WO1996014561A1

WO1996014561A1 - A method and a device for testing the tightness of a closed container

Info

Publication number: WO1996014561A1
Application number: PCT/SE1994/001104
Authority: WO
Inventors: Bo Harde
Original assignee: Volvo AB
Current assignee: Volvo AB
Priority date: 1994-11-04
Filing date: 1994-11-21
Publication date: 1996-05-17
Anticipated expiration: 1997-05-04
Also published as: SE9403781D0; SE515496C2; SE9403781L

Abstract

The invention relates to a method for testing the air-tightness of a closed container (3), comprising an evacuation of the container (3), measuring the pressure (p) in the container (3), measuring the time and providing an indication depending on values of the pressure (p), wherein the container (3) can be evacuated during a certain predetermined time period (t0). The method according to the invention is characterized in that it comprises: a first level detection, wherein the current value of the pressure (p) is compared with a first limit value (p2), the evacuation being switched off if the pressure is below said limit value (p2); and a second level detection after said time period (t0) has elapsed, wherein the current value of the pressure (p) is compared with a second limit value (p3), an indication regarding a leakage being provided if the second limit value (p3) has been exceeded. The invention provides a simplified and cheap, but still safe, testing of the air-tightness of closed containers, in particular fuel tanks for motor vehicles. The invention also relates to a device intended for use in the above-mentioned tightness testing of a container.

Description

A method and a device for testing the tightness of a closed container

TECHNICAL FIELD:

The present invention relates to a method for testing the air-tightness of a container in accordance with the preamble of claim 1. The invention also relates to a device for such testing in accordance with the preamble of claim 4.

BACKGROUND OF THE INVENTION:

In the field of motor vehicles it is of great importance, not least from a safety point of view, that no leakage of fuel from the fuel tank occurs. To this end, there is a demand for devices and methods for testing and controlling the air-tightness of the fuel tank. This type of testing can for example be carried out during a manufacturing process for motor vehicles or during service of motor vehicles in a workshop.

A system for air-tightness testing of a fuel tank is previously known from US-A-5261379. The basic principle of this system is that air is pumped out of the tank by means of a vacuum pump, after which the pressure in the tank is measured and compared with a certain threshold level. The device is intended to be used in a motor vehicle which is running, and is not directed towards the problem of preventing leakage by controlling the air-tightness of a tank as early as during the manufacture of the vehicle.

SUMMARY OF THE INVENTION:

A main object of the present invention is to provide a method and a device which provide a simplified and cheap, although safe, testing of the air-tightness of closed containers, in particular fuel tanks for motor vehicle, in connection with manufacture or service of the motor vehicle. This is accomplished by means of a method, the features of which will become apparent from the characterizing portion of claim 1. The invention also relates to a device for such testing, the features of which will become apparent from the characterizing portion of claim 4.

In accordance with a preferred embodiment, the pressure in the tank is lowered by means of a pump. Subsequently, a first level detection is carried out, wherein the prevailing pressure in the tank is compared with a first limit value and the pump is switched off if the pressure is below this limit value, and a second level detection after a certain time period has elapsed, wherein the pressure is compared with a second limit value and an indication of a leakage is issued if the second limit value is exceeded. In this manner, a safe and reliable measurement is achieved in which the result of the measurements is not presented until said time period has elapsed.

In accordance with a further embodiment, the invention comprises a further level detection, wherein the value of the pressure is compared with a third limit value which corresponds to an overpressure in the fuel tank. An indication is issued if said third limit value is exceeded. In this manner, an advantage as regards safety is provided in the case where an unallowable overpressure exists in the fuel tank.

In accordance with a preferred embodiment of a device according to the invention, a current switch in the form of a thyristor is used, which thyristor is influenced by the above-mentioned level detections so that the pump is switched off when the pressure is below said first pressure level and can only be switched on again after a new manual triggering of the device.

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will be described in greater detail in the following with reference to the annexed drawings, in which

Fig. 1 shows the invention in a schematic form.

Fig. 2 is a flow chart showing the operation of a device according to the invention,

Fig. 3 shows a graph of the operation according to Fig. 2, and

Fig. 4 shows a detailed electrical circuit of the invention according to a preferred embodiment.

PREFERRED EMBODIMENT:

Fig. 1 shows a device according to the present invention in the form of a block diagram. In accordance with a preferred embodiment, the device comprises a central testing unit 1, the design of which will be described in detail below with reference to Fig. 4. A pressure sensor 2 is connected to the testing unit 1, which sensor is in turn arranged at a measuring object in the form of a tank 3. According to the preferred embodiment, the tank 3 is the fuel tank of a motor vehicle, but the invention can also be used in connection with other types of containers and vessels, the air-tightness of which it is desired to test.

The pressure sensor 2 supplies a signal to the testing unit

1, which signal constitutes a measure of the pressure p which is present in the tank 3. The testing unit 1 is connected to a display 4 which shows a measurement of the signal which is supplied from the pressure sensor 2. Furthermore, the testing unit 1 comprises a first indicator 5 and a second indicator 6, which preferably are in the form of light emitting diodes having different colours, for example red and green, respectively. The indicators 5, 6 indicate different pressure levels, in accordance with that which will be described in greater detail below. Also, a third indicator 7 in the form of a yellow light emitting diode is provided, which indicator indicates that a testing procedure is in progress.

A supply voltage is supplied to the testing unit 1 by means of a power supply unit 8 of conventional kind. A push button 9 in the form of a spring-back switch is also connected to the testing unit 1. The push button 9 is used to trigger the measuring operation for the air-tightness testing. The testing unit 1 is also connected to a pump 10, the operation of which can be controlled electrically by means of signals which are supplied by the testing unit 1. The purpose of the pump 10 is to pump out air from the tank 3 so as to lower the pressure in the tank 3. The pump 10 is connected to the tank 3 via a first hose 11 and to the surrounding air via a second hose 12.

The basic operation of the device will now be explained with reference to Figs. 2 and 3. Fig. 2 shows a flow chart for a method for tightness testing according to the invention. It should be noted that the numerals shown in Fig. 3 correspond to the reference numerals in Fig. 2.

When the tightness of the tank is to be tested, the pressure sensor 2 is first connected to the testing unit 1. Furthermore, the pump 10 is connected to the tank 3. The testing of the tightness of the fuel tank is triggered by pressing the start button 9 to initiate a measuring cycle (box 13). This causes the pump 10 to be activated by means of the testing unit 1 (box 14). Simultaneously, a time measurement is initiated in a time measuring circuit which forms part of the testing unit (box 15). The time measurement proceeds during a certain time period t₀. When the pump 10 is activated, the pressure p in the tank 3 will decrease due to the fact that air is pumped out of the tank 3. This operation is shown in Fig. 3, which illustrates the relationship between time t (x-axis) and pressure p (y- axis). During the entire testing operation, the pressure p in the tank 3 is measured by means of the pressure sensor 2 and a measuring signal is supplied to the testing unit 1. When a certain testing round is initiated it is assumed that the value supplied by the pressure sensor corresponds to the normal atmospheric pressure, which is denoted by "NORM" in Fig. 3.

If an overpressure is present initially in the tank, which can be the result of an increased temperature of the fuel or if the pump 10 has been connected in a wrong manner, this will be registered in the testing unit 1 by an abnormally high value of the signal being supplied by the pressure sensor 2. The value of the current pressure p is thus higher than, or equal to, a certain predetermined limit value p_x (box 16), and the time t which has elapsed since the measurement was initiated not having exceeded t₀. This causes the testing unit 1 to issue a warning signal by activating a light emitting diode (box 17).

Furthermore, the testing unit 1 is adapted to control whether the time t₀ has elapsed (box 18). As long as this has not yet occured, a control as to whether the magnitude of the pressure is less than or equal to a predetermined value p₂ is carried out (box 19, see also Fig. 3). If no underpressure in the tank has arisen, i.e. if the pressure p has not decreased to the limit value p₂ after the time t₀ has elapsed (in spite of the fact that the pump 10 is activated) , this is an indication of the fact that a major leakage is present in the tank 3 (box 20). As a consequence, the testing unit 1 supplies a signal to the pump 10 so as to switch off said pump. Furthermore, an indicator 5 on the testing unit will be switched on, which indicator 5 preferably is a red light emitting diode which is lit so as to indicate "leakage" (box 21). Simultaneously, the present pressure is indicated on the display 4, after which the testing procedure is completed. Thereafter, the system adopts a "stand by" mode, and a new testing procedure can be started when the start button 9 is pressed again.

If an underpressure arises in the tank 3 before the time period t₀ has elapsed, i.e. if the value of the present pressure has dropped to p₂ (box 19), the pump 10 will be switched off (box 22). Thereafter, the pressure will rise slightly due to the fact that the pressure in the tank is equalized. If there is no leakage, the pressure will be stabilized at a level which is slightly higher than p₂. When the time period t₀ has elapsed (box 23), the result of the pressure measurement will be presented on the display 3 and by the indicators 5 and 6, respectively. If the pressure p is lower than a certain maximum allowed pressure p₃, an indicator 6, preferably in the form of a green light emitting diode, lights up. This indicates "no leakage" (box 25). However, if the pressure p is higher than said limit value p₃, another indicator 5, which is a red light emitting diode, will be lit. This indicates a "leakage" (box 26) .

In accordance with the preferred embodiment, the result of the measurement is not shown until the measuring time t₀ has elapsed. In this manner, mistakes can be avoided when the test first appears to provide an acceptable result and then a change from "no leakage" to "leakage" occurs towards the end of the measuring period. Such errors can otherwise occur when the leakage is low. Furthermore, it should be noted that the time period t₀ can be chosen in an arbitrary manner. According to the preferred embodiment, the time period t₀ is set at 30 seconds but if more accurate measurements is required the time period t₀ can be longer, for example 30 or 60 minutes.

The detailed design of the invention will now be described with reference to Fig. 4, which shows the components which form part of and are connected to the testing unit 1. A mains voltage 27 is connected to the power supply unit 8, which is in the form of a transformer, which in turn is connected to a rectifier 28.

The rectifier 28 supplies a rectified voltage to a voltage stabilizer, which comprises a standard component in the form of a voltage regulator 29, preferably of 12 V, and two capacitors 30 and 31, respectively. The stabilized voltage is supplied to a second voltage regulator which comprises a regulator circuit 32 in the form of a standard component and a capacitor 33 which is used for filtering the voltage from the regulator circuit 32. The second voltage regulator supplies a voltage of 5 V via a resistor 34 to a first connection terminal 35 for the pressure sensor 2. Furthermore, a second connection terminal 36 and a third connection terminal 37 for the pressure sensor 2 are provided. The pressure sensor 2 detects the pressure in the fuel tank 3 and supplies a measurement signal to the third connection terminal 37. From the resistor 34, a connection to a light emitting diode 38 is also provided, which is connected to earth via a further resistor 39. The light emitting diode 38 is normally lit, but is extinguished if any error occurs in the pressure sensor 2, for example if the connection terminals 35 and 36 are shortcircuited. The light emitting diode 38 also provides an indication of whether the regulator circuit 32 is operating, since the light emitting diode 38 is extinguished if the regulator circuit 32 ceases to supply a voltage of 5 V.

The display 4 is connected to the third connection terminal of the pressure sensor 2, which display presents a measurement of the pressure which is registered by means of the pressure sensor 2.

In order to start a testing procedure, the push button 9 is pressed. The push button 9 is of the spring-back type and is used in order to close two switches 40 and 41, respectively. When the first switch 40 is closed, a time measuring circuit or "timer" circuit 42 is activated. The timer circuit is preferably formed by a known standard component. Between the supply voltage and earth, a capacitor 43 is connected which functions as a protection against unintentional triggering of the timer circuit 42 due to current transients.

If the push button 9 is pressed, the timer circuit 42 is activated. This causes a relay 44 to be activated, which in turn causes two contacts 45 and 46, respectively, to be closed, i.e. they adopt the positions shown in Fig. 4. Furthermore, the relay 44 is provided with a diode 47 for protection against reverse voltages. Furthermore, the contact 45 is connected to a thyristor 48, which in turn is connected to a second relay 49.

When the second contact 41 of the start button 9 is closed, the supply voltage is connected as a gate voltage to the gate terminal 50 of the thyristor. A current-limiting resistor 51 is connected between the contact 41 and the gate terminal 50 of the thyristor. When the start button 9 is pressed, the timer 42 will be activated, whereby the relay 44 moves the contact 45 to the position which is shown in Fig. 4 and a current is supplied to the thyristor 48. Simultaneously, the thyristor 48 will be conductive due to the fact that the contact 41 is closed. This causes the relay 49 with its contacts 52 to be activated, whereby the mains voltage 27 is connected to the pump 10. The pump 10 is connected to the fuel tank 3 via a first hose 11 and to the surrounding air by means of a hose 12. When the pump 10 is activated, air is pumped out of the tank 3, which can be viewed on the display 4 as a lowered value of the pressure.

The signal from the pressure sensor 2 influences three level-detecting switching elements in the form of transistors 53, 54 and 55, respectively, each of which is provided with a current-limiting resistor 56, 57 and 58, respectively. A relay 59, which is provided with a protecting diode 60 and a capacitor 61 which is used for filtering current transient, is connected to the collector of the first transistor 53. A diode 62, which together with the voltage drop across the transistor 53 defines a voltage level at which the transistor 53 is conductive, is connected between the emitter of the transistor 53 and earth. This voltage level corresponds to a certain pressure level p₃ (see also Fig. 3) which constitutes a limit level for determining whether a leakage is present or not. The relay 59 comprises a contact 63 which can adopt two different positions. In the respective position, one of two light emitting diodes 64 and 65, respectively, is connected into the circuit provided that the contact 46 is in its "lower" position (see Fig. 4), i.e. a position corresponding to the timer circuit being inactive. The light emitting diode 64 is preferably in the form of a red light emitting diode (the purpose of which is to indicate that a leakage is present), whereas the light emitting diode 65 preferably is in the form of a green light emitting diode (the purpose of which is to indicate that no leakage is present) .

A further light emitting diode 66 indicates that a testing procedure is in progress. According to the embodiment, the light emitting diode 66 emits a yellow light. A resistor 67 is connected between the light emitting diode 66 and earth.

As mentioned above, the signal from the pressure sensor 2 also affects the transistor 54, which is connected to a further relay 68 having a protecting diode 69. Furthermore, a capacitor 70 is connected between earth and the cathode of the diode 69. The capacitor 70 constitutes a filter against current transients. The relay 68 can influence a contact 71 which is connected in series with the thyristor 48. When the signal from the pressure sensor 2 reaches a certain limit value corresponding to the pressure level p₂ (see also Fig. 3), the transistor 54 will cease to conduct current, causing the relay 68 to release the contact 71 so that the latter becomes open. This causes the current to the thyristor 48 to be disconnected, which in turn causes the pump 10 to be switched off.

If the contact 71 is open, the thyristor 48 returns to its cut off position, i.e. it does not conduct current. If subsequently the contact 71 is closed (as a result of the signal from the pressure sensor 2 again rising above its limit value) the pump 10 can however not be started again without pressing the start button 9, which once again triggers the thyristor 48 to conduct current.

Furthermore, the signal from the pressure sensor 2 also affects the transistor 55, the collector of which is connected to a relay 72 having a protecting diode 73. A diode arrangement 74 comprising three diodes defines, together with the voltage across the transistor 55, a voltage level corresponding to a certain limit value p_: (see also Fig. 3) of the pressure. This limit value p_x corresponds to the condition of an abnormally high overpressure being present in the fuel tank 3. Furthermore, the relay 72 is connected to a light emitting diode 75, which is connected to earth via a resistor 76 and a pair of contacts 77 which cooperate with the relay 72. If the signal from the pressure sensor 2 reaches a certain value which corresponds to the condition that an unallowable overpressure is present in the fuel tank 3, the relay 72 will affect the pair of contacts 77 to adopt the position shown in Fig. 4. Otherwise, the pair of contacts 77 is normally in a position where the thyristor 48 is connected to earth. If the relay 72 is activated, the current through the thyristor 72 will be cut off, which in turn causes the pump 10 to be switched off. Simultaneously, the light emitting diode 75 is activated, which functions as an indicator of any overpressure in the fuel tank 3. In the case that an overpressure has been acknowledged and the pressure p in the tank 3 again decreases to a value which is below the limit value p the pump cannot be activated until the start button 9 is pressed again.

Finally, it should be noted that the device comprises two fuses 78 and 79, respectively.

The procedure for the air-tightness testing of the tank 3 according to the present invention will now be described, mainly with reference to Fig. 4. When the push button 9 is pressed, the contacts 40 and 41 are closed. This causes the timer circuit 42 to be activated, which in turn causes the relay 44 to be activated, i.e. the contacts 45 and 46, respectively, are moved to the positions which are shown in Fig. 4. This implies that the contact 46 activates the light emitting diode 66 so that the latter indicates that a testing procedure is in progress. The timer circuit 42 keeps the relay 44 in the same position until the time period t₀ has elapsed. Also, the contact 71 is maintained in its closed condition, and the contact 77 is in a condition where the thyristor 48 is connected to earth via the relay 49. As a consequence of the fact that the contact 41 is closed, the thyristor will be conductive, which causes the relay 49 to activate the pump 10. In this manner, air is pumped out of the tank 3.

As long as a measurement of the pressure p takes place, a value of the current pressure will be supplied by the pressure sensor 2 and be shown on the display 4.

In the case that an initial overpressure occurs, i.e. if the pressure p in the tank 3 exceeds a certain predetermined limit value p_lf the voltage across the transistor 55 will be so high that it will be conductive. This causes the relay 72 to be activated and forces the relay 72 with its contact 77 to the position shown in Fig. 4. As a consequence, the current through the thyristor 48 is cut off, whereby the pump 10 is switched off. Simultaneously, the light emitting 75 is lit. Even if the pressure in the tank 3 again falls below the limit value P_J, the pump 10 will not be activated if not the start button 9 is pressed again.

During the measurement, the transistor 54 senses the signal from the pressure sensor 2. If the pressure p in the tank 3 becomes so low that it is below the limit value p₂, the transistor 54 will cease to be conductive, which causes the relay 68 to be released and its contact 71 to be open. This causes the current through the thyristor 48 to be switched off, which in turn causes the pump 10 to be switched off. Even if the pressure again increases and exceeds the limit value p₂, the pump 10 cannot be activated until the push button 9 is pressed again.

Furthermore, when the time t₀ the timer circuit 42 is set to has elapsed, the relay 44 will re-position the contacts 45 and 46 to their open positions. In this regard, the contact 46 will be moved from the position shown in Fig. 4 and will instead activate any one of the light emitting diodes 64 or 65 (via the contact 63). If the signal from the pressure sensor 2 is higher than a certain limit value p₃, the transistor 53 will be conductive. This causes the relay 59 to be activated in such a manner that the contact 63 adopts the position shown in Fig. 4. This causes the red light emitting diode 65 to be activated, which is an indication that a leakage is present. If however the signal from the pressure sensor 2 is below the limit value p₃, the transistor 53 will not be conductive. This causes the relay 59 to adopt its second position in which the light emitting diode 64 is activated. Consequently, this is an indication that there is no leakage in the tank 3. Simultaneously, a value of the pressure p is shown on the display 4.

The invention is not limited to the described embodiments, but may be varied within the scope of the appended claims. In addition to fuel tanks for motor vehicles, the invention can be used for testing the air-tightness in other types of closed containers, for example cooling systems, braking systems or filler tubes. The air-tightness of the respective components can be tested either when the component is provided as a separate article or when it is arranged as a built-in part of, for example, a motor vehicle.

Furthermore, the switching elements do not have to be transistors, but may be in the form of other level- detecting switches . The pressure sensor 2 itself can be provided as a pre- mounted sensor in certain motor vehicles. In such case, a low cost testing device is provided.

Claims

CLAIMS :

1. Method for testing the air-tightness of a closed container (3), comprising an evacuation of the container (3), measuring the pressure (p) in the container (3), measuring the time and providing an indication depending on values of the pressure (p), wherein the container (3) can be evacutated during a certain predetermined time period (t₀), c h a r a c t e r i z e d i n that the method comprises: a first level detection, wherein the current value of the pressure (p) is compared with a first limit value (p₂) , the evacuation being switched off if the pressure is below said limit value (p₂); and a second level detection after said time period (t₀) has elapsed, wherein the current value of the pressure (p) is compared with a second limit value (p₃), an indication regarding a leakage being provided if the second limit value (p₃) has been exceeded.

2. Method according to claim 1, c h a r a c t e ¬ r i z e d i n that a third continuous level detection is carried out during said time period (t₀), wherein the current value of the pressure (p) is compared with a third limit value (p_x) which corresponds to a pressure (p) which is higher than said first and second limit values (p₂, p₃), an indication being issued if said third limit value p_{) is exceeded.

3. Method according to claim 1 or 2, c h a r a c t e - r i z e d i n that an indication regarding a leakage is provided if the pressure is not below said first limit value (p₂) before said time period (t₀) has elapsed.

4. Device for testing the air-tightness of a closed container (3) comprising means (2) for measuring the pressure (p) in the container (3) and means (10) for evacuating the container (3) depending on signals from a control unit (1) to which said means (2) is connected, and comprising a time measuring device (42) and at least one indicator (5, 6; 64, 65) for providing indications depending on measured values of the pressure (p) , said means (10) for evacuation being adapted to be activated during a certain time period (t₀), c h a r a c t e ¬ r i z e d i n that the device comprises first level- detecting means (54) which is adapted to switch off the evacuation if the pressure (p) is below a first limit level (p₂) , and second level-detecting means (53) which is connected to said at least one indicator (64, 65) and adapted to indicate whether the pressure (p) exceeds a second limit value (p₃) after said time period (t₀) has elapsed.

5. Device according to claim 4, c h a r a c t e r i z e d i n that it comprises a third indicator (75) for indication of an unallowable high pressure and third level- detecting means (55) adapted to activate said third indicator (75) if the current value of the pressure (p) exceeds a third limit value (pj) .

6. Device according to any one of claims 4 or 5, c h a r a c t e r i z e d i n that the respective level- detecting means (53, 54, 55) comprises a transistor.

7. Device according to any one of claims 4-6, c h a r a c t e r i z e d i n that it comprises a controllable current switch (48) which can be set in a conductive state when a switch associated with the control unit (1) is activated, whereby the current switch (48) in its conductive state activates said means (10) for evacuation.

8. Device according to claim 7, c h a r a c t e ¬ r i z e d i n that said current switch (48) is adapted to be controlled to a non-conductive state depending on the condition of any one of said first and third level- detecting means (54, 55), wherein said means (10) for evacuation can be set in a non-active state.

9. Device according to claim 8, c h a r a c t e ¬ r i z e d i n that said current switch (48) is adapted to adopt its non-conductive state if the first level-detecting means (54) detects that the current value of the pressure (p) is below said first limit value (p₂) or if the third level-detecting means (55) detects that the current value of the pressure (p) exceeds said third limit value (Pi) .

10. Device according to any one of claims 7-9, c h a r a c t e r i z e d i n that said current switch (48) is a thyristor.

11. Device according to any one of claims 7-10, c h a r a c t e r i z e d i n that said switch (9) is also adapted for activating said time measuring means (42) .

12. Device according to any one of claims 4-11, c h a r a c t e r i z e d i n that it comprises control means (35, 36, 38) for detecting and indicating if said means (2) for measuring the pressure (p) is connected in a wrong manner. AMENDED CLAIMS

[received by the International Bureau on 3 April 1996 (03.04.96); original claims 1-12 replaced by amended claims 1-10 (3 pages)]

CLAIMS:

1. Method for testing the air-tightness of a closed container (3), comprising an evacuation of the container (3) by means of a pump (10), measuring the pressure (p) in the container (3), measuring the time and providing an indication depending on values of the pressure (p), wherein the container (3) can be evacutated during a certain predetermined time period (t₀), c h a r a c t e r i z e d i n that the method comprises: a first level detection, wherein the current value of the pressure (p) is compared with a first limit value (p₂), the evacuation being switched off if the pressure is below said limit value (p₂); a second level detection after said time period (t₀) has elapsed, wherein the current value of the pressure (p) is compared with a second limit value (p₃), an indication regarding a leakage being provided if the second limit value (p₃) has been exceeded; and an indication regarding a leakage if the pressure is not below said first limit value (p₂) before said time period (t₀) has elapsed.

2. Method according to claim 1, c h a r a c t e ¬ r i z e d i n that a third continuous level detection is carried out during said time period (t₀), wherein the current value of the pressure (p) is compared with a third limit value (pj) which corresponds to a pressure (p) which is higher than said first and second limit values (p₂, p₃), an indication being issued if said third limit value (p is exceeded. 3. Device for testing the air-tightness of a closed container (3) comprising means (2) for measuring the pressure (p) in the container (3) and a pump (10) for evacuating the container (3) depending on signals from a control unit (1) to which said means (2) is connected, and comprising a time measuring device (42) and at least one indicator (5, 6; 64, 65) for providing indications depending on measured values of the pressure (p), said means (10) for evacuation being adapted to be activated during a certain time period (t₀), c h a r a c t e ¬ r i z e d i n that the device comprises first level- detecting means (54) which is adapted to switch off the evacuation if the pressure (p) is below a first limit level (p₂), second level-detecting means (53) which is connected to said at least one indicator (64, 65) and adapted to indicate whether the pressure (p) exceeds a second limit value (p₃) after said time period (t₀) has elapsed, and a controllable current switch (48) which can be set in a conductive state when a switch (9, 40, 41) associated with the control unit (1) is activated, whereby the current switch (48) in its conductive state activates said pump (10).

4. Device according to claim 3, c h a r a c t e r i z e d i n that it comprises a third indicator (75) for indication of an unallowable high pressure and third level- detecting means (55) adapted to activate said third indicator (75) if the current value of the pressure (p) exceeds a third limit value (Pi).

5. Device according to any one of claims 3 or 4, c h a r a c t e r i z e d i n that the respective level- detecting means (53, 54, 55) comprises a transistor.

6. Device according to any one of claims 3-5, c h a r a c t e r i z e d i n that said current switch (48) is adapted to be controlled to a non-conductive state depending on the condition of any one of said first and third level-detecting means (54, 55), wherein said means (10) for evacuation can be set in a non-active state.

7. Device according to claim 6, c h a r a c t e ¬ r i z e d i n that said current switch (48) is adapted to adopt its non-conductive state if the first level-detecting means (54) detects that the current value of the pressure (p) is below said first limit value (p₂) or if the third level-detecting means (55) detects that the current value of the pressure (p) exceeds said third limit value (pi .

8. Device according to any one of claims 3-7, c h a r a c t e r i z e d i n that said current switch

(48) is a thyristor.

9. Device according to any one of claims 3-8, c h a r a c t e r i z e d i n that said switch (9) is also adapted for activating said time measuring means (42) .

10. Device according to any one of claims 3-9, c h a r a c t e r i z e d i n that it comprises control means (35, 36, 38) for detecting and indicating if said means (2) for measuring the pressure (p) is connected in a wrong manner.