Abstract:
A dispensing installation for fuel or other volatile liquids comprises a storage tank, a pipe and a pump for dispensing fuel, a vapor recovery pipe and pump for recovering vapor and delivering it back to the tank, and a controller maintaining the vapor delivery rate approximately equal to the liquid delivery rate. In order to check that the vapor recovery system is operating correctly, the vapor delivery rate is constantly detected and compared with a value of the liquid delivery rate. If the result of the comparison is outside a predetermined range, an alarm is triggered in order to indicate a malfunction. The predetermined range may be adjustable.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of checking that a system recovering vapour emitted in a liquid dispensing installation, in particular when dispensing fuel to the interior of a motor vehicle tank, is operating correctly. 
     BACKGROUND OF THE INVENTION 
     Fuel dispensing installations conventionally comprise a fuel storage tank, a pipe for dispensing liquid incorporating a delivery pump enabling the fuel to be circulated between the storage tank and a dispensing gun at a liquid delivery rate QL, as well as counting means connected into the liquid dispensing pipe and fitted with a liquid measuring unit linked to a pulse generator or coder enabling a computer to ascertain the volume and price of the fuel dispensed, which then appear in plain text on a display. 
     For reasons of safety (risk of explosion) and environmental protection, installations of this type ate generally fitted with a system for recovering vapour emitted when the tank is being filled; such a system comprises a pipe for recovering vapour incorporating a recovery pump which enables the vapour to be circulated between the dispenser gun and the storage tank at a vapour delivery rate QV when the tank is being filled. 
     In order for a system of this type to operate efficiently, the delivery rate of the vapour QV at any instant must be approximately the same as the liquid delivery rate QL. 
     In order to achieve this performance, the recovery system is fitted with control means which are able to maintain this balance. 
     In smaller installations having only one or two dispenser guns, these control means are provided in the form of simple means whereby the vapour delivery rate QV is calibrated beforehand on the maximum liquid delivery rate QLmax, which is generally in the order of 40 litres per minute. 
     In larger, more sophisticated installations, the control teams consist of an electronic control unit fitted with a microprocessor, connected to counting means which supply the value of the liquid delivery rate QL instantaneously and co-operate either with the recovery pump if it is of the variable delivery type and hence operates a variable delivery rate, or with an electronically operated control valve connected into the vapour recovery pipe if the recovery pump operates at a fixed rate. In a system of this type, the values governing opening of the electronically operated control valve or the speed of the recovery pump corresponding to a vapour delivery rate QV are stored in the memory of the microprocessor during the initial calibration process. 
     Vapour recovery system of the type outlined above are generally efficient immediately after they have been calibrated. After a period in service operation, however, the results become leas certain, not to say totally erratic. 
     This situation is generally attributable to ageing of the equipment: wear on the pumps, clogged pipelines, stretching in the belts leading to a reduction in pumping rates, blocked pumps, etc. 
     Currently used installations are not fitted with units to detect when operation is poor and incapable of maintaining equality between the liquid delivery rate QL and the vapour delivery rate QV and the period between two service inspections on the installation may be very long (one to three years), which represents a source of pollution in particular and is therefore harmful to the air quality. 
     It should be pointed out that an earlier document, U.S. Pat. No. 5,332,008, discloses (column 4, lines 13-18) a fuel dispensing installation incorporating a vapour recovery system which is fitted with a sensor detecting operation of the recovery pump, which means that the speed normally expected of this pump can be checked and distribution disabled in the event of an anomaly. 
     However, this detection system is not always able to react if the pump is exhibiting mechanical wear (changes in its characteristics), which may render it incapable of attaining a vapour delivery rate QV equal to the liquid delivery rate QL. 
     The same applies if the suction or delivery pipes of the recovery pump become partially or totally blocked (due to encrustation or by accidental means); if an installation is fitted with an electronically operated control valve, its timing will initially have been programmed after calibration, thereby preventing an adequate delivery rate from being achieved and the vapour delivery rate QV is always lower than the liquid delivery rate QL and may even fall to zero under extreme circumstances unless the detection system disclosed in this earlier publication triggers an alarm to indicate that there is a malfunction. 
     In document U.S. Pat. No. 5,857,500, it was also suggested that automatic checks be made on the recovery pump for wear, when not dispensing fuel, by means of a command issued to electronically controlled valves upstream and downstream of the pump to be checked and to do so by providing two pressure sensors to measure the active or negative pressures attained when the pump is rotating. The pressures measured during an opening/closing cycle of the electronically controlled valves can be compared with the measurements taken when the system was installed in order to determine the extent of wear on the recovery pump. 
     According to this earlier document, another test was to measure the drop in pressure on the auction side what dispensing In order to evaluate the degree of encrustation or blockage at the level of the vapour recovery pipe. 
     However, these are nothing more than pressure measurements which depend both on an instantaneous delivery rate and resistance in the line in which changes are evaluated as compared with the initial situation as recorded on the date of installation. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention in to remedy the above-mentioned disadvantages by proposing a method of checking that the system used to recover vapour in a liquid dispensing installation, in particular when dispensing fuel to the interior of a motor vehicle tank, is operating correctly, providing a reliable indication of any malfunction in the vapour recovery system, regardless of the source of this malfunction 
     Accordingly, the method proposed by the invention is characterised in that: 
     the vapour delivery rate QV is constantly detected by detection means, 
     the value of the vapour delivery rate QV thus detected is transmitted to comparison means which compare it with a value of the liquid delivery rate QL and 
     if the result of this comparison is outside a predetermined range, which may or may not be adjustable, an alarm is triggered in order to indicate a malfunction. 
     In a first embodiment of the invention adapted to a vapour recovery systen having an electronic control unit co-operating with an electronically operated control valve or a variable delivery pump, the value of the liquid delivery rate QL determined by the counting means is constantly transmitted to the comparison means and it is compared with the value of the vapour delivery rate QV detected by the detection means. 
     It should be pointed out that in the case of this embodiment, the vapour delivery rate QV is compared with the liquid delivery race QL by the electronic control unit if this function has been programmed in the microprocessor incorporated therein, although this is not always the case with existing systems which would have to be modified accordingly. 
     In addition, if the microprocessor of the electronic control unit is able to interact with the computer of the counting means, the alarm could also be transmitted via this computer to the service station manager or remotely transmitted to a maintenance company which could then respond more rapidly. 
     In a second embodiment of the invention adapted to a simplified recovery system which does not have an electronic control unit and in which the control means correspond co a prior calibration of the vapour delivery rate QV to the maximum liquid delivery rate QLmax, the maximum value QLmax of the liquid delivery rate QL in stored in the comparison means and the value of the vapour delivery rate QV detested by the detection means in compared with this maximum value QLmax, 
     With regard to this second embodiment, it should be pointed out that the threshold triggering the alarm indicating a malfunction may be based on a specific mechanical structure or alternatively on a fluid-related phenomenon. 
     By virtue of another feature of the invention, also relating to this second embodiment, the alarm indicating a malfunction is disabled for a predetermined period after the liquid dispensing pump has been activated and it is then re-activated for a predetermined time so that it can be disabled again until the end of the tank-filling operation. 
     It is often necessary to disable the system in this manner, particularly at the end of the filling protest when the user finishes the operation at a low delivery rate or alternatively at the start of filling: accordingly, the invention enables the alarm to be disabled for a time to after detecting the first pulses indicating the start of liquid delivery QL, after which the alarm may be active for a time ti and finally disabled again after t 0 +t 1  until the end of filling, which is of particular advantage in the case of pre-payment. 
     It should be pointed out that the fuel dispensing system can be fitted with an additional device such as a calibrated detector (for example a detector with paddles or vanes which move with the liquid flow QL) co-operating with an alarm switch which allows the alarm to be disabled if the liquid delivery rate QL is below the maximum liquid delivery rate QLmax. 
     As a result of a preferred feature of the invention, the detection means and the comparison means are selected so that any fault in these means will also trigger the alarm to indicate a malfunction. 
     This essential characteristic, which corresponds to an active safety system, allows the alarm to be triggered to indicate a malfunction irrespective of the source of this malfunction. 
     It should be pointed out that a delivery rate measurement based on measuring a pressure difference at the terminals of a membrane by means of a pressure sensor susceptible to drift, can nor be regarded as an active safety system of the type mentioned above whereas a detector, on the other hand, transmitting an alternating signal depending on the flow rate will almost always be seen as an active safety feature. 
     The invention also relates to an installation enabling the above-mentioned method to be implemented. 
     For the purpose of the invention, such an installation conventionally comprises: 
     a storage tank for the fuel to be dispensed, 
     a dispensing pipe for the liquid incorporating a delivery pump which enables the fuel to be circulated between the storage tank and a dispenser gun at a liquid delivery rate QL, 
     a vapour recovery pipe incorporating a recovery pump enabling the vapour emitted when filling the tank to be circulated between the dispenser gun and the storage tank at a vapour delivery rate QV, 
     counting means connected into the liquid dispensing pipe and having a liquid measuring unit linked to a pulse generator or coder so that a computer can ascertain the volume and price of the fuel dispensed, which will appear in plain text on a display and 
     control means enabling the vapour delivery rate QV to be held more or less at the same level as the liquid delivery rate QL at any instant. 
     For the purpose of the invention, this installation is characterised in that it comprises. 
     detection means enabling the vapour delivery rate QV to be constantly detected, 
     comparison means sensitive to the vapour delivery rate QV detected by the detection means and enabling this delivery rate QV to be compared with a value of the liquid delivery rate QL and 
     alarm means which, if the result of this comparison is outside a predetermined range, which may be or not be controllable, triggers an alarm alerting either to a fault in the vapour recovery system, in particular the control means, or a failure of the detection means or comparison means. 
     In accordance with the invention, the signal transmitted by the alarm means may be an optical signal or an electric signal emitted, as is the case, by a detector mounted on the tracker of a magnetic member. 
     It should be pointed out that the alarm may be given simply by interrupting the delivery of fuel. 
     The configuration of the detection means and the comparison means may vary to a large degree depending on the characteristics of the fuel dispensing installation and in particular depending on whether it is adapted to the first or second of the embodiments mentioned above. 
     By way of example and in accordance with another feature of the invention the detection means may be a flow detector of the fluid oscillator type such as a flow meter with an oscillating jet or an eddy flow meter. 
     In flow meters of this type, the alternating passage of the vapour jet in front of two orifices connected to a differential pressure sensor, for example, generates an alternating pressure detected by the sensor and amplified; only the frequency of the phenomenon is taken into account not its amplitude, which is susceptible to shifts in the pressure sensor. The frequency F of the signal emitted by the amplifier is directly proportional to the vapour flow rate; this frequency F compared with a pre-established reference frequency FO enables an alarm to be triggered, for example as soon as 1.1≦F/F 0 ≦0.9. 
     If the vapour recovery system in managed by a microprocessor, this comparison operation is easy and can be set up without any additional expense. 
     An operating fault in the sensor or the amplifier or any damage at the orifices at which the differential pressure measurement is taken correspond to an absence of any signal and hence to a zero flow rate. Consequently, any malfunction in a detection system of this type will cause an alarm to be triggered and is therefore also an active safety feature. 
     By virtue of another feature of the invention, the detection means are provided in the form of a mechanical oscillator. 
     A flow detector based on the movement of a mechanical oscillator whose frequency depends on the flow rate can also be regarded as an active safety system for the same reasons as those described above. 
     In accordance with another characteristic of the invention, the detection means are provided in the form of a constrictive element, in particular of the Venturi type, connected to a system that is sensitive to pressure and provided with a mechanical memory. 
     In accordance with another feature of the invention, the detection means ray be a constrictive member, in particular of the venturi type, which do not operate except above a f low threshold which may or tray not be adjustable. 
     In accordance with another feature of the invention, the detection means are a turbine. 
     A turbine gives accurate information about flow rate and above all enables an alternating signal to be generated, for example as its vanes pass in front of a detector (optical, field-effect, etc.), and is therefore an active safety feature. 
     Any slowing down due to untimely friction or blockage of the turbine triggers an alarm. Clearly, reliable usage of a turbine would only be conceivable if dust had been totally removed from the gases. 
     By virtue of another feature of the invention, the detection means are provided in the form of a paddle or obstacle. 
     In accordance with another feature of the invention, the detection means co-operate with alarm means via optical transmission units. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The characteristics of the method and the installation proposed by the invention will be described in more detail with reference to the appended drawings, in which: 
     FIG. 1 shows a fuel dispensing installation incorporating a vapour recovery system fitted with an electronic control unit of the type used in the prior art, 
     FIG. 2 is an installation corresponding to a first embodiment of the invention, 
     FIG. 3 is a first variant of an installation corresponding to the second embodiment of the invention, 
     FIG. 4 is a detail from FIG. 3, 
     FIG. 5 is a second variant of an installation corresponding to the second embodiment of the invention, 
     FIG. 6 is an example of detection means and comparison means used with an installation corresponding to the second embodiment of the invention as illustrated in FIGS. 3,  4  and  5 , 
     FIGS. 7 a ,  7   b  and  7   c  give an example of the layout of detection means provided in the form of a mechanical oscillator, 
     FIGS. 8 and 8 a  illustrate a different operating mode of these detection means. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated in FIG. 1, the fuel dispensing installation essentially comprises a storage tank  1  for the fuel to be dispensed in Which a liquid dispensing pipe  2  is immersed enabling the fuel to be circulated to a dispenser gun  10  by means of a suction/pressure delivery pump  3  And to be so at a liquid delivery rate QL, as well as a vapour recovery pipe  16  comprising a suction/pressure recovery pump  8  enabling the vapour emitted when filling the tank to be circulated between the dispenser gun  10  and the storage tank and to be so at a vapour delivery rate QV. 
     The volume of fuel dispensed is determined by means of a liquid measuring unit  4 , connected into the dispensing pipe  2  and linked to a pulse coder  5  which emits a pulse with every one hundredth of a litre. These pulses are counted by a computer  6  in order to determine the volume dispensed and the corresponding price so that this information can be transmitted to the consumer on a display  7 . 
     The gun  10  on the one hand dispenses the liquid fuel from its end-piece  12  and on the other recovers the vapour emitted during filling by means of a suction inlet  11 . 
     To this end, it is mounted at the end of a coaxial pipe  1 , in which the fuel is conveyed through an annular section whilst the vapour are sucked in via the circular section at the centre. 
     This coaxial pipe  13  connects directly into the liquid dispensing pipe  2  whilst a separator  17  enables the vapour to be fed in the direction of the tank  1  via the vapour recovery pipe  16 . 
     In the example illustrated in FIG. 1, the recovery pump  8  is a fixed speed pump driven by a motor  9  co-operating with an electronically operated control valve  14 , the opening of which is controlled by an electronic control unit  15  fitted with a microprocessor, so as to maintain the vapour delivery rate QV equal to the liquid delivery rate QL at any instant: to this end, the electronic control unit  15  is corrected to the pulse coder  5  or to the computer  6 , so as to be supplied with the instantaneous value of the liquid delivery rate QL. This value may be transmitted either directly by the computer  6  or in the form of a number of pulses per unit of time by the pulse coder  5  then computed by the electronic control unit  15 . 
     In all cases, the value controlling opening of the electronically operated valve  14  which enables the delivery rates QL and QV to be kept equal is determined on the basis of a table stored in the microprocessor memory of the electronic control unit  15  beforehand, during a calibration process, in order to take account of the installation conditions (drops in pressure) and the actual performance of the recovery pump  8  at the time of installation. 
     As may be seen from FIG. 2, the installation illustrated in FIG. 1 is additionally equipped with detection and comparison means  20  comprising a flow meter  21  fitted on the vapour recovery pipe  16  downstream of the recovery pump  8  as well as a flow comparator  22  provided with a microprcessor. 
     The flow comparator  22  is connected to the pulse coder  5  or, as may be the case, the computer  6  so as to be supplied with an instantaneous value for the liquid delivery rate QL either directly or derived from a computation. 
     Using this value of the liquid delivery rate QL as well as the value of the vapour delivery rate QV transmitted to it by the flow meter  21 , the flow comparator  22  computes; at any instant the QV/QL ratio and, if this ratio moves outside a predetermined range stored in the microprocessor memory (for example 0.9/1.1), it transmits a signal to alarm means  20 ′ enabling an alarm to be triggered drawing attention either to a fault in the vapour recovery system or to failure of the flow meter  21  or flow comparator  22 . 
     As illustrated in FIG. 3, the fuel dispensing installation does not have an electronic control unit and the recovery pump  8  is driven by a hydraulic motor  23 , the rate of which is imparted by the passage of fuel in he dispensing pipe  2 , the energy being supplied by the delivery pump  3 . 
     A shaft  24  provides a rigid link between the hydraulic motor  23  and the recovery pump  8 , which therefore rotate at the same speed. 
     The maximum speed of the hydraulic motor  23  corresponds to a vapour delivery rate QV which is greater than the maximum liquid delivery rate QLmax. 
     This installation is calibrated on the basis of the maximum liquid delivery rate QLmax, In order to bring the vapour delivery rate QV and the liquid delivery rate QL into line, the speed of the hydraulic motor  23  is adjusted by diverting some of the liquid flow QV with the aid of a mechanically controllable hydraulic shunt  25 . 
     As illustrated in FIG. 4, a gas counter or a flow meter  26  co-operating with a check valve  27  inserted in the vapour recovery pipe  16  upstream of the recovery pump  8 , fitted during the calibration process, enables the detection and comparison means  20   a  to be controlled. These means are set up by linking a flow meter  21   a  and a flow comparator  22   a  fitted with a mechanical storage system pre-set to the maximum liquid delivery rate QLmax in a manner that will be described in more detail below. Accordingly, a signal can be forwarded to the alarm means  20 ′ a  which triggers an alarm indicating a malfunction if the ratio QV/QLmax is below an adjustable predetermined threshold. 
     As illustrated in FIG. 5, the recovery pump  8  is driven not by a hydraulic motor such as that  23  illustrated in FIG. 3 but by an independent motor  9  and the installation is initially calibrated on the maximum value of the liquid delivery rate QLmax by a mechanically adjustable pressure reducer  28 , which acts on the vapour delivery rate to obtain QV=QL. 
     In addition, the detection and comparison means  20   b  are established by connecting a flow meter  21   b  to a flow comparator  22   b  co-operating with means for disabling  29  alarm means  20 ′ b.    
     These alarm-disabling means  29  consist of a calibrated liquid flow detector  29   1  branching into the liquid dispensing pipe  2  and co-operating with an alarm switch  29   2 ; consequently; the alarm means  20 ′ b  can therefore be disabled if the liquid delivery rate QL is below a predetermined fraction of its maximum value QLmax. 
     As illustrated in FIG. 6, the detection and comparison means are established by connecting a flow detector  100  to a flow comparator  150  having a mechanical memory. 
     In this embodiment, the flow detector  100  consists of a constrictive member of the Venturi type mounted on the vapour recovery pipe  16  and provided with two pressure taps  101 ,  102 , located respectively on a level with the Venturi neck  100  and on a level with the outlet 
     It is clear that the pressure difference between the taps  101  and  102  will depend on the vapour flow rate QV. 
     The flow comparator  150 , which is an element sensitive to the pressure difference ΔP between the taps  101  and  102 , is made up of a membrane  151  with an effective surface S, which is clamped at its periphery between two half-housings  152  and  153 , to provide a tight seal. 
     The half-housings  152  and  153  are respectively provided with pressure taps  154 ,  155 , each being linked to one of the pressure taps  101 ,  102  of the Venturi  100 , 
     The membrane  151  therefore sub-divides the casing comprising the two joined half-housings  152 ,  153  into two chambers  152 ′,  153 ′. 
     The pressure on a level with the neck of the Venturi  100  prevails in chamber  152 ′ which is connected to the pressure tap  101  whilst the pressure on a level with the outlet of the Venturi  100  prevails in chamber  153 ′ which is connected to the pressure tap  102 . 
     Furthermore, the membrane  151  is joined to and bears a plate  156  on which a rod  157  is fixed, extending inside a cylindrical appendage  157   1  extending the chamber  153 ′ connected to the pressure tap  102 . 
     The cylindrical appendage  157   1  is provided with two windows  160 ,  161  made from a transparent material positioned respectively facing two optical fibers  158 ,  159 , one of which  158  is linked to a light source whilst the other  159  is linked to a photo-receiver, not illustrated, which is connected to an amplifier allowing the alarm to be triggered, indicating malfunction if the photo-receiver is not receiving any light. 
     The presence of the rod  157  between the windows  160 ,  161  prevents the light from being transmitted from the optical fibre  158  to the optical fibre  159 , thus triggering the alarm. 
     Furthermore, the chamber  1521  connected to the pressure tap  101  encloses a spring  162  which is very flexible but compressed across a long length by means of an adjusting screw  162 ′ to allow the plate  156  joined to the membrane  151  to be applied against the walls of the half-housing  153  with a force F when in the position illustrated in FIG. 6, in which the rod  157  obscures the windows  160  and  161 . 
     From this position, when the vapour delivery rate QV increases, the pressure differential ΔP between the taps  101  and  102  also increase until the membrane  151 , due to the effect of the pressure prevailing in chamber  153 ′ connected to the pressure tap  102 , exerts a force SΔP greater than the force F and opposing the latter At this instant, the membrane  151  is suddenly retracted and the rod  157  exposes the windows  160 ,  161 ; light is then able to pass between the optical fibres  158  and  159  towards the photo-receiver. 
     It should be pointed out that when the installation is calibrated, the flow comparator  150  is calibrated by means of the adjusting screw  162 ′ to allow light to pass through, starting from a threshold value of the ratio between the vapour delivery rate QV and the maximum liquid delivery rate QLmax (for example when QV/QLmax≧0.9). 
     The system described above affords active safety features because: 
     the light is only transmitted during normal operation and the alarm is triggered if the light source is no longer emitting or if the photo-receiver is out of service, 
     if the membrane  151  is punctured or cracked, it will not allow light to pass between the optical fibres  158  and  159 , 
     a connection fault between the pressure taps  101 ,  154  and  102 , 155  corresponds to the same effect. 
     This type of system is therefore, in effect, a system of mechanical memory for the maximum liquid pressure QLmax. 
     It should be pointed out that optical detection of a malfunction has advantages in terms of safety (hazardous atmosphere) although it would alto be possible to replace the rod  157 , in a manner not illustrated in the drawings, with a magnetic element connected to a Hall-effect detector or a “Reed” or pneumatic relay or more simply to set up the rod  157  so that any displacement observable from the exterior corresponds to a change of colour to the observer. 
     It should also be pointed out that the Venturi  100  illustrated in FIG. 6 is assumed to have an angle of 7°±2° so that the function ΔP=f(QV) is a continuous function. 
     An angle shift in excess of 14°, for example, would render the phenomenon discontinuous. In practice, at a low delivery rate, the jet leaving the neck  101  of the Venturi  100  may not open out and cling to the walls thereof, which would make it impossible to obtain a pressure differential ΔP between the pressure taps  101  and  102 . 
     Over and above a certain flow rate, the jet might cling to the walls of the Venturi and cause a pressure differential. The rate at which this phenomenon occurs can be adjusted by placing an obstacle in the outlet path of the vapour with an adjustable position. 
     Adding this feature would make it possible to obtain a trigger threshold based on a fluid-related phenomenon and an inexpensive commercially sold pressure sensor would suffice to trigger the alarm on an “all or nothing basis”. 
     In the example illustrated in FIGS. 7 a ,  7   b  and  7   c , the detection means consist of an oscillator of the mechanical type. 
     The oscillator illustrated in FIG. 7 b  consists of a cylindrical disc B one the one hand suspended by a torsion wire C embedded by its ends d and d′ and on the other hand having two shoulders E 1  and E 2 . 
     In FIG. 7 a , the cylinder B, illustrated in cross section, has two curved passages C 1  and C 2  bored through it, each hating an inlet orifice G 1 , G 2  and an outlet orifice H 1 , R 2  opening to tho outside on a level with the shoulders E 1  and E 2 . 
     The passages C 1  and C 2  each have a straight section adjacent to the inlet orifice G 1 , G 2  as well as a curved section adjacent to the outlet orifice E 1 , H 2   
     The two straight sections extend substantially parallel in immediate proximity with one another whilst the two curved sections are divergent. 
     As shown in FIG. 7 a , the inlet orifices G 1 , G 2  of the passages C 1  and C 2  of the cylinder B are positioned facing a fixed piece A mounted on the vapour recovery pipe  16  which has an incoming passage C 0  for the vapour flow QV. 
     If the vapour flow QV is zero, the cylinder B is in the non-operating position and the inlet orifice G 1  of the passage C 1  is located facing the passage C 0  of piece A as illustrated in FIG. 7 a.    
     When the vapour flow QV starts, the jet entering the passage C 1  via the inlet orifice G 1  leaves this passage by means of the outlet orifice H 1  located on a level with the shoulder E 1 . 
     Because of the specific geometry and mounting of the cylinder B, this flow causes it to rotate at an angular velocity ω. 
     As a result of this rotating notion, the inlet orifice G 2  of the passage C 2  is displaced in front of the passage C 0  of piece A, thereby driving the cylinder B in rotation at a velocity ω in the opposite direction and so on. 
     An oscillating motion is therefore produced which can be detected by an optical sensor, not illustrated, allowing the alarm to be triggered. 
     As illustrated in FIG. 7 c , the angular velocity ω applied to this oscillating system significantly modifies the natural oscillation frequency T 0  of piece B producing an oscillation frequency T 1  directly related to the vapour flow QV. 
     In the example illustrated in FIGS. 8 and 8 a , the vapour flow QV to be detected is channelled through an end-piece  101  mounted directly on the vapour recovery pipe  16  so that it enters a casing  102  with an outlet orifice  103  as a jet. 
     In FIG. 8, the median part of the casing  102  is provided with two metal blades  104  and  105  disposed symmetrically and attached to the walls of the casing at points  106  and  107 . 
     In FIG. 8 a , each of the blades  104 ,  105  has a flexible part  104   a ,  105   a  close to the points of attachment  106 ,  107  as well as a thicker part  104   b ,  105   b  of a curved shape which extends freely. 
     The two curved parts  104   b  and  105   b  form between them a Venturi of sorts. 
     Because of the design described above, as it passes between the two plates  104 ,  105 , the vapour jet QV causes a drop in pressure compared with the rest of the volume of the casing  102 , causing these two plates  104 ,  105  to be displaced towards one another until they touch one another and locally interrupt the flow QV, which causes the plates to return to their initial position and so on. 
     Accordingly, an oscillating system is obtained whose frequency depends on the vapour flow QV This frequency may be measured by the interruption caused in a light beam, not illustrated, when the plates  104 ,  105  come into contact. 
     Again, this is an active safety feature given that the alternating signal disappears as soon as oscillation is no longer possible or the light beam is interrupted for some accidental reason.