PRESSURE SENSOR, IN PARTICULAR FOR PRESSURES OF MORE THAN 100 BAR

Abstract

A pressure sensor for measuring high pressures includes: a housing having a sealed volume filled with a compressible fill liquid, wherein the housing includes an elastically deflectable diaphragm having a rear side facing the filled volume and a front side opposite the rear side such that the diaphragm transmits a pressure applied to the front side of the diaphragm to the fill liquid; and a measuring unit adapted to ascertain a variable dependent on velocity of sound in the fill liquid and, based on the measured variable, to determine a pressure measured value representing the pressure on the front side of the diaphragm, wherein the fill liquid contains an organic compound, which is present in the fill liquid at a volume fraction of greater than 99%.

Claims

1-14. (canceled)

15. A pressure sensor, comprising: a housing, which includes a sealed volume filled with a compressible fill liquid, wherein the housing includes an elastically deflectable diaphragm, which has a rear side facing the volume filled with the fill liquid and a front side opposite the rear side, the diaphragm operable to transmit a pressure applied to the front side of the diaphragm to the fill liquid; and a measuring unit configured to ascertain a variable dependent on a velocity of sound in the fill liquid and, based on the measured variable, to determine a measured pressure value representing the pressure applied to the front side of the diaphragm, wherein the fill liquid comprises an organic compound, which is present in the fill liquid at a volume fraction of greater than 99%.

16. The pressure sensor according to claim 15, wherein the organic compound is a nonpolar compound.

17. The pressure sensor according to claim 15, wherein the organic compound is in the liquid state in an intended pressure measuring range of the pressure sensor at least in a temperature range between 0 C. and 50 C.

18. The pressure sensor according to claim 15, wherein the organic compound is a saturated alcohol, an ester, an ether, or an aliphatic hydrocarbon.

19. The pressure sensor according to claim 15, wherein the organic compound is thermally stable at least up to a temperature of 100 C.

20. The pressure sensor according to claim 15, wherein the measuring unit comprises: at least one ultrasonic transducer, which is arranged on or in a wall of the housing and configured to transmit and/or receive ultrasonic waves propagating along a measurement path; and a measuring circuit configured to excite the at least one ultrasonic transducer as to transmit ultrasonic waves and/or to receive and to process output signals of the at least one ultrasonic transducer as to ascertain a value of the variable dependent on the velocity of sound of the ultrasonic waves in the fill liquid.

21. The pressure sensor according to claim 20, wherein: the housing includes the measuring unit and a diaphragm unit; the volume filled with the fill liquid includes a measuring chamber, a capillary tube, and a pressure chamber, each formed in the diaphragm unit and sealed by the diaphragm; the measuring chamber and the pressure chamber communicate with each other via the capillary tube; and the measurement path extends through the measuring chamber.

22. The pressure sensor according to claim 20, wherein the measuring circuit is further configured to determine the measured pressure value from the ascertained value of the variable dependent on the velocity of sound.

23. The pressure sensor according to claim 22, further comprising a temperature detector, which is operable to generate an electrical measurement signal dependent on a temperature of the fill liquid.

24. The pressure sensor according to claim 23, wherein the measuring circuit is configured to use measurement signals of the temperature detector for temperature compensation when determining the measured pressure value.

25. The pressure sensor according to claim 15, wherein the variable dependent on velocity of sound in the fill liquid is a travel time of an ultrasound pulse along the measurement path.

26. The pressure sensor according to claim 15, wherein the variable dependent on velocity of sound in the fill liquid is a resonant frequency, or another variable dependent on a resonant frequency, at which a standing wave forms along the measurement path.

27. The pressure sensor according to claim 15, wherein a wall of the housing surrounding the measuring chamber has a thickness of at least 2 mm.

28. The pressure sensor according to claim 27, wherein the thickness of the wall of the housing surrounding the measuring chamber is 2 to 6 mm.

29. The pressure sensor according to claim 15, wherein the elastically deflectable diaphragm is composed of a metal alloy.

30. The pressure sensor according to claim 30, wherein the metal alloy comprises at least one of a stainless steel, a duplex steel, tantalum, titanium, silver, brass, or bronze.

31. The pressure sensor according to claim 15, wherein the pressure sensor is configured to be operable at measured pressures greater than 100 bar.

32. The pressure sensor according to claim 15, wherein the pressure sensor is configured to be operable at measured pressures of at least 2000 bar.

33. The pressure sensor according to claim 15, wherein the measuring circuit includes measuring electronics.

Description

[0037] The invention will now be explained in greater detail based on examples of embodiments illustrated in the figures. Equal reference characters refer in the figures to identically embodied components of the illustrated sensors or sensor components. The figures of the drawings show as follows:

[0038] FIG. 1 a schematic view of a pressure sensor according to a first example of an embodiment;

[0039] FIG. 2 a schematic view of a longitudinal section of the housing of the pressure sensor shown in FIG. 1;

[0040] FIG. 3 a schematic view of the measurement path of the pressure sensor shown in FIG. 1 when pressure measurement is by means of ultrasound according to a travel time method;

[0041] FIG. 4 a graph of velocity of sound in a liquid based on triglyceride as a function of pressure at constant temperature;

[0042] FIG. 5 a schematic, longitudinally sectioned illustration of a measuring unit of a pressure sensor according to a second example of an embodiment;

[0043] FIG. 6 a schematic, longitudinally sectioned illustration of a measuring unit of a pressure sensor according to a third example of an embodiment;

[0044] FIG. 7 a graph of impedance as a function of frequency for the measuring units illustrated in FIGS. 5 and 6; and

[0045] FIG. 8 a graph of phase difference as a function of frequency for the measuring units illustrated in FIGS. 5 and 6.

[0046] FIG. 1 shows schematically a pressure sensor according to a first example of an embodiment. FIG. 2 shows the housing of the pressure sensor in a schematic, longitudinally sectioned illustration. The pressure sensor includes a measuring unit 1, a diaphragm unit 2, and a capillary tube 3. These components are provided in a housing 4, which surrounds a hermetically sealed, closed volume. This volume is completely filled by a compressible fill liquid. The volume is composed of a measuring chamber 5 arranged in the measuring unit 1, a pressure chamber 6 formed in the diaphragm unit 2 and the internal volume of the capillary tube 3, via which the measuring chamber 5 and the pressure chamber 6 are in communication with one another.

[0047] The diaphragm unit 2 comprises a platform, which has on a side a recess, which is covered by an elastically deflectable diaphragm 7, such that the pressure chamber 6 is formed between the diaphragm 7 and the floor of the recess. The platform can serve at the same time as a connecting means, e.g., as a flange, for connecting the diaphragm unit 2 to a container, e.g., a process container or a pipe, whose internal pressure is to be measured by means of the pressure sensor. The platform can, alternatively, be connected to such a connecting means. If the platform is connected to the container as described, the diaphragm 7 is then in communication with the interior of the container, such that the pressure prevailing in the container lies on the pressure chamber 6 far, front side of the diaphragm 7. Depending on the pressure prevailing in the container, which is applied to the front side of the diaphragm 7, the diaphragm is correspondingly elastically deflected. The pressure prevailing in the container is, thus, transmitted via the diaphragm 7 to the fill liquid. In the case of pressure changes in the container, the deflection of the diaphragm 7 changes and, thus, the volume filled with the compressible fill liquid changes.

[0048] Diaphragm 7 can be formed in the present example of a metal or metal alloy, e.g., a stainless steel such as 1.4435, 1.4404, duplex steel 1.4462, tantalum, titanium, silver, brass, bronze, or others. The selection of the material of the diaphragm 7 can depend on the intended application in which the pressure sensor is to be applied. For monitoring hydrogen tanks, for example, a diaphragm 7 of bronze is especially advantageous. Diaphragm 7 can be connected with the platform by a soldered or welded connection. The material of the platform and the total housing can likewise be a metal or metal alloy, especially those mentioned above as diaphragm materials.

[0049] The measuring unit 1 has a solid wall, which surrounds the measuring chamber 5. The capillary tube 3 passes through the wall into the measuring chamber 5. The velocity of sound in the compressible fill liquid filling the measuring chamber 5 depends on the pressure prevailing in the fill liquid. In the equilibrium state, such pressure equals the ambient pressure lying on the front side of the pressure dependently deflected diaphragm 7, thus the pressure in the container to which the pressure sensor is secured for measurement. This ambient pressure is, thus, ascertainable by measurement of the velocity of sound in the fill liquid. The measuring principle on which the pressure sensor works is based on this.

[0050] The measuring unit 1 can have one or more ultrasonic transducers. In the present example, two mutually opposing ultrasonic transducers 8, 9 are placed externally on opposing side walls of the housing 4. In an alternative embodiment, the ultrasonic transducers can also be integrated into the wall of the housing, such that the transducers are in direct contact with the fill liquid in the measuring chamber. The two ultrasonic transducers 8, 9 lie in the present example opposite one another in such a manner that a measurement path extending between them extends in the longitudinal direction of the measuring chamber 5 and therewith through the fill liquid filling the measuring chamber 5.

[0051] The ultrasonic transducers 8, 9 are connected with a measuring circuit 10. Measuring circuit 10 can be an analog measuring circuit. In advantageous manner, the measuring circuit 10 is, however, a measuring electronics, which comprises a processor, memory and, stored in the memory, operating programs, which can be executed by the measuring electronics for operating the ultrasonic transducer 8, 9 and for registering and processing measurement signals of the ultrasonic transducers 8, 9 for determining pressure measured values. The measuring electronics 10 can show the pressure measured value via a display and/or communicate such via a communication interface by wire or wirelessly, e.g., by radio, to a superordinated unit or to a service or operating device.

[0052] FIG. 3 shows schematically the measurement path 11 extending between the ultrasonic transducers 8, 9, in order to illustrate the principle of pressure measurement with the pressure sensor according to the first example of an embodiment according to FIGS. 1 and 2. The measurement path 11 extends between the transmitting transducer 8 and the receiving transducer 9. The ultrasonic transducers 8, 9 serving as transmitting transducer 8 and receiving transducer 9 can be embodied essentially identically. They can be piezoelectric transducers, for example; however, also magnetostrictive transducers can be used.

[0053] Measuring circuit 10 is adapted to excite the transmitting transducer 8 with an electrical signal such that via the wall of the housing it couples an ultrasound pulse 12 into the measuring chamber 5. After having traveled the measurement path 11, the ultrasound pulse 12 passes through the wall of the housing to reach the receiving transducer 9. This converts received ultrasonic signals into an electrical measurement signal, which is received and processed by the measuring circuit 10, in order to ascertain a travel time of the ultrasound pulse 12 through the fill liquid from the transmitting transducer 8 to the receiving transducer 9. The actual travel time t.sub.L along the measurement path of length L extending through the fill liquid can be ascertained using the echo signals arising at the interfaces between the housing wall and the liquid.

[0054] From the ascertained travel time t.sub.L, the velocity of sound c of the ultrasonic wave in the fill liquid transmitted by the transmitting transducer 8 can be ascertained according to the formula:

c=L/t.sub.L.

[0055] This is temperature and pressure dependent. In order to ascertain a pressure measured value from the velocity of sound, consequently, the temperature in the fill liquid must be known and be kept as uniform and constant as possible during the measuring. In the present example, the housing 4 has, consequently, in the region of the measuring unit 2 a substantial wall to assure a uniform temperature distribution. An additional advantage of this substantial wall is that it resists the high pressures, for which the pressure sensor is intended, without deforming.

[0056] In the present example, the pressure sensor includes a temperature detector 13, which can be applied for temperature compensation. Temperature detector 13 is connected with the measuring circuit 10 to provide such with temperature measurement signals. Measuring circuit 10 is adapted to receive and to process the temperature measurement signals, especially in order to ascertain from the measured travel time a pressure measured value normalized to a reference temperature. For this, calibration data can be stored in a memory of the measuring circuit 10.

[0057] In the present example, the measurement path 11 extending between the ultrasonic transducers 8, 9 has a cylindrical shape with a diameter of 2-4 millimeters and a length between 15 and 30 mm. In order to keep the volume of the fill liquid as small as possible, the length amounts advantageous to only between 15 and 20 mm. Used as fill liquid is a triglyceride, which is liquid at room temperature. In order to achieve sufficient accuracy of the pressure measurement, it is advantageous to apply a temperature detector 13 with an accuracy of 0.05 to 0.01 K.

[0058] FIG. 4 shows velocity of sound of the ultrasound transmitted from the transmitting transducer 8 at 20 C. in a triglyceride-based liquid as a function of pressure. Such characteristic lines/curves of velocity of sound as a function of pressure and temperature are stored in a memory of the measuring circuit 10, for example, in functional or tabular form, and the measuring circuit 10 is adapted, based on such characteristic lines/curves, to ascertain from the observed travel time or from the velocity of sound derived from the travel time a measured value of the pressure lying on the diaphragm 7 of the pressure sensor.

[0059] Suitable compressible fill liquids are primarily nonpolar organic liquids, i.e., such organic compounds that are liquid in an application temperature range lying, e.g., in an interval between 40 C. and 200 C. Such organic substances have typically main or side chains with greater than 2 and less than 10 carbon units. For example, these are saturated alcohols such as n-octanol, propylene glycol or paraffins such as n-decane, esters such as propylene glycol dicaprate, 1,2-propylene glycol diacetate, propylene carbonate, and short chain polysiloxanes such as decamethyltetrasiloxane. Advantageously, these materials are thermally stable in the application temperature range, e.g., between 40 C. and 200 C., or in an interval, which lies within limits, e.g., 0 C. to 100 C.

[0060] If an oil containing a mixture of different chemical compounds is used, it is required, or at least advantageous, to ascertain the characteristic lines/curves for the velocity of sound as a function of pressure and temperature, in each case, for a given batch of the oil by calibration in the production of the pressure sensor and to store such in a memory of the pressure sensor. Such is, most often, required because the exact composition of the oil in terms of the individually contained chemical compounds can differ from batch to batch. In the case of some oils, e.g., silicone oils, the molecular composition of the oil can change over the lifetime of the pressure sensor and therewith the physical properties of the fill liquid. Such can require re-calibrations of the pressure sensor during its service life.

[0061] It is therefore advantageous to use as fill liquid a single substance having a purity of at least 99 vol-% because, in such case, the corresponding characteristic lines/curves of the pure substance can be stored in the measuring electronics of the pressure sensor and do not differ from one another among different batches of the fill liquid. A first calibration in the production of the pressure sensor is, consequently, not required.

[0062] Since the measurement path 11 and the fill liquid composed of a pure material remain unchanged during the lifetime of the pressure sensor, the measuring unit 1 of the pressure sensor exhibits no aging phenomena. In contrast with the case of pressure sensors known in the state of the art, an aging or creep of the diaphragm 7 plays no role because these effects have no noticeable effects on the compressing of the fill liquid and on the sound velocity measurement along the measurement path in the measuring unit. In contrast with the case of the silicone oil as fill liquid in DE 10 2005 009818 A1, a thermally stable pure material as fill liquid remains unchanged during the total service life. The pressure sensor described here thus enables a maintenance free operation of the pressure sensor over a length of time of operation, which is longer by orders of magnitude than the maintenance free length of time of operation of conventional pressure sensors. On the whole, also the production effort for the pressure sensor described in the present example of an embodiment and the pressure sensors described in the additional examples of embodiments is significantly less than for piezoresistive sensors.

[0063] The housing of the pressure sensor can be made vacuum tight, for example, of metal or a metal alloy. That prevents, for an optimally designed sensor construction, a failure of the joint location. The simple construction enables use of the sensor for high pressure applications from 100 bar to 1500 bar or more, e.g., for pressure monitoring of hydrogen tanks. For such purpose, a diaphragm 7 sealing against hydrogen diffusion can be used. The sensor can be used for the total pressure range between 100 bar and up to 2000 bar or even up to 5000 bar.

[0064] FIG. 5 shows a measuring unit 21 for a pressure sensor according to a second example of an embodiment and enabling pressure measurement based on measurement of the velocity of sound. Measuring unit 21 serves for ascertaining a variable representing the pressure-dependent velocity of sound using a resonance method. FIG. 5 shows only the measuring unit 21 and a section of the capillary tube 3, which connects the measuring unit 21 with the diaphragm unit (not shown) of the pressure sensor. The diaphragm unit of the pressure sensor according to the second example of an embodiment can be embodied identically to the diaphragm unit of the pressure sensor according to the first example of an embodiment, which was described based on FIGS. 1 to 3.

[0065] Measuring unit 21 includes a housing 4, which surrounds the measuring chamber 5. Such is quite analogous to the first example of an embodiment filled with a compressible fill liquid, which also fills the capillary tube 3 and a pressure chamber formed in the diaphragm unit. A deflection of the diaphragm in the direction of the pressure chamber effects, thus, a compression of the fill liquid, which results in a change of the velocity of sound in the fill liquid. For producing a measurement signal dependent on velocity of sound, the measuring unit 21 includes a single piezoelectric transducer in the form of ultrasonic transducer 28. Additionally, the measuring unit includes (not shown in FIG. 5) a measuring circuit, which excites the piezoelectric transducer with a continuous excitation signal, whose frequency continuously increases or decreases as a function of time, this being referred to in the following as frequency sweep. The frequency range of this frequency sweep can have a width of about 0.3 to 0.5 MHz. When the separation K between the opposite walls of the measuring chamber 5 corresponds to a whole numbered multiple n of half wavelength of the ultrasonic wave produced from the transducer 28, a standing wave forms in the measuring chamber 5. The following equation holds for the frequencies F.sub.n, at which this resonance state is fulfilled:

[00001]$F_n=\frac{\mathrm{nc}}{2K}.$

[0066] These resonance frequencies F.sub.n, at which such a standing wave forms in the measuring chamber 5, can be ascertained based on the impedance of the ultrasonic transducer 28 and/or the phase difference (referred to herein as phase for short) between excitation and output signals of the ultrasonic transducer 28. The typical behavior of the impedance of the ultrasonic transducer 28 in the measuring unit 21 of FIG. 5 as a function of frequency of the excitation signal is shown in FIG. 7 as the dashed curve (1). The measuring was performed using a triglyceride-based fill liquid. The resonance frequencies are at the abrupt changes of the impedance. Typical behavior of phase as a function of frequency of the excitation signal of the ultrasonic transducer 28 for the measuring unit 21 according to FIG. 5 appears in FIG. 8 as the dashed curve (1). At the resonance frequencies, an abrupt rise of the phase is to be observed. The abrupt changes of the impedance and/or phase as a measurement signal can be used by the measuring circuit for detecting the resonance frequencies. Based on the above set forth equation relating resonance frequency and velocity of sound, which, in turn, depends on pressure and temperature, a measuring circuit or measuring electronics of the pressure sensor can derive a pressure measured value from the ascertained resonance frequencies.

[0067] A change of the velocity of sound in the fill liquid due to a pressure change and/or temperature change causes a change of the resonance frequency. In ascertaining a pressure measured value, it is, consequently, advantageous, to ascertain at least two neighboring resonance frequencies F.sub.n and F.sub.n+1, in order to determine the value of n according to the formula:

[00002]$n=\frac{F_n}{F_{n+1}-F_n}.$

[0068] The time and calculative effort associated with the finding and evaluating two neighboring resonance frequencies can be avoided by using two ultrasonic transducers. Such an example of an embodiment will now be described using FIG. 6. FIG. 6 shows a measuring unit 31 of a pressure sensor according to a third example of an embodiment, in the case of which the pressure measurement likewise occurs based on a measurement of the velocity of sound. As in the case of the measuring unit 21 of the second example of an embodiment, measuring unit 31 includes a housing 4 having a wall, which surrounds a measuring chamber 5. Measuring chamber 5 is filled with a compressible fill liquid. The interior of the measuring chamber communicates via the capillary tube 3 with a pressure chamber of a diaphragm unit (not shown), which can be embodied analogously to the diaphragm unit 2 of the pressure sensor illustrated in FIGS. 1 to 3 showing the first example of an embodiment. By deflection of the diaphragm of the diaphragm unit and corresponding pressure change in the compressible fill liquid, the velocity of sound changes in the fill liquid.

[0069] The velocity of sound of ultrasound in the fill liquid is ascertained in the measuring unit 31 of this example of an embodiment by means of an alternative of the previously described example of a resonance method. Here, the measuring unit 31 includes a first ultrasonic transducer 38, which is placed on a first wall of the housing 4 in such a manner that it can transmit and/or receive ultrasonic waves along a measurement path extending through the measuring chamber 5. Additionally, measuring unit 31 includes a second ultrasonic transducer 39, which is placed on a wall opposite the first wall of the housing 4 in such a manner that it can transmit and/or receive ultrasonic waves along the measurement path in the opposite direction to the ultrasonic waves transmitted from the first ultrasonic transducer 38. A measuring circuit of the measuring unit 31 connected with the two piezoelectric transducers is adapted to excite the transducers 38, 39 with equal phase and to measure the impedance and/or phase of the first and second ultrasonic transducers 38, 39, which are electrically connected in parallel. The excitation frequency is, in such case, varied quite analogously to the above described second example of an embodiment as a function of time in the form of a frequency sweep. A standing ultrasonic wave forms, when the separation K between the opposite walls of the measuring chamber 5 corresponds to an even numbered multiple n (with n=2N) of half the wavelength of the ultrasonic waves transmitted from the transducers 38, 39. In the case of exciting the mutually opposing ultrasonic transducers 38, 39 with equal phase, no resonances occur for uneven numbered multiples of half the wavelength corresponding to (n=2N+1).

[0070] Shown in FIGS. 7 and 8 are the curves of impedance and phase of one of the transducers 38, 39 as a function of frequency, in each case, as the solid curve (2). As in the case of the second example of an embodiment, the measurement signals were registered for a triglyceride-based liquid. If the above set forth resonance condition is fulfilled, the impedance and the phase each exhibit an abrupt change. It can be seen that resonances ascertained with the measuring unit according to the third example of an embodiment coincide always with each second of the resonances detected with the measuring unit 21 of the second example of an embodiment. In manner analogous to the second example of an embodiment, the measuring circuit of the pressure sensor can according to the third example of an embodiment be adapted to ascertain resonance frequencies based on the behavior of impedance or phase and to determine a pressure measured value based on the relationships set forth above. Advantageous in measuring unit 31 is that the frequency separations between the resonance frequencies are sufficiently large that the frequency range, in which a resonant frequency shifts as result of a pressure and/or temperature change in the fill liquid, is typically less than frequency separation between two neighboring resonance frequencies. Thus, as a rule, a pressure measured value can be determined reliably based on ascertaining a single resonant frequency.

[0071] In the case of both examples of embodiments for pressure measurement by means of a resonance method, the measuring units 21, 31 can, in each case, have a temperature sensor, whose measurement signal serves for temperature compensation of the measured values of the measuring circuit. Such can happen in manner analogous to the first example of an embodiment described based on FIGS. 1 to 3.

[0072] The invention is not limited to the above-described examples of embodiments. Thus, it is possible to ascertain the velocity of sound in the fill liquid based on further, alternative methods known in the state of the art. Also, it is possible to use, instead of one or more piezoelectric transducers, other means for producing standing waves in the fill liquid, e.g., magnetostrictive transducers.

REFERENCE CHARACTERS

[0073] 1 measuring unit [0074] 2 diaphragm unit [0075] 3 capillary tube [0076] 4 housing [0077] 5 measuring chamber [0078] 6 pressure chamber [0079] 7 diaphragm [0080] 8 transmitting transducer [0081] 9 receiving transducer [0082] 10 measuring circuit [0083] 11 measurement path [0084] 12 ultrasound pulse [0085] 21 measuring unit [0086] 28 ultrasonic transducer [0087] 31 measuring unit [0088] 38 ultrasonic transducer [0089] 39 ultrasonic transducer