Fumagalli, Alessandro (Via per Intimiano 1, Cantù_CO, I-22063, IT)
Cattaneo, Massimo (Via Pietro Nenni 4/C, Bollate MI, I-20021, IT)
Giannantonio, Roberto (Via Montagnina 9, Oleggio NO, I-28047, IT)
Fumagalli, Alessandro (Via per Intimiano 1, Cantù_CO, I-22063, IT)
Cattaneo, Massimo (Via Pietro Nenni 4/C, Bollate MI, I-20021, IT)
| 1. | Method for measuring the pressure (P) of the residual gases in a vacuum environment (7), characterized by comprising the following operating steps: arranging in this environment (7) a sensor comprising a control circuit (6) connected to at least one resistor (2) suitable for dissipating heat through the residual gases; transmitting a first electromagnetic signal (9) to a first inductor (8) connected to said control circuit (6); heating through Joule effect the resistor (2) with an electric current (I) supplied by said control circuit (6); determining the pressure (P) of the residual gases as a function of the variation of the resistance of the resistor (2) due to its heating. |
| 2. | Method according to claim 1, characterized in that the first electromagnetic signal (9) is transmitted by a second inductor (20) of an apparatus arranged outside the vacuum environment (7). |
| 3. | Method according to one of the previous claims, characterized in that said sensor transmits outside the vacuum environment (7) a signal which varies as a function of the variation of the resistance of the resistor (2). |
| 4. | Method according to claim 3, characterized in that the frequency (f) of said signal transmitted outside the vacuum environment (7) varies as a function of the variation of the resistance of the resistor (2). |
| 5. | Method according to claim 4, characterized in that said frequency (f) is comprised between 0,1 kHz and 1 MHz. |
| 6. | Method according to claim 3 or 4, characterized in that said signal transmitted outside the vacuum environment (7) is an acoustic signal. |
| 7. | Method according to one of claims 3 to 5, characterized in that said signal transmitted outside the vacuum environment (7) is a second electromagnetic signal (11) transmitted by a third inductor (10) connected to said control circuit (6). |
| 8. | Method according to claim 1 or 7, characterized in that the first (9) and/or the second (11) electromagnetic signal are substantially sinusoidal. |
| 9. | Method according to claim 1,7 or 8, characterized in that the first (9) and/or the second (11) electromagnetic signal are generated by resonant circuits. |
| 10. | Method according to claim 9, characterized in that the first (9) and/or the second (11) electromagnetic signal are generated by underdamped LC circuits comprising the second (20) and/or the third (10) inductor, respectively. |
| 11. | Method according to one of claims 7 to 10, characterized in that the apparatus arranged outside the vacuum environment (7) controls the frequency of the first electromagnetic signal (9) according to the frequency of the second electromagnetic signal (11). |
| 12. | Method according to one of the previous claims, characterized in that the electric current (I) which crosses the resistor (2) is constant. |
| 13. | Method according to one of the previous claims, characterized in that the electric power (Qe) supplied to the resistor (2) is constant. |
| 14. | Sensor for measuring the pressure (P) of the residual gases in a vacuum environment (7), which comprises at least one resistor (2) suitable for dissipating heat through the residual gases in this environment (7), characterized in that said resistor (2) is connected to a control circuit (6) which is in turn connected to a first inductor (8) suitable for being electromagnetically coupled with a second inductor (20) arranged outside the vacuum environment (7) for supplying an electric current (I) to the resistor (2). |
| 15. | Sensor according to claim 14, characterized in that the resistor (2) is made up of a wire of a conductive material arranged in a housing (1) which is connected with the vacuum environment (7). |
| 16. | Sensor according to claim 15, characterized in that the housing (1) is permeable to the gases. |
| 17. | Sensor according to claim 16, characterized in that the housing (1) is made up of a tube of a nonporous material provided with a plurality of holes. |
| 18. | Sensor according to claim 16, characterized in that the housing (1) is made up of a tube of a porous material. |
| 19. | Sensor according to one of claims 15 to 18, characterized in that the wire (2) is made of a metal having a high temperature coefficient of the resistance au and a low emissivity (f. |
| 20. | Sensor according to claim 19, wherein said metal is selected among nickel, platinum or tungsten. |
| 21. | Sensor according to one of claims 15 to 20, characterized in that the housing (1) has a substantially cylindrical shape with an inner diameter dl>>d2, where d2 is the diameter of the wire (2). |
| 22. | Sensor according to claim 21, characterized in that the ends of the housing (1) are provided with two closing members (3,3') crossed by conductive terminals (4,4') in which the ends of the wire (2) are inserted, so as to result taut in the middle of the housing (1) in a coaxial way. |
| 23. | Sensor according to claim 22, characterized in that the conductive terminals (4,4') are made of a conductive material with a low thermal conductivity. |
| 24. | Sensor according to claim 14, characterized in that the resistor (2) is a PTC thermistor. |
| 25. | Sensor according to claim 14, characterized in that the resistor (2) is a restorable fuse. |
| 26. | Sensor according to one of claims 14 to 25, characterized in that said control circuit (6) comprises a voltage generator (15) connected to the first inductor (8). |
| 27. | Sensor according to claim 26, characterized in that the voltage generator (15) is connected to the first inductor (8) through a rectifier (13). |
| 28. | Sensor according to claim 26 or 27, characterized in that the voltage generator (15) is connected to the first inductor (8) through a lowpass filter (14). |
| 29. | Sensor according to one of claims 26 to 28, characterized in that the voltage generator (15) is connected to a current generator (16) which supplies the electric current (I) to the resistor (2). |
| 30. | Sensor according to claim 29, characterized in that the current generator (16) supplies a constant electric current (I). |
| 31. | Sensor according to claim 29, characterized in that the current generator (16) supplies an electric current (I) at a constant power. |
| 32. | Sensor according to one of claims 14 to 31, characterized in that said control circuit (6) comprises means for indicating outside the vacuum environment (7) the variation of the resistance of the resistor (2). |
| 33. | Sensor according to one of claims 14 to 32, characterized in that said control circuit (6) is connected to means for transmitting outside the vacuum environment (7) a signal which varies as a function of the variation of the resistance of the resistor (2). |
| 34. | Sensor according to claim 33, characterized in that said means for transmitting said signal outside the vacuum environment (7) comprise a transducer suitable for sending an acoustic signal which varies as a function of the variation of the resistance of the resistor (2). |
| 35. | Sensor according to claim 33, characterized in that said means for transmitting said signal outside the vacuum environment (7) comprise a third inductor (10) suitable for sending an electromagnetic signal (11) which varies as a function of the variation of the resistance of the resistor (2). |
| 36. | Sensor according to claim 35, characterized in that said control circuit (6) comprises a voltagetofrequency converter (17) suitable for transforming the voltage at the ends of the resistor (2) into an electromagnetic signal which is transmitted to the third inductor (10). |
| 37. | Sensor according to claim 36, characterized in that said voltageto frequency converter (17) comprises an oscillator which generates an electromagnetic signal oscillating at a frequency depending on the voltage at the ends of the resistors (2). |
| 38. | Sensor according to one of claims 35 to 37, characterized in that the third inductor (10) is connected in parallel to at least one capacitor so as to make up an underdamped LC system which works substantially in resonance conditions. |
| 39. | Sensor according to one of claims 14 to 38, characterized in that the control circuit (6), the first (8) and/or the third (10) inductor are arranged inside a protective airtight closure (12). |
| 40. | Sensor according to one of claims 14 to 39, characterized in that the control circuit (6), the first (8) and/or the third (10) inductor are drowned in a material characterized by a very low degassing rate. |
| 41. | Apparatus for measuring the pressure (P) of the residual gases in a vacuum environment (7), characterized by comprising means (18,19) for powering a second inductor (20) suitable for transmitting an electromagnetic signal (9) to a first inductor (8) of a sensor arranged in the vacuum environment (7). |
| 42. | Apparatus according to claim 41, characterized in that said means (18, 19) for powering the second inductor (20) comprise a power supply (18) which supplies to a power switch (19) an electric current variable according to an electric control signal received from the outside. |
| 43. | Apparatus according to claim 42, characterized in that the power switch (19) is an active member comprising power transistors with negative resistance feedback. |
| 44. | Apparatus according to one of claims 41 to 43, characterized in that the second inductor (20) is connected in parallel to at least one capacitor so as to make up an underdamped LC system working substantially in resonance conditions. |
| 45. | Apparatus according to one of claims 41 to 44, characterized in that the amplitude of the voltage supplied by an oscillator (26) is controlled by the output voltage of a feedback circuit (24,25, 26,27) which is tuned with the amplitude of the first electromagnetic signal (9). |
| 46. | Apparatus according to claim 45, characterized in that the frequency of the voltage supplied by the oscillator (26) is controlled by the output voltage of a PLL (24) which is tuned with the amplitude of the first electromagnetic signal (9). |
| 47. | Apparatus according to one of claims 42 to 46, characterized in that the current circulating in the second inductor (20) is measured by a current meter (21) connected to a current loop (22) which subtracts the amplitude of the electromagnetic field generated by the second inductor (20) from the amplitude of a reference electric signal supplied by a current generator (23), so as to send said electric control signal to the power supply (18). |
| 48. | Apparatus according to claim 47, characterized in that the current meter (21) is connected to a feedback circuit comprising a PLL (24), a shifter (25), an oscillator (26) and a frequency divider (27), mutually connected in a loop. |
| 49. | Apparatus according to claim 48, characterized in that the frequency divider (27) is connected to the power switch (19). |
| 50. | Apparatus according to claim 49, characterized in that the frequency divider (27) is connected to the power switch (19) through an optoelectronic circuit (28). |
| 51. | Apparatus according to one of claims 41 to 50, characterized by comprising means for receiving a signal transmitted by a sensor arranged in the vacuum environment (7). |
| 52. | Apparatus according to claim 51, characterized in that said means for receiving said signal comprise a Halleffect sensor. |
| 53. | Apparatus according to claim 51, characterized in that said means for receiving said signal comprise an acoustic sensor. |
| 54. | Apparatus according to claim 51, characterized in that said means for receiving said signal comprise a fourth inductor (29) suitable for receiving the electromagnetic signal (11) transmitted by an inductor (10) of the sensor. |
| 55. | Apparatus according to claim 54, characterized in that the fourth inductor (29) is connected to a bandpass filter (30) which is centered on the transmission frequency of the sensor. |
| 56. | Apparatus according to claim 54 or 55, characterized in that the fourth inductor (29) is connected to a signal preamplifier (31) which amplifies the signal received by this inductor (29) and sends it to an analogtodigital converter (32) which in turn transforms it into a format compatible with a memory (33) which stores the signal and supplies it to shifter (25). |
| 57. | Apparatus according to claim 56, characterized in that the amplified signal transmitted by the signal preamplifier (31) is sent to a trigger (34) which transforms the received signal into a squarewave signal, which is in turn transformed by a frequencytovoltage converter (35) into a signal, the voltage of which is proportional to the frequency of the input signal. |
| 58. | Apparatus according to claim 57, characterized in that said frequency tovoltage converter (35) is connected to a display (36). |
| 59. | Evacuated panel, characterized by comprising a sensor according to one of claims 14 to 40. |
It is known that the quality of evacuated panels depends on the vacuum degree inside them, so that it is necessary, during the manufacture, to measure the pressure of the residual gases in some samples. These measurements normally employ invasive devices so that the tested specimens are destroyed, and are generally carried out manually in laboratory, with consequent high costs and long times. Moreover, because of its sampling nature, this quality control cannot exclude a single failure in a series of evacuated panels.
The object of the present invention is therefore to provide a method free from said drawbacks, i. e. a method which allows to measure the vacuum, within a suitable pressure range, in short times and without destroying the evacuated panel.
Said object is achieved with a method, a sensor, an apparatus and an evacuated panel, the main features of which are specified in claims 1,14, 41 and 59, respectively, while other features are specified in the remaining claims.
Thanks to the electromagnetic coupling obtained through the inductors of the inner sensor with the inductors of the outer apparatus, the method according to the present invention allows to carry out in a simple, quick and cheap manner measurements of the pressure of residual gases not only in an evacuated panel, but also in all the vacuum environments which cannot be or are not provided with electric and/or mechanic contacts with the outside for pressure measurements.
Furthermore, the inner sensor and the outer apparatus according to the present invention can comprise more or less complex electronic and/or electric circuits according to the required use. It is for instance possible to carry out the method according to the present invention by means of simple and cheap sensors to be inserted into all the evacuated panels, so as to carry out carpet-controls
among them and to guarantee their quality. On the contrary, when suitable, more complex sensor can be produced, for example when the measure conditions are more critical.
According to a particular aspect of the invention, the sensor and the apparatus can mutually exchange electromagnetic signals in resonance conditions, so as to maximize the transfer of electromagnetic energy, with consequent greater precision, accuracy and/or speed of the measure. The apparatus and the sensor can further mutually exchange electromagnetic signals in conditions close to the resonance but anyway in a sinusoidal alternated condition, so as to reduce the signal absorption of barriers, such as for example the metallic or metallized films which enclose the evacuated panels.
For this purpose, the apparatus according to the present invention can be provided with particular circuits and devices for alternatively obtaining and/or controlling the frequency and/or the amplitude of the oscillations from and/or to the sensor, so as to avoid interferences.
According to another aspect of the invention, the apparatus and/or the sensor can be provided with a VCO (Voltage Controlled Oscillator) or PWM (Pulse Width Modulation) oscillator and/or other devices suitable for generating a sinusoidal alternated condition with a high suppression of harmonics.
According to a further aspect of the invention, the sensor can be provided with a circuit suitable for storing, transforming, filtering and stabilizing the energy transmitted by the apparatus. With this arrangement, the criticality of the electromagnetic coupling between the inductors of the apparatus and those of the sensor is reduced.
According to a further aspect of the invention, the determination of the variation of the resistance in the sensor can be obtained with transmitters not only electromagnetic but also of other kinds, for instance acoustic.
A particular use of the method according to the present invention consists in the measurement of the vacuum degree in the evacuated panels, in which a sensor according to the present invention can be inserted for this purpose.
Further advantages and features of the method, the apparatus, the sensor and
the evacuated panel according to the present invention will become clear to those skilled in the art from the following detailed and non-limiting description of an embodiment thereof, with reference to the attached drawings, wherein: - figure 1 shows a partial cross-section top view of the sensor according to said embodiment of the invention; - figure 2 shows a schematic view of the sensor; - figure 3 shows a block scheme of the control circuit of the sensor of figure 2; - figure 4 shows a block scheme of the apparatus according to said embodiment of the invention; and - figures 5 and 6 show two working diagrams of the sensor of figure 2.
Referring to figure 1, it is seen that the sensor according to the present embodiment of the invention comprises a housing 1 preferably having a cylindrical shape, in which a resistor 2, in particular a wire of a conductive material, is arranged. The inner volume of housing 1 is much greater than the volume of wire 2 and in particular the inner diameter dl of housing 1 is much greater than the diameter d2 of wire 2, i. e. d>>d2. The interior of housing 1 is suitably connected with the vacuum environment in which it is arranged so as to exchange gases with it. In particular, housing 1 is permeable to the gases and can consist of a tube made of a non-porous material, for example glass, which is provided with a plurality of holes, or of a tube made of a porous material, for example ceramics or alumina. Wire 2 is preferably made of nickel, or of platinum or tungsten, i. e. metals having a high temperature coefficient ot of the resistance and a low emissivity er. The ends of housing 1 are provided with two closing members 3,3', for example substantially conical or frustoconical-shaped. The external ends of the closing members 3,3'are in turn crossed by two conductive terminals 4,4'in which the ends of wire 2, which is therefore taut in the middle of housing 1 preferably in a coaxial way so as to be exposed to gases contained in housing 1 for a length L, are inserted. Terminals 4,4'are preferably made of a conductive material having a low thermal conductivity, such as steel.
In the present embodiment of the invention, housing 1 is arranged inside an evacuated panel comprising a discontinuous or porous filling material 5 enclosed
between two barrier sheets mutually joined along the edges, for example by means of welding, but it is obvious that in other embodiments housing 1 may be arranged in other vacuum environments.
With reference to figure 2, it is seen that wire 2 is electrically connected to at least one control circuit 6 which is suitable for generating an electric current I crossing wire 2 and is arranged in the closed environment 7, for instance in the above mentioned evacuated panel, in which the pressure P of the residual gases is to be measured. The control circuit 6 is suitably connected to at least one inductor 8 suitable for receiving an electromagnetic signal 9, preferably at a constant frequency, which is transmitted from outside the vacuum environment 7 for powering the control circuit 6. The latter is preferably connected to another inductor 10 suitable for transmitting outside of the vacuum environment 7 an electromagnetic signal 11 which varies as a function of the variation of the resistance of wire 2, i. e. of the pressure P. In particular, the frequency of signal 11 depends on the value of pressure P. In other embodiments of the present invention, signals 9 and 11 can be alternatively transmitted and received by a same inductor. The control circuit 6 and inductors 8 and 10 are preferably arranged inside a protective airtight closure 12 and/or are drowned in a material characterized by a very low degassing rate, for instance glass, epoxy or polyimide resins.
Referring to figure 3, it is seen that inductor 8 is connected to a rectifier 13, for instance comprising a diode bridge or a Zener diode, as well as to a low-pass filter 14, for instance comprising a capacitor, which are suitable for rectifying and smoothing, respectively, the alternated voltage at the ends of inductor 8, so as to suitably power a known switching generator 15 of constant voltage. The voltage generator 15 is in turn connected to a known current generator 16 which supplies a constant current I to wire 2. In another embodiment of the invention the current generator 16 can work at a constant power, instead of a constant current, so as to supply to wire 2 a current I which varies as a function of the voltage at its ends for obtaining a constant power.
For transmitting the electromagnetic signal 11, wire 2 is connected also to a
voltage-to-frequency converter 17, which is powered by the voltage generator 15 and transforms the voltage at the ends of wire 2 into an electromagnetic signal which is transmitted to inductor 10 and varies as a function of this voltage, i. e. to pressure P in vacuum environment 7. In particular, the voltage-to-frequency converter 17 comprises a VCO or PWM oscillator which generates an electromagnetic signal which oscillates at a frequency depending on the voltage at the ends of wire 2. Inductor 10 is preferably connected in parallel to a capacitor (not shown in the figures), so as to make up a underdamped LC system which, working substantially in resonance conditions, amplifies the signal received from the voltage-to-frequency converter 17, thereby circulating within inductor 10 a current much greater than current I in wire 2. The electromagnetic signal 11 generated by inductor 10 is therefore sinusoidal and has a high power.
An alternative embodiment of the sensor according to the present invention can comprise not the above mentioned means, as inductor 10, for transmitting outside the vacuum environment 7 a signal depending on pressure P therein, but comprises instead means for directly supplying the value of said pressure P, for instance a visual indicator.
Another embodiment of the sensor according to the present invention can comprise means, for instance an acoustic transducer, for transmitting outside the vacuum environment 7 a signal not electromagnetic but of another kind, for example acoustic, even in the ultrasonic band, provided that it can vary as a function of the resistance of wire 2, that is to pressure P.
With reference to figure 4, it is seen that the electromagnetic signals 9 and 11 received and transmitted by the sensor according to the present embodiment of the invention can be transmitted and received, respectively, by at least one apparatus arranged outside the vacuum environment 7. In particular, this apparatus comprises a known power supply 18, for example connected to the electric network, which supplies to a power switch 19 a continuous electric current variable according to an electric control signal. The power switch 19 is a known active member, for example comprising power transistors with negative resistance feedback, which supplies an electric current to an inductor 20 which
sends signal 9 suitable for powering the sensor by means of an electromagnetic coupling with inductor 8. Inductor 20 is preferably connected in parallel to a capacitor (not shown in the figure) so as to make up, along with the power switch 19, an underdamped LC circuit. The current circulating within inductor 20 is measured by a current meter 21 which sends to a current loop 22 a signal variable according to said current. The current loop 22 subtracts the amplitude of the electromagnetic field generated by inductor 20 from the amplitude of a reference electric signal supplied by a known current generator 23, so as to send to the power supply 18 the above said electric control signal. The feedback circuit comprising the power supply 18, the power switch 19, the current meter 21 and the current loop 22 can therefore automatically compensate possible variations of the electromagnetic field generated by inductor 20. The current meter 21 sends its signal also to a feedback circuit comprising a PLL (Phase Locked Loop) 24, a shifter 25, a VCO or PWM oscillator 26 and a frequency divider 27, mutually connected in a loop. Oscillator 26 generates a signal oscillating at a high frequency which, besides being employed as a synchronism signal for the different devices of the apparatus, is modified by the frequency divider 27 which decreases its frequency to a value suitable for powering the sensor. PLL 24 subtracts the frequency of the signal measured by the current meter 21 from the frequency of the signal modified by the frequency divider 27, so as to generate an error signal which, passing through shifter 25, keeps oscillator 26 constantly tuned. Shifter 25 acts as a switch for circulating only the last frequency measure, so as to reduce the measurement time, since PLL 24 does not perform anymore a complete scan of the frequency band at every measurement. The frequency divider 27 controls the power switch 19 through a known optoelectronic circuit 28 acting as a control unit and electric de-coupling unit.
The apparatus according to the present invention can comprise also a receiver for the signal transmitted by the sensor arranged in the vacuum environment 7. In the present embodiment of the invention said receiver comprise an inductor 29 suitable for receiving the electromagnetic signal 11 transmitted by inductor 10 of the sensor. In other embodiments, said receiver can comprise a
Hall-effect sensor or an acoustic sensor, also in the ultrasonic band.
Inductor 29 is connected to a band-pass filter 30 which is centered at the transmission frequency of the sensor and is in turn connected to a signal preamplifier 31 which amplifies the signal received from inductor 29 to a level suitable for being processed by the apparatus. This amplified signal is transmitted to an analog-to-digital converter 32 which transforms it into a format compatible with a known digital memory 33 that stores the signal and supplies it to shifter 25, so as to obtain a feedback system provided with a very slow drift which keeps almost unvaried the frequency of oscillator 26 between two next measurements.
The amplified signal transmitted by the signal preamplifier 31 is further sent to a trigger 34 which transforms the received signal into a square-wave signal. The latter signal is transformed by a known frequency-to-voltage converter 35 into a signal, the voltage of which is proportional to the frequency of the input signal, i. e. to pressure P in the vacuum environment 7, so as to be displayed by a known display 36 or similar means.
The flux lines of the electromagnetic field generated by inductor 20 are suitably shaped by means of a ferromagnetic core for conveying the same lines to inductor 8 of the sensor. For example, all the flux lines can be substantially linked together by using a ferromagnetic core with three columns for inductor 20 and a wide coil or a planar bobbin for inductor 8, provided that said coil or bobbin covers a sufficiently wide area, that is greater than the area of the cross-section of the middle column of the core. Obviously, it is important that said coil or bobbin be substantially concentric with the middle column of the ferromagnetic core.
It is obvious that in other embodiments of the present invention the components transmitting the electromagnetic signal to the sensor can be arranged in a first housing, while those receiving the signal from the sensor and furnishing the value of pressure P in the vacuum environment 7 can be arranged in a second housing separated from the first one.
If the vacuum environment 7 consists of an evacuated panel, the thin metallic or metallized film which encloses it forms a barrier to the electromagnetic field generated by inductor 20 and is a location for eddy currents
induced by the same electromagnetic field, so that the frequency and/or the power of the signal transmitted by the apparatus must be sufficiently high for going beyond this barrier and at the same time sufficiently low for not overheating it.
For this purpose, the electromagnetic signals transmitted and received by the sensor and by the apparatus according to the present invention are substantially sinusoidal and generated by underdamped LC circuits, so as to be substantially resonant with a resonance pulse coo = l/(L-C) t/2, where L is the inductance of inductors 10 and/or 20 and C the capacitance of the capacitors connected in parallel to them. The resonance frequency to = (wo/ (2-7))) corresponding to said resonance pulse wo is preferably comprised between 0.1 kHz and 1 MHz.
In the measurement method according to the present embodiment of the invention, wire 2 is powered by the control circuit 6 which supplies a constant current I = I2. When at time t = 0 the current begins to flow in wire 2, the latter warms up due to the Joule effect. If pressure P of the residual gases in housing 1 is relatively low, in particular lower than 0. 1 hPa, the thermal exchange due to these gases is very low and the temperature T of wire 2 progressively rises from the initial value Ti up to a high final temperature Tf, which becomes stable when the dissipated thermal power Qf, G, depending on the existing thermal gradient between wire 2 and the gaseous mass in housing 1, is equivalent to the electric power Qe supplied from the outside. If pressure P of the residual gases in housing 1 is relatively high, in particular higher than 1 hPa, when current 12 begins to flow in wire 2 mechanisms of convective thermal exchange are immediately established, that keep the final temperature Tf of wire 2 substantially equal to the initial temperature Ti.
Therefore, with low values of pressure P, wire 2 reaches the stationary conditions by absorbing the maximum electric power Qe and revealing the maximum voltage drop AV at its ends, since the electric resistance R of the wire raises with high temperatures Tf. On the contrary, with high values of pressure P the electric resistance R and the temperature Tf, and consequently the absorbed electric power Qe and the voltage drop AV, have the minimum values.
Figure 5 shows a diagram from which it can be seen how the variation of the
voltage difference AV at the ends of wire 2, measured in stationary conditions, varies as a function of pressure P of the residual gases present in housing 1, i. e. in the vacuum environment 7.
Figure 6 shows instead a diagram from which it can be seen how the voltage difference AV measured at the ends of wire 2 develops with time at a pressure P of the residual gases equal to 0.1 hPa. As it can be noticed, the stationary conditions are reached very quickly, in particular in about 5 seconds, which is therefore the time necessary for carrying out the pressure measurement.
In the present embodiment of the invention, wire 2 is powered with an electric current I2 constant in time, thereby measuring at the same time the voltage difference AV at the ends of wire 2. In this case, the electric power Qe supplied to wire 2 in stationary conditions is a function of pressure P and of final temperature Tf, since Qe = R (Tr)-I22 and temperature Tf reached in stationary conditions depends on the thermal exchange mechanisms, that is also on pressure P.
In another embodiment of the present invention, wire 2 can be powered by means of a circuit capable of supplying a constant electric power Qe In this case, the maximum temperature Tf reachable by wire 2 in stationary conditions can be determined and controlled beforehand.
By powering wire 2 with a constant current Iz, the stationary conditions are established by means of a self-adjustment mechanism which may require a certain oversize of the control circuit 6. As a matter of fact, the latter must be capable of supplying the electric power Qe corresponding to the maximum thermal dissipation of wire 2, while by powering wire 2 with a constant power it is possible to precisely size the control circuit 6, while limiting, if necessary, the maximum thermal power that can be dissipated by the same wire. The working principle of wire 2 powered with the constant current I2 or with the constant electric power Qe is anyway substantially identical. It is therefore evident that, by keeping an electric power Qe constant or anyway determinable through the measurement of the voltage difference AV at the ends of wire 2, pressure P of the residual gases present in the vacuum environment can be obtained.
Housing 1 is preferably made of a heat-resistant material, since wire 2 may
reach a high temperature Tf, for example comprised between 300 and 350 °C.
More in detail, when wire 2 is crossed by current 12, it warms up due to the Joule effect to a final temperature Tf higher than the initial temperature Ti of housing 1, which is equal to the initial temperature of the residual gases contained in the vacuum environment 7, be it an evacuated panel or any other vacuum environment in which the sensor according to the present invention is inserted.
Because of their thermal conductivity, said residual gases remove a thermal power Qf, G from wire 2 with a rate which depends on pressure P of the same gases and on the difference between the final temperature Tf and the initial one T,. When pressure P of residual gases is low, i. e. when the gases are in molecular flow conditions, the thermal power Qf, G removed from wire 2 by them is: Qf,G= ar x Ao x P x (273/T1)1/2#(T2-T1), where αr, Ao (Wxcm-2 xK-1 xhPa-'), P (hPa), Tl (K), T2 (K) are respectively the accommodation coefficient, the free-molecular conductivity, the pressure, the temperature of the inner surface of housing 1 and the temperature of wire 2. If the temperature of wire 2 is quickly brought from the initial value T ; to the final value Tf, the temperature of housing 1 substantially coincides with the initial value Ti. In these conditions it is: Qf,G=α#A0#(273/Ti)1/2#(Tf-Ti), that is: Qf, G = Qf, G (P, Tf)- When pressure P of the residual gases is high, that is when the gases are in viscous flow conditions, the thermal power removed from wire 2 is calculated with the usual equations which describe the thermal exchange by natural convection, that is expressions of this type: Qf, G=##d2#L#h#(Tf-Ti), where d2 (m) and L (m) are the diameter and the length of the exposed portion of wire 2 licked by the gases and h (Wxm~2xKl) is the liminal coefficient. As it is known, h = h (Nu) where Nu is the Nusselt number, Nu = Nu (Ra), where Ra = Gr x Pr is the Rayleigh number, Gr is the Grashof number and Pr is the Prandtl number. Since Gr = Gr (P, Tf), when the pressure of the residual gases is high it is
again :- Qf, G = yt, G (P, Tf).
When pressure P of the residual gases is intermediate, that is when the gases are between the molecular and viscous flow conditions, the thermal exchange between the hot wire 2 and the residual gases is due to a complex combination of Knudsen free-molecular conductivity, natural convection and conduction through the gas film licking wire 2. However, also in this condition it results that: Qf, G = Qf, G (P, Tf)- In fact, when the electric current 12 starts to flow through wire 2, the electric power Qe supplied to wire 2 is partly stored therein, thereby raising its temperature, and partly dissipated. The energy balance on wire 2 is the following: 7T X (d2/2) 2 x L x p X Cp x dT/dt = Qe-Qf,T=Qe-(Qf,G+Qf,R+Qf,C), where d2 (cm), L (cm), p (gxcm-3) and Cp (J#G-1#K-1) are the diameter, the length, the density and the thermal capacity, respectively, of the exposed part of wire 2, Qf, T is the total thermal power dissipated by wire 2, Qf, R is the thermal power dissipated by irradiation and Qf, C is the thermal power dissipated by conduction through the conductive terminals 4, 4'.
Wire 2, according to the present embodiment of the invention, is relatively long, thin and connected to the conductive terminals 4,4'so as to maximize the thermal exchange with the gases and to minimize the irradiating area and the dispersion by conduction through the terminals. With this arrangement, that is with Qf, G » Qf, R + Qf, CS pressure P can be measured with the maximum sensibility since it results that the dissipated thermal power Qf, C depends essentially on pressure P of the residual gases.
The electric power Qe supplied to wire 2 is equal to: Qe=I2X tV = R (T) x I22, where AV (Volt) is the voltage difference at the ends of wire 2 and R = R (T) (Ohm) is the resistance of wire 2, which depends on temperature T of the same wire. By controlling the electric power supplied to wire 2, in stationary conditions it results that: Qe= Qf, T,
i. e. : I2 x AV = R (Tf) X 12 2=-QfG (P, Tf) +-QfR (Tf) + Qf, C (Tf) that is the dissipated thermal power Qf, T is equal to the electric power Qe supplied to wire 2.
The measure range of pressures P can be adjusted by suitably modifying the inner diameter of housing 1. In fact, the fluid dynamic conditions of the residual gases inside housing 1 depends on a dimensionless quantity called Knudsen number, defined as Kn = k/d, where X is the mean free path of the gas molecules and d is the characteristic diameter of the canalization in which the gas is present, i. e. the inner diameter d of housing 1, so that Kn = k/dl in the present embodiment of the invention.
In general, if Kn > 0.5 the gases are in molecular flow conditions, if Kn < 0.01 are in viscous flow conditions and if 0.5 > Kn > 0.01 they are in intermediate conditions. Since k depends on pressure P, on temperature T and on the nature of the residual gases, once these quantities have been fixed, the fluid dynamic conditions of the residual gases depend only on inner diameter d, of housing 1.
Hence, the variation of voltage difference AV at the ends of wire 2 as a function of pressure P in housing 1 can be influenced by the choice of inner diameter d of housing 1.
The present embodiment of the invention employed a housing 1 having an inner diameter dl = 0.8 mm and a wire 2 with length L = 43 mm and diameter d2 = 0.017 mm. With these sizes, pressures P comprised between 0.01 hPa and 10 hPa could be accurately measured. For measurements of lower pressures P, for example comprised between 0.001 hPa and 1 hPa, it would have been sufficient to increase the inner diameter dl of housing 1. On the contrary, for measurements of higher pressures P, for example comprised between 0.1 hPa and 100 hPa, it would have been sufficient to decrease the inner diameter d of housing 1.
In other embodiments of the invention, wire 2 can be substituted by other resistors, such as a PTC thermistor with a positive temperature coefficient or a restorable fuse, for example the PolySwitch RXE device, produced by the companies Raychem Corp. and Tyco Electronics Corp. , which have a positive
temperature coefficient and the functional features of variable resistors. Therefore, if the surface temperature T of these particular resistors is lower than a given threshold value T*, their resistance is equal to a value Ri. On the other hand, if their surface temperature exceeds the threshold value T*, their resistance quickly raises up to the value Re » R ;. By applying a constant voltage V = V* at their ends, an electric current I = V*/R, begins to flow therein, so that the surface temperature of these resistors begins to raise due to the Joule effect from the initial value Ti equal to the temperature in housing 1. When their surface temperature exceeds the threshold value T*, current I quickly drops to the value I = Ie = V/Re due to the transition of their resistance. The time t which elapses between the beginning of the current circulation and the exceeding of the threshold value T depends on the dynamics of the thermal exchange between these resistors and the residual gas in the vacuum environment 7. Time t is thus the solution of the thermal balance on the resistor, i. e. t = t (Qin, Qout, T), where Ojn is the electric power going into the resistor and Qout is the dissipated thermal power. By powering the resistor with a constant voltage V = V, if I = Is is the current absorbed by this member, Q ; n = Qin (Ij, V) and Qout = Qout (P) since the resistor dissipates heat proportionally to the value of pressure P in the vacuum environment 7, the functional relation t = t (P) can be exploited for measuring pressure P, by measuring the time period T elapsing from the beginning of the current circulation and the moment in which the sharp transition from I = Ij to I = Ie occurs. The threshold temperature T* and the current Ie therefore depend on pressure P while current li is substantially independent from pressure P.
This measurement method is particularly interesting since the value of pressure P is measured in a consistent manner by determining not only the time T which can be calculated by observing the time profile of current I, i. e. of the transition from I = li to I = Ie, but also the absolute value of current Ie. By inserting in the sensor a circuit which converts the current into a signal variable in frequency, it is thus possible to control a transducer supplying a signal electromagnetic, acoustic or of another kind, having a frequency f = f [I (P) ], i. e. variable as a function of pressure P in the vacuum environment 7.