raising the head so as to allow the octane contained in said reservoir to enter the capillary. The body 213 of the shutter is threaded at its upper end for fixing in a threaded bore 208 of the reservoir.

 Appropriate O-rings (not referenced) provide a suitable seal between the rod 211 and the body 213 of the shutter. Finally, a return spring 215, here a helical compression spring, is provided around the head 212 so that, at rest, said head is applied against the seal 204.

 The rod 211 and its head 212 are crossed centrally by an axial passage (not visible) which makes it possible to establish communication between the capillary tube 100 and the outside.

 This communication makes it possible, for example, to perform the suction draining and the drying of the capillary, to balance the pressures prevailing in the measurement chambers 300 provided at the lower end of each capillary, and finally to scan the capillary tube and the measurement chamber using a neutral gas such as nitrogen in the case where the medium to be studied must be exposed to an anaerobic environment.

 There is also provided at the base of the head 212 of the shutter a short needle (not visible) oriented axially and intended to form a baffle effect during suction through the axial passage, in order to avoid any reflux of liquid from said passage to the capillary.

 The axial passage of the shutter is connected at its upper part to a multi-way solenoid valve 218 allowing, in the closed position of said shutter, to make the connection of the capillary 100 either on a vacuum source, for drying purposes, either to a gas source such as nitrogen for the purpose of conditioning the environment, or to the interior atmosphere of the other measurement units, for pressure balancing purposes (using solenoid valve 221).

 According to an alternative embodiment, and now with reference to FIGS. 8 and 9, the reservoir 200 as described with reference to FIG. 4 is replaced by a structure cut from a block of PMMA 200. In the lower part, each capillary tube 100 penetrates into the block and opens into a free volume or load compartment 201 whose diameter is slightly greater than the outside diameter of the capillary tube. This volume 201 is open on the upper surface of the block 200 to be connected to a first solenoid valve which allows the charging compartments 201 of each measuring unit to be placed in communication with a common air volume or internal buffer located in a reservoir 205 formed in the block, via a passage 207.

As can be observed, the upper end of the tube 100 is here bevelled at 450 in order to allow, as will be seen below, better drying efficiency.
At the base of the load compartment 201 opens an oblique transverse channel 203 connected by a second two-way solenoid valve, either to a reserve of charge liquid, contained in the bottom of the tank 205, or to a third solenoid valve, common to all the measurement units and allowing the communication of the load compartments, and therefore of the capillaries r either with a vacuum source for drying purposes, or on a fourth solenoid valve allowing pressure balancing and charging the measurement units With a gas such as nitrogen, the internal buffer mentioned above is constituted by the closed volume of air located above the level of the liquid in the reservoir 205. Thus, the situation in which the first solenoid valve is open and the second solenoid valve connects the volume 201 to the liquid in the reservoir 205 is equivalent to the case where the shutter 210 is open in the embodiment of FIG. 4.

 Still with reference to FIGS. 8 and 9, the device can include means for keeping the level of liquid in the reservoir constant. These means preferably include another capillary tube 230, the lower end of which is immersed in the liquid in the reservoir and the upper end of which is open to the air. An optical detector 232, preferably operating by emission of infrared radiation and reception of the reflected radiation, can be positioned in an adjustable manner at a given height along the tube 230. When the column of liquid in the tube 230 crosses the detector 232 , the change of electrical signal at the output thereof causes the opening of a fifth solenoid valve making it possible to empty the reservoir 205 until the column of liquid in the tube 230 has dropped sufficiently, On the contrary, when the liquid level drops below the level of optical detector 232, a sixth solenoid valve is opened to fill reservoir 205 with an external reserve (not shown) of liquid. These operations are carried out via passage 209 illustrated in FIG. 8.

 Referring now to Figures 2, 3 and 6, the optical scanning detector 400 comprises a mechanical part and an electronic part.

 The mechanical part comprises, for each measurement unit, a block 402, for example made of plastic, which is crossed by a vertical opening 404 through which the respective capillary 100 passes. This block receives, in association with each capillary, a device optical consisting of an infrared radiation transmitter / receiver, schematically indicated in 406.

The blocks are mounted in a common bar 408 so as to be able to move relative to the latter in rotation and in translation, in order to be able to match any variations in shape of the capillary 100 along its length.

 The bar 408 is integral with a carriage 410 which is movable in translation vertically and which is driven via a toothed belt 412 by a reversible geared motor 414, via a toothed pulley 415.

 The electronic part of the detector 400 preferably comprises means for adjusting the sensitivity of each optical device 406. The state of each optical device (presence of liquid or gas in the associated capillary tube) can also be visually indicated by the using a light-emitting diode 416.

 Each optical device 406 operates in the following manner: when the capillary tube 100 is empty of any liquid at the level of the device, it turns out to constitute a barrier to the transmission of infrared radiation. On the contrary, the presence of liquid removes this barrier and allows the receiver of the device to be excited by the infrared radiation emitted by the transmitter.

 The magnetic rule 500 associated with the scanning optical detector 400 is for example of the "tMagnescale" type manufactured and sold by the Japanese company SONY. This rule includes a cursor 502 which is integral with the carriage 410 and which moves along a part detection 504. Such a magnetic rule also conventionally comprises an electronic card comprising in particular a counter containing a digital value representative of the position of the cursor 502 along the detector 504.

 Via an appropriate interface, the computer is systematically informed of the passage of the optical detector respectively associated with each capillary via the liquid / gas interface in this capillary. This change of state constitutes an order for the abovementioned counter of the magnetic rule 500 to be read by the computer, for the latter's acquisition, almost immediately, of information representative of the height of the optical detector considered 406. The instant of this change of state is also stored in the computer.

 Thus, the computer has a set of information, each information item comprising the number of the column considered, the instantaneous height of the liquid / gas interface in this column, and the time.

 The measurement chamber 300 is a cylindrical element, for example made of stainless steel, which can have a diameter of the order of 16 mm and a height of the order of 30 mm. An upper axial recess 300a of the part 300 receives a removable receiving part 302, also made of stainless steel and intended to receive the substrate to be studied, for example in an amount of a few tens of microliters. The bottom of this recess 300a constitutes a well capable of receiving a support loaded with a liquid defined by the measurement protocol concerned (for example potassium solution, water, etc.).

 Finally, the base of the cylinder 300 receives a plastic part 304 forming thermal insulator for the purposes explained below.

 The conditioning module 600 is constituted by a fixed part 602 forming a casing and a set of mobile drawers 610 (here seven in number), which according to the embodiment, each define either a single cylindrical compartment 610c of vertical axis (case of Figure 5), or two identical cylindrical compartments 610a and 610b (case of Figures 7a and 7b).

 With reference to FIG. 5, the fixed part 602, made for example of brass, comprises in its upper part a set of thermostated cylinders 602a (here seven in number) each provided with a side vent 602b and each closed at their apex by a metal part 604 fitted with an O-ring 606. The lower end of the capillary tube 100 penetrates a short distance into a central passage of the part 604. A lower cable gland 603 ensures the fixing of the lower end of tube 100 on part 602.

 The fixed part 602 further comprises in its lower part housings 602c for the respective drawers, while pneumatic cylinders 608 are provided in the base of the part 602, coaxially with each cylinder 602a, in order to push up a measurement 300 initially housed in the cylindrical compartment 610c, as far as the cylinder 602a, so as to put the measurement chamber in sealed communication with the base of the capillary tube 100 via the part 604 and its seal 606. It can be noted here that the insulating part 304 described above makes it possible to ensure satisfactory thermal insulation between the measurement chamber 300 and the rod of the jack.

 Preferably, the fixed part 602 is produced in the form of two blocks 6021 and 6022 of brass, respectively upper and lower, the first of which incorporates the cylinders 602a and the vents 602b, and the second of which incorporates the housings 602c for the drawers.

These two blocks are jointly regulated in temperature using a thermostatically circulated fluid circulation system. this system comprises an external reservoir (not shown) of thermostatically controlled liquid, an inlet 624 and an outlet 626 for this liquid, and a circulation pump 628. Each of the two blocks 6021 and 6022 is crossed by a plurality of internal channels, respectively 6021a and 6022a for the circulation of the liquid. This system makes it possible to maintain the conditioning module 600 and the elements which it houses at a predetermined constant temperature, using an appropriate thermostat in conjunction with means for controlling the temperature of the heat-sealed liquid (not illustrated). .

 Thus the samples to be studied can be subjected to a very homogeneous thermal environment, which conditions the precision and the interpretability of the measurements carried out. Besides1 independently of any measurement, the compartments of the drawers are in an environment allowing a well controlled thermal preparation (incubation) of the samples.

 Each drawer 610 is made of a thermally conductive material such as brass, in order to expose its compartments the thermostatically controlled environment as described above.

 All the parts of the packaging module 600 are produced with such precision that gas packaging can be produced; the vent 602b, and where appropriate the capillary 100, allows, during the various manipulations, a flow of gases towards the outside in order to avoid any overpressure.

 There is also provided, in association with each drawer, a limit switch microswitch 614 which authorizes the lifting of the measurement chamber considered 300 by the associated jack 608 only when the drawer has been fully inserted in its housing. .

 Furthermore, means are provided for magnetic agitation of the medium to be studied in each measurement chamber in the active position (that is to say in the cylindrical housing 602a), these means comprising for each unit of the apparatus a magnetic stirring micro-bar (not illustrated) and specific means for creating a variable magnetic field in the region of the upper block 6021 of the conditioning module. These specific means may include a geared motor 630 which drives in a horizontal guide 632 a ball cursor 634 which supports an element 636 of magnetic steel, generator of the stirring field.

 In the variant of FIGS. 7A and 7B, it can be observed that each drawer 610 comprises two juxtaposed compartments formed in a block 616 capable of sliding longitudinally relative to the body of the drawer, being guided by the latter. A pull tab 618 associated with block 616 and passing through the front wall of the drawer makes it possible to position plumb with the heat-sealed cylinder either the compartment 610a (case of FIG. 7A), or the compartment 610b (case of FIG. 7B).

The computer is preferably designed, being equipped with appropriate software and an interface, to ensure:
- automatic sequencing of the procedures for loading the samples to be studied and activating the various measurement units (optical detectors, magnetic ruler, etc.);
- acquisition, processing, visualization and archiving of measurement data; and
- monitoring the proper functioning of the device.

 In a particularly appropriate manner, all of the constituent elements of the device described above, with the exception, however, of the computer, which may be a standard microcomputer on the market, are received in a box organized and equipped so as to ensure the structural assembly and the functional connection of all these elements, and generally indicated in 800.

 The housing 800 comprises a base or base 802 in which are housed on the one hand an electrical supply card for the various elements (in particular electropneumatic elements, solenoid valves and electronic cards), the transformer 804 of which can be observed in FIG. 5, and on the other hand a battery of seven pneumatic cylinders 608 with vertical displacement, used as described above to bring the measurement chambers into communication with the base of their respective capillary tube.

 The compressed air circuit for the various pneumatic cylinders of the device includes an inlet 805 for compressed air from an external compressor, as well as an electropneumatic distributor 806 to ensure from this single source l 'supply of various pneumatic elements.

 As already indicated, one of the seven measurement units is a reference unit and allows the regulations imposed by the measurement protocol considered. The optical detection device 406a associated with this unit is independent of the other optical devices, that is to say not integral with the carriage 410 whose movements are controlled by the magnetic rule 500.

The optical device 406a is immobilized at a height chosen on the associated capillary tube 100a, using a locking screw 407, and the open or closed state of a regulating solenoid valve 220 (FIG. 4) is controlled by the output signal from the device 406a, so as to keep as high as possible the height of the column of liquid in the capillary tube 100a
To this end, the regulating solenoid valve 220 is a three-way solenoid valve mounted between an air space situated above the octane tank 200, which is also closed, a source of pressurized air and a source of vacuum. , so as to provide or withdraw air in this space in order to keep the pressure essentially constant, and therefore to keep the height of the octane column in the reference tube 100a also constant.

Preferably, the source of pressurized air is a diaphragm pump (not shown), while the vacuum source is advantageously created by effect
Venturi at the temperature control system with liquid circulation, not shown.

The other six measurement units are available for carrying out microrespirometry measurements in a completely independent manner, in particular thanks to the presence:
- at the top, individual shutters, and
- in the lower part, thanks to the presence of drawers 610, measuring chambers 300 and individual cylinders 608.

 The procedure for using the device described above can be as follows.

 When the device is started, the vacuum source is operational. All the shutters 210 are lowered, to isolate the capillary tubes from the reservoir 200. Furthermore, no measurement chamber 300 is in position at the lower end of its tube 100. The solenoid valves 218 are actuated so as to connect the tubes 100 at the vacuum source, for their drying.

 When it is desired to study a sample, it is conditioned either outside the apparatus, or even in one of the drawers 610, for a period defined by the operator.

 Prior to or during this period, the device is brought to the desired temperature, set on the regulation system 620, then the measurement chamber or chambers 300 are placed in their respective drawers 610, then open. The drawers are then pushed into the position where their limit switch 614 is actuated, and informs the computer of the presence of the associated drawer.

 It is possible at this stage to carry out a particular gaseous conditioning such as the anaerobic setting of the environment of the measurement chambers 300, by introducing via the solenoid valves concerned 218 and the axial passages for example a flow of nitrogen. To this end, said solenoid valves 218 are switched to the appropriate gas source. It should be noted here that the vents 602b formed in the fixed part 602 of the conditioning module allow the gas initially present (air) to be expelled under the effect of the introduction of the flow of nitrogen or the like.

 At this stage, the interior atmosphere of the measurement units can be checked using, for example, a polarographic oxygen probe.

 Then the solenoid valves 218 are controlled so as to isolate the reaction media from the vacuum or from the gas source, as the case may be, for example using an appropriate command via the computer keyboard.

 On the operator's order, the measurement chambers 300 contained in the drawers 610 are then mounted using the jacks 608 in the cylindrical receptacles 602a of the upper thermostatically controlled block 6021, and can be left there waiting, for example for a few minutes, in order to perfect the thermal and hygrometric balances, the quality of which conditions the precision of the measurements that will be carried out.

 It is important to note here that the relationship between each measurement chamber 300 and the part 604 which receives the lower end of the capillary tube 100 is essentially without elasticity. For this purpose, the air pressure applied to the cylinders 608 is high, for example of the order of 15 to 20 × 105 Pascals, so as to compress the seal 606 sufficiently so that the metal parts are in direct contact. This ensures thermal balancing in this region.

 The ramp of optical detectors 406 can then be activated, and an initial control, namely a complete scanning of the entire length of the capillary tubes 100, can be carried out to inform the computer of the fact that the actual measurement procedure can be started. . More precisely, if this instant is detected the accidental presence of liquid in one of the capillary tubes, the measurement operation is delayed and the tube in question is rewired, as indicated above, after removing the associated room 300.

 When the initial control has not detected any anomaly, the computer then automatically triggers the lifting of each shutter 210, which authorizes octane to enter each tube 100, by capillarity and by gravity.

 The column of. liquid stabilizes at a height which depends on the residual volume of the associated chamber. It should be noted here that no prior calibration is necessary, insofar as the equilibrium height reached itself validly constitutes the reference for the measurements which will follow.

 The ramp of optical detectors 406 is then driven back and forth by the geared motor 414, over the entire extent of the capillary tubes, and at each rise and fall, the value of the height of the liquid / gas interface for each tube is transmitted to the computer.

 The reference measurement unit, which, as indicated above, has a construction similar to that of the other measurement units (with the exception of its optical device 406), does not contain a sample to be studied. It is therefore neutral and there is no production or absorption of gas. By regulating the height of the octane column in the reference unit of measurement, as indicated above, it is thus ensured that any variation in height of the octane column in an effective unit of measurement can be fully charged to a change in the amount of gas in that unit.

 The data collected by the computer can be processed, displayed and / or printed in any suitable manner, for example in real time. They can also be memorized to carry out these operations offline.

 The end of the measurement operation is either a normal end, at the end of a determined period of time during which the computer has collected measurement values, or an anticipated end, triggered by an alarm integrated into the computer.

At the end of the actual measurement operation, the closing operations are sequenced, for all or part of the measurement units involved, is carried out automatically. In particular, for each measurement unit concerned, the shutter 210 is lowered to isolate the capillary 100 from the reservoir 200, and the solenoid valve 218 is tilted to the vacuum source, to empty the capillary from its column of liquid
The lower cylinder 608 is then lowered to lower the measurement chamber 300, the vacuum then being eliminated. The chamber 300 can then be recovered by pulling the associated drawer 610. The vacuum is then restored to proceed with the wiping of the capillary. The unit is then ready for a new measurement.

 The apparatus according to the present invention can be used very advantageously, but not exclusively, for microbiological study and analysis, and more particularly for bacterial counting and sterility checks in the food and medical fields.

 The apparatus according to the invention can also be used advantageously in the field of fermentations and decarboxylations, as well as for monitoring tissue and bacterial cultures.

 The device is also useful for carrying out antibiograms in extremely fast times.

 We will indicate below some concrete examples of applications of the device.

Sterility test
the device is used in this case in all or nothing mode. On the basis of a high hypothesis of a germ present and a doubling time of 40 min, an incubation of 12 hours outside the device allows detection of contamination of the medium studied in a few minutes. This result should be compared to the 36 hours generally required to carry out the same test using conventional methods.

Bacterial count
The device can be used for dynamic counting in order to determine the total microflora, for example of milk. The apparatus makes it possible to determine the doubling time of the microflora in approximately 1 hour. The device can be applied advantageously on the farm, in a dairy or in a consumer quality control test.

Mastitis detection
The device allows immediate detection.

Selection of cheese strains
An anaerobic study on homofermentative strains used in cheese making has made it possible to recognize certain strains responsible for a decarboxylating activity degrading cheeses.

Fermentation detection
The device allows immediate detection.

 In the medical sector, the device can be used in particular for the control of urine infection and for the early detection of germs in blood cultures.

 In cosmetology, the device can be used on aqueous solutions, creams, emulsions, ...

 In pharmacology, the effects of broad or narrow spectrum antibiotics are immediately noticeable using the device.

Finally we can cite
- studying the metabolism of tissues and cells in culture in vitro, as well as growth factors;
- studying the metabolism of small pests of stored food: mites, insects (in the form of an egg, larva or adult), as well as allometric curves of oxygen consumption and the respiratory quotient;
- tests of pesticides, insecticides and bactericides;
- the study of the microbial activity of waste (faeces) and the recovery of organic waste (pre- and post-methanised manure);
- the study of the metabolic adaptations of animals to climatic factors and their ecological environment, applicable in particular to polar, desert, tropical fauna, etc.

 Of course, the present invention is in no way limited to the embodiment described above and shown in the drawings, but a person skilled in the art will know how to make any variant or modification in accordance with his spirit.

 Thus, in order to minimize the temperature gradient at the base of the capillary tubes, it is possible to sweep this region of the tubes with air at an appropriate temperature.

 In addition, the optical detection means as described above can be replaced by a fixed camera, provided with sufficient resolution and in the field of which the capillary tubes are located, in combination with means of image analysis to determine the position of the liquid-gas interface in each tube.

 Finally, the removable measuring chambers can be replaced by single-use parts, for example made of plastic, adapted to specific application protocols which are implemented by incorporating specific products (reagents, antibiotics, etc.) .

Claims (20)

 1. Apparatus for measuring small variations in the quantities of gas generated by a sample, characterized in that it comprises at least one measurement unit comprising:  - a capillary tube (100);  - a reservoir (200, 200a, 201, 205) located in the region of the upper end of the capillary tube and filled with liquid (L);  shutter means (210) for selectively placing the upper end of the capillary tube in communication with the reservoir or for isolating it from said reservoir,  - a measurement chamber (300) receiving the sample;  - Means (600, 608, 610) for manipulating the measurement chamber to selectively apply the latter against the lower end of the capillary tube and extract it from the measurement unit; and  - scanning electro-optical detection means (400, 406, 500) for producing at least periodically electrical information representative of the height of a liquid / gas interface in the capillary tube, said information making it possible to deduce the activity d absorption or production of sample gas.  2. Apparatus according to claim 1, characterized in that the liquid (L) is octane.  3. Apparatus according to one of claims 1 and 2, characterized in that the internal diameter of the capillary tube (100) is between about 0.1 and 1 mm.  4. Apparatus according to one of claims 1 to 3, characterized in that the shutter means (210) comprise a shutter head (212) controlled to come to apply sealingly against the upper end of the capillary tube ( 100), which opens into a liquid reservoir (200a) common to all the measurement units.  5. Apparatus according to claim 4, characterized in that the closure head (212) is traversed by a passage which can be selectively connected to a vacuum source or to a gas source.  6. Apparatus according to one of claims 1 to 3, characterized in that the shutter means comprise solenoid means capable of selectively isolating a load compartment (201) specific to the measurement unit, and into which opens l 'upper end of the capillary tube (100), relative to a common reservoir (205).  7. Apparatus according to one of claims 1 to 6, characterized in that it further comprises level regulation means (230, 232) to keep the liquid level in the common tank constant.  8. Apparatus according to claim 7, characterized in that the level control means comprise an additional capillary tube (230) whose lower end is immersed in the liquid of the common reservoir (205) and whose upper end is the open air, an optical detector (232) whose position on the additional capillary tube is adjustable, and a solenoid valve means for selectively filling or emptying said reservoir according to information supplied by the optical detector  9. Apparatus according to one of claims 1 to 8, characterized in that the scanning electro-optical detection means comprise a set of individual optical detectors (406) mounted on a common support (410), means (412, 414) to move the support parallel to the capillary tubes of each measurement unit, and means (500) for measuring the displacements of said support.  10. Apparatus according to claim 9, characterized in that the means for moving the common support comprise a geared motor (414) and a toothed belt (412).  11. Apparatus according to one of claims 9 and 10, characterized in that the optical detectors (406) are transmitters / receivers of infrared radiation operating by reflection.  12. Apparatus according to one of claims 9 to 11, characterized in that the displacement measurement means comprise a magnetic rule (500) comprising a cursor (502) secured to the common support and a magnetic measurement part (504) disposed parallel to the capillary tubes and along which said cursor moves.  13. Apparatus according to one of the preceding claims, characterized in that the measurement chamber comprises a cylindrical part (300) with a vertical axis having in its upper face a recess (300a), and a receiving part (302) of the sample housed in said recess.  14. Apparatus according to one of claims 1 to 13, characterized in that the means for manipulating the measuring chamber comprises a drawer (610) in which is formed at least one housing (610c; 610a, 610b) capable of receiving tightly the measuring chamber and capable of taking an open position, in which the housing can be loaded and unloaded from outside the device, and a closed position in which the housing is located directly below the lower end of the capillary tube (100), and in which a displacement means (608) can push the measuring chamber upwards to apply it sealingly against the lower end of the capillary tube.  15. Apparatus according to claim 14, characterized in that the displacement means comprises a pneumatic cylinder (608).  16. Apparatus according to one of claims 14 and 15, characterized in that the drawer (610) comprises two housings (610a, 610b) arranged side by side and means (616, 618) for selectively bringing one or the other of the two housings plumb with the lower end of the capillary tube (100) when the drawer is in the closed position.  17. Apparatus according to one of the preceding claims, characterized in that the handling means are included in a massive thermostatically controlled packaging block (602).  18. Apparatus according to claim 17, car terized in that the conditioning block (602) is made of a material with good thermal conductivity and in that it is provided in the channel block (6021a, 6022a ) circulation of a heat-sealed fluid.  19. Apparatus according to one of the preceding claims, characterized in that one of the measurement units is a reference unit comprising an electro-optical detection means (406a) capable of being fixed at an adjustable height on the tube capillary (100a) and providing information from which solenoid valve means is controlled to keep the gas pressure above the liquid in the reservoir (200) constant.  20. Apparatus according to one of the preceding claims, characterized in that there are provided magnetic stirring means (630-636) of the contents of the measurement chamber.