FR2672995A1 - Sensor allowing the continuous measurement of the particle concentration in a liquid medium - Google Patents

Abstract

The invention relates to a measurement sensor making it possible to assess the particle concentration, and more particularly the microorganism concentration, in a liquid medium. The sensor is composed of two parts: - a chamber (1) connected to a system making it possible to create in it a reduced pressure measured by the torque exerted by the motor (11) which drives the movement of a sealed bellows (18); - a membrane (2) separating this chamber from the suspension or the solution to be studied; this membrane is of organic or inorganic (sterilisable) nature and has variable porosity, matched to the suspension or to the solution to be studied. A relationship is set up between the particle concentration and the progressive clogging of the membrane by these particles. With a reduced pressure having been created in the chamber (1), the time necessary to reach equilibrium is measured. This time will be commensurately longer as the particle concentration is small. Two measurement principles can be used: - measuring the time necessary to reach a fixed pressure (close to equilibrium); - measuring the pressure reached at a given time. The invention may be used both for measuring the particle distribution in a suspension and for determining the viscosity of polar solutions; in the latter case, it is the presence of the polymers which leads to the clogging of the membrane.

Description

TECHNIOUE DESCRIPTION
The present invention relates to methods for measuring the concentration of particles in suspension in a liquid medium.

Currently, this determination is made by other methods based on different principles such as: - Measuring volume or mass; this is a classic method requiring the separation of
the sample in two parts: the particles whose concentration one seeks to measure on the one hand and
the liquid medium on the other hand In the case of a volume measurement, the sample is subjected to a
centrifugation to accelerate the sedimentation of particles. Depending on the method used, we
then proceeds to measure the volume or the dry matter of the particles after filtration of the
suspension on a membrane. This method has the disadvantage of being insensitive and of
thereby necessitating the use of large volumes.

- The measurement of the variation of the electrical resistance. In this case, the measuring device consists
a cylindrical tube pierced with a micro-orifice on either side of which two electrodes are placed.

Each particle passing through the orifice displaces an electrolyte volume equal to its own volume, which
creates a variation in impedance. In a given time interval, the number of pulses
recorded corresponds to the number of particles having passed through the orifice. The major drawback of this
system is to work well only with large cells.

- Optical methods, the operation of which is as follows: when passing a light ray
through a suspension, there is light scattering by the particles, absorption of part of
the incident light intensity (and transmission from the other party) and finally diffraction. Different
methods based on these principles are used for the measurement of particle concentration
of a medium:
- We can first of all measure the intensity of light scattered by a
suspension made from variable angles. The wavelength used is a function of
the size of the particles analyzed. However, the intensity of the light scattered by a
particle is weak and poses a technological problem for suspensions
concentrated. To increase the sensitivity of the measurement, a laser beam can be used to
light source.

- We can also exploit the following phenomenon: when a particle is sent,
analysis of the transmitted light makes it possible to determine the concentration of particles at
constant volume. A disadvantage of this technique is that there is no correlation between
light absorption and particle concentration only in a limited area.

- We can finally, given that the particles in suspension in a liquid diffract the
light, record the diffraction. This allows, after analysis and treatment by
appropriate algorithms, to determine the particle size distribution. n is however
the refractive index of the liquid must be different from that of the particles. The
optical methods have the disadvantage of requiring a clear medium, containing no
for example no bubbles.

Another method is also used; it is about image analysis. Images from a microscope are processed so that they can be further processed by computer.

All of these methods have drawbacks. First of all, measuring volume or mass is long and tedious. The methods of varying the electrical resistance and the optical methods are very expensive.

The invention relates to a device for measuring the concentration of particles in a liquid medium comprising: - a chamber (I) in contact with the liquid medium via a semi-membrane
permeable or of suitable porosity (2); - Means (4), (18), (19), (22) making it possible to create a depression in the chamber (1); - means (3), (12) for measuring the volume of liquid which enters the chamber (1)
before clogging of the membrane (2); - Means (7), (24), (25) suitable for controlling the vacuum in the chamber (1).

The characteristics and advantages of the invention will emerge more clearly from the description which follows, referenced in the appended drawings, in which: - Figure I schematically represents the principle of measurement with the use of a circuit
pneumatic for sucking the liquid; - Figure 2 shows an embodiment in which the suction is carried out by means of a piston
waterproof; - Figure 3 shows an embodiment in which the suction in the chamber (I) is carried out by
the action of a deformable membrane (19); - Figure 4 shows an embodiment in which the suction in the chamber (1) is carried out
means of a piston sealed by means of a bellows (18); - Figure 5 shows the electric motor supply circuit (11); - Figure 6 shows the intensity measurements (and as a result of depression) and displacement (and by
volume suite) made at the terminals of the electric motor (11).

Referring to Figure I, we can understand the principle of measurement.

Or a solution contained in a container (9) which contains suspended particles whose concentration is proposed to be measured. If this liquid is filtered, by passing it through a permeable membrane (2), capable of allowing the liquid to pass while retaining the particles on its surface, it is easily conceivable that a clogging phenomenon may occur after the membrane has let a certain volume of liquid pass. In fact, it will be the volume of liquid relative to the membrane surface which is truly representative of the concentration of particles. We will agree to designate it by the term "specific volume".

To carry out this measurement, the procedure is as follows: first a certain vacuum is applied, for example 0.1 bar, in a cavity or chamber (I) formed between the part (10) supporting the membrane (2) and this last one. This depression is achieved by means of a vacuum pump or any depressor (4) which, if the valve (5) is open, sucks in the chamber (1) whose volume is as small as possible.

The liquid then rises in the suction pipe and enters a glass bulb (3), suitably graduated, to allow a measurement of the volume of liquid sucked. At the start of the measurement, a very rapid rise in the liquid is observed, then it decreases and finally the level almost stabilizes. The vacuum is maintained at a constant value thanks to an adjustable pressure reducer (7). During this suction phase, the valve (6) is kept closed. When the measurement is finished, the valve (5) is closed and the valve (6) is opened, allowing the chamber (1) to be subjected to an overpressure this time. This causes the liquid contained in the measuring pipette (3) to be ejected and the layer of agglomerated particles on the surface of the membrane (2) to come off. These are sprayed inside the solution and this results in a cleaning of the membrane (2) .The device is then ready to carry out a new measurement by carrying out the previous cycle.

The disadvantage of this method comes from the fact that it requires an operator to visually perform the measurement at the level of the liquid in (3). It is therefore advantageous to envisage other embodiments of the invention in which the measurement can be entirely automatic.

This is the case for example of the system of FIG. 2 where the vacuum in the chamber (1) is produced by means of a piston (22) sliding in leaktight manner in a cylinder (21) with friction as reduced as possible . To obtain a good seal, it is possible to use a lip seal or better, a ferromagnetic fluid seal.

The piston (22) is actuated by means of a rack (23) or any other means making it possible to convert the rotational movement of the geared motor (11), (13) into a translational movement. In this principle, the vacuum is continuously monitored and measured by the current flowing in the DC motor (11), the operation of which will be detailed later. The volume of liquid introduced into the chamber (1) is measured by the position of the piston which can be obtained by means of a potentiometer or by measuring the angle of rotation of the motor, which can advantageously be achieved by means of a angular encoder (12) fixed at the end of the motor shaft.

FIG. 3 describes an embodiment where the suction into the chamber (1) is effected by deformation of an elastic membrane (19), under the effect of a traction exerted by the screw-nut system (15), ( 16) which converts the rotational movement of the motor (11), equipped with a possible reduction gear (13), into a translational movement. The system comprises a body (20) integral with the motor (11), supporting the elastic membrane (19) and the semi-permeable membrane (2). As before, the supply current of the DC motor (11) imposes a torque, therefore a tensile force and finally a vacuum in the chamber (1). The volume is also measured from the angle of rotation of the motor, measured by the angle encoder (12), by means of a calibration curve. The processing of these two signals makes it possible to dispense with the effort required to deform the membrane (19).

It will be noted that this very simple arrangement makes it possible to obtain an excellent tightness of the chamber 1 with respect to leaks other than those of the membrane (2) and that the system can be very easily cleaned by dismantling the membrane (21 ). The membrane (2) is cleaned at the end of the cycle by reversing the direction of the motor, which ejects the liquid contained in the chamber (1).

FIG. 4 represents another variant of these embodiments of automatic sensors, in which the vacuum is obtained by the deformation of a cylindrical bellows (18).

Here, the permeable membrane (2) has been given a shape of a thimble which can be fixed on the body (14) of the device by means of a nut (17); the seal being obtained by an O-ring. It is thus easy to disassemble the sensor to clean it periodically.

The depression is achieved by the deformation of the bellows (18) which is driven in translation by a nut (16) of which it is integral. The movement is carried out by means of a screw (15), driven in rotation by an electric motor (11), possibly equipped with a speed reducer (13) as well as an encoder (12) providing proportional information displacement of the nut (16) and therefore proportional to the volume of liquid sucked in.

The motor is supplied by means of a voltage supply, the polarity of which can be reversed in order to allow motor operation in both directions. During the measurement phase, the polarity of (25) is such that the current generator (24) supplies the motor (11) with constant current During the phase of return to the initial position where the liquid contained in the chamber (1) is discharged into the tank (9) which ensures the unclogging of the semi-permeable membrane (2), the polarity of (25) is intense and the current flows through the diode D.

The reverse polarity of the power supply (25) uses one of the many known methods based on reversing relays or electronics.

FIG. 6 ideally represents the variations of the current in the electric motor during the measurement phase. It will be noted that it takes a certain time C before reaching its set value, this due to the absence of a resistive torque when the membrane is not yet clogged. Curve 1 relates to a solution more highly charged with particles than that of curve 2.

It will be noted that the volumes debited V1 and V2 are only reached asymptotically due to the relative tightness of the clogging. Of course, the highest concentration corresponds to the lowest volume.

The membranes can be organic in nature or, on the contrary, metallic or ceramic so as to be sterilizable. Their porosity is variable and, in any case, suitable for the suspension in solution to be studied.

The invention can be used: - for measuring the distribution of particles in a suspension: if the particles are the same
size: measurement of particle concentration (biomass, plant cells, animal cells,
lactose, in the fields of biotechnology, immunology, biomedical or
pharmaceutical); - if the particles are of different sizes: measurement of the particle load (dust
especially); - for measuring the viscosity of polymer solutions: food applications (ketchup,
mayonnaise, syrup, jam, cookie, chocolate, cream, cheese, ...); chemical applications
(paint, varnish, gel, ...).

Claims (10)

 1. Device for measuring the concentration of particles in a medium, characterized in that it comprises: - a chamber (1) in contact with the liquid medium via a semi membrane  permeable or of suitable porosity (2); - Means (4), (18), (19), (22) making it possible to create a depression in the chamber (1); - Means (3), (12) for measuring the volume of liquid entering the chamber (1)  before clogging of the membrane (2); - Means (7), (24), (25) suitable for controlling the vacuum in the chamber (1). 2. Device according to claim 1, characterized in that it comprises a depressor or vacuum pump (4) which can be connected to the chamber (1) by means of an electric or manual valve (5). 3. Device according to claim 1 or 2, characterized in that it comprises a valve (6), electric or manual, suitable for injecting pressurized air into the chamber (1), the source of pressurized air possibly being the "discharge" output of the depressor or vacuum pump (4). 4. Device according to claim 1, characterized in that the suction of the liquid in the chamber (1) is effected by means of an elastic, metallic or elastomeric membrane of cylindrical (18) or planar (19) shape . 5. Device according to claim 1, characterized in that the means making it possible to obtain the vacuum in the chamber (1) consist of a piston (22) sliding in leaktight manner, in a chamber (21). 6. Device according to claims 4 or 5, characterized in that the means used to actuate the membranes (18), (19) or the piston (22), consist of an electric motor (11) optionally equipped with a reduction gear ( 13) with transformation of the movement by a screw (15), a rack (23) or any known means allowing a translational movement to be obtained. 7. Device according to claim I, characterized in that the means making it possible to measure the volume of liquid sucked in are constituted by a graduated pipette (3) with visual reading. 8. Device according to claim 1, characterized in that the means for measuring the volume of liquid sucked into the chamber (1), consist of an encoder (12) placed at the end of the motor shaft (11). 9. Device according to claim 1, characterized in that the means making it possible to control or measure the vacuum in the chamber (1) are constituted either by a pressure reducer (7), or by a control of the engine torque (11) at by means of a current generator (24). 10. Device according to any one of claims 1 to 9, characterized in that it is used for measuring the viscosity of polymer solutions.