Patent Application
20180180504
The present disclosure relates to a pressure-measuring sensor for measuring a pressure of a liquid medium, with an electrically-conductive separating diaphragm closed off to the outside, to whose exterior surface a liquid medium under the pressure to be measured can be applied.
Pressure sensors are used in industrial pressure-measurement equipment to measure pressures, e.g., in diaphragm seals or in pressure- or differential pressure-measuring instruments.
Pressure sensors normally have an interior chamber which is closed to the outside by a metal separating diaphragm, to whose exterior surface the pressure to be measured by the pressure-measuring sensor can be applied when in operation. Its interior chamber is filled with a pressure-transmitting liquid which is used for transferring the pressure acting upon the separating diaphragm from the outside to a destination connected to the interior chamber.
The separating diaphragms of pressure-measuring sensors are regularly made of metal, such as stainless steel, and have a predetermined thickness which depends upon the design of the pressure-measuring sensor and the pressure range in which it is to be used. Typical values for the thickness of the separating diaphragm fall within the 20 μm to 150 μm range. Due to the functionally determined, low thickness of the separating diaphragm, there is a risk of the separating diaphragm being damaged, either mechanically or chemically. Should damage of this kind penetrate the separating diaphragm at any point, e.g., by corrosion eating through locally, the term used for this is diaphragm rupture.
When a diaphragm rupture occurs, the pressurized medium penetrates into the pressure-measuring sensor at the rupture location and changes the pressure transfer properties of the pressure-measuring sensor. This leads to failure of the pressure-measuring sensor after some time at least. In addition, there is a risk of the liquid-transferring pressure escaping from the pressure-measuring sensor and contaminating the medium in contact with the separating diaphragm.
In DE 101 31 855 A1, a pressure-measuring sensor is described which has a device for detecting a diaphragm rupture. This comprises a sensor which is arranged in a measuring chamber connected to the interior chamber of the pressure-measuring sensor and which, by metrological means, measures and monitors a conductivity or a dielectric constant of the pressure-transmitting medium. This device detects a diaphragm rupture as soon as the liquid medium acting upon the separating diaphragm from the outside penetrates into the pressure-measuring sensor due to a diaphragm rupture, there penetrating as far as the measurement chamber, and thereby resulting in a change in the measured variable being monitored.
In order to minimize as far as possible the adverse consequences of a diaphragm rupture, which may go beyond replacing the defective separating diaphragm, and also to make it possible to take any safety measures which may be necessary, it is important to detect a diaphragm rupture as early as possible.
It is an object of the present disclosure to provide a pressure-measuring sensor with which a diaphragm rupture can be detected early on.
To this end, the present disclosure comprises a pressure-measuring device for measuring a pressure of a liquid medium, with an electrically-conductive separating diaphragm closed off to the outside, to whose exterior surface a liquid medium under the pressure to be measured can be applied, which is characterized in that at least one electrode is arranged on an interior surface of the separating diaphragm which faces the interior chamber, said electrode being electrically insulated from the separating diaphragm by an insulating layer arranged between the electrode and the separating diaphragm and mechanically connected to the separating diaphragm, and a measuring circuit connected to a capacitor formed by the separating diaphragm and the electrode, said measuring circuit measuring and monitoring, by metrological means, a measured variable dependent upon an electrical property of the capacitor, which is changed by a medium penetrating into the area of the capacitor when a diaphragm rupture of the separating diaphragm occurs.
A first variant of the present disclosure is characterized in that the medium is electrically conductive, the measured variable is an ohmic resistance of the capacitor, and the measuring circuit comprises a resistance-measuring circuit for measuring the ohmic resistance of the capacitor formed by the separating diaphragm and the electrode.
A development of the first variant is characterized in that the electrode is connected to the measuring circuit by a connecting lead routed through the interior chamber of the pressure-measuring sensor, and the separating diaphragm is connected to the measuring circuit by an electrically-conductive connection in particular, a connection running via a metal support of the pressure-measuring sensor, said support enclosing the interior chamber and being closed to the outside by the separating diaphragm.
A second variant of the present disclosure is characterized in that the measured variable is a measured variable dependent upon a phase shift between alternating current and alternating voltage caused by the capacitor in an alternating current circuit in particular, a dielectric power loss of the capacitor or a tangent of a loss angle dependent upon the phase shift and the measuring circuit contains a measuring circuit in particular, a power-loss-measuring circuit which registers this measured variable by metrological means.
A third variant of the present disclosure is characterized in that two electrodes are arranged on the interior surface of the separating diaphragm which are electrically insulated from each other and from the separating diaphragm by the insulating layer and mechanically connected to the separating diaphragm, each of the electrodes, together with the separating diaphragm, forms a capacitor, the measured variable is a measured variable dependent upon a phase shift between alternating current and alternating voltage caused by the two capacitors connected in series in an alternating current circuit in particular, a dielectric power loss of the series-connected capacitors or a tangent of a loss angle dependent upon the phase shift and the measuring circuit contains a measuring circuit connected to the electrodes in particular, a power-loss-measuring circuit which registers this measured variable by metrological means.
An embodiment of the present disclosure is characterized in that the measuring circuit detects a diaphragm rupture when the measured variable deviates from a reference value measured with an intact separating diaphragm.
A development of the second or third variant is characterized in that the measuring circuit determines the measured variable at at least two different frequencies of the alternating voltage applied via the measuring circuit, and the measuring circuit detects a diaphragm rupture when the two measured variables determined at different frequencies exhibit a frequency dependence.
A development of the development of the second or third variant is characterized in that the frequencies lie below a cut-off frequency above which the measured variable is frequency-dependent when the separating diaphragm is intact, and the frequencies fall within frequency range of 100 Hz to 100 kHz, and, in particular, cover the frequency range from 100 Hz to 100 kHz in particular, by at least one frequency falling within a range of less than one kilohertz, at least one frequency falling within a range of a few kilohertz, and/or at least one frequency falling within a range of more than 10 kHz.
According to a preferred embodiment of the pressure-measuring sensor, the electrode is made of metal in particular, made of silver.
A development of the pressure-measuring sensors according to the present disclosure is characterized in that the insulating layer comprises one or more superimposed layers in particular, a layer of silicon carbide arranged on the separating diaphragm and a layer of diamond-like carbon arranged thereon and each layer consists of a dielectric in particular, made of silicon carbide, diamond-like carbon, or silicon dioxide.
A further development of the pressure-measuring sensors according to the present disclosure is characterized in that the insulating layer comprises at least one layer in particular, a layer of silicon carbide, diamond-like carbon, or silicon dioxide which forms a diffusion barrier to hydrogen.
A further development of the pressure-measuring sensors according to the present disclosure is characterized in that the separating diaphragm has a thickness in the 20 μm to 150 μm range, the insulating layer has a layer thickness in the 0.05 μm to 50 μm range, and/or the electrode has a layer thickness in the 0.1 μm to 10 μm range.
A development of the pressure-measuring sensors according to the present disclosure is characterized in that the interior chamber is filled with a pressure-transmitting liquid which transmits the pressure acting from outside upon the separating diaphragm to a destination connected to the interior chamber in particular, a pressure-measuring chamber equipped with a pressure sensor and forming part of a pressure- or differential pressure-measuring instrument.
The present disclosure further comprises a pressure- or differential pressure-measuring instrument with a pressure-measuring sensor according to the present disclosure which is characterized in that a pressure sensor arranged in a housing is provided, to which pressure acting from outside upon the separating diaphragm can be applied via the pressure-measuring sensor.
In addition, the present disclosure includes a device for detecting a diaphragm rupture of an electrically-conductive separating diaphragm which closes off an interior chamber to the outside and to which a pressurized liquid medium can be applied from the outside, characterized in that at least one electrode is arranged on an interior surface of the separating diaphragm which faces the interior chamber, said electrode being electrically insulated from the separating diaphragm by an insulating layer arranged between the electrode and the separating diaphragm and mechanically connected to the separating diaphragm, and a measuring circuit is provided which is connected to a capacitor formed by the separating diaphragm and the electrode, said measuring circuit measuring and monitoring, by metrological means, a measured variable dependent upon an electrical property of the capacitor, which is changed by a medium penetrating into the area of the capacitor when a diaphragm rupture of the separating diaphragm occurs.
The present disclosure and its advantages will now be explained in greater detail on the basis of the figures, which illustrate two embodiments; identical elements are provided with identical reference numerals in the figures.
The support 1 and the separating diaphragm 3 are made of metal. As protection against corrosion, the separating diaphragm 3 is preferably made of a corrosion-resistant alloy, such as stainless steel or Hastelloy, or has, on its exterior surface, a corrosion-resistant coating 6 which, as an optional feature, is indicated in
The interior chamber 3 of the pressure-measuring sensor is connected by a pressure-transmission line 7 to a destination to which the sensed pressure p is to be transmitted. In the case of pressure-measuring sensors used in pressure-measuring instruments, the destination is a pressure-measurement chamber 11 arranged within a housing 9 of the pressure-measuring instrument, with a pressure sensor 13 being arranged within said pressure-measurement chamber, said pressure sensor measuring the pressure p applied to it via the pressure-measuring sensor. The interior chamber 3 of the pressure-measuring sensor, the pressure-transmission line 7, and the pressure-measurement chamber 11 are filled with a pressure-transmitting liquid which transmits to the destination the pressure p acting upon the separating diaphragm 5. The pressure-transmitting liquid is a non-conductive, dielectric fluid which is as incompressible as possible and preferably has a low vapor pressure. Suitable here are, for example, the silicone oils or paraffin oils used for this purpose in pressure-measurement technology.
The present disclosure can be used fully analogously even with differential pressure-measuring instruments, which are preferably equipped with two pressure-measuring sensors according to the present disclosure, each of which registers one of the two pressures whose pressure difference is to be measured with the differential pressure-measuring instrument and transmits said pressures to a differential pressure sensor. In precisely the same way, the present disclosure can, of course, be used with pressure-measuring sensors used for other purposes. These include, in particular, pure diaphragm seals, which transmit to a destination a pressure acting upon the pressure-measuring sensor either directly or via a pressure-transmission line connected to the interior chamber of the pressure-measuring sensor.
Pressure-measuring sensors according to the present disclosure are equipped with a device for detecting a diaphragm rupture of the separating diaphragm 5. This comprises at least one electrode 15 arranged on an interior surface of the separating diaphragm 5 so as to face the interior chamber 3 and which, in conjunction with the separating diaphragm 5, forms a capacitor. The electrode 15 is electrically insulated from the separating diaphragm 5 by an insulating layer 17 arranged between the electrode 15 and the separating diaphragm 5 and mechanically connected to the separating diaphragm 5.
The insulating layer 17 consists of a dielectric, such as, for example, silicon carbide (SiC), diamond-like carbon (DLC), or silicon dioxide (SiO2), and, depending upon the selection of dielectric, is applied to the interior surface of the separating diaphragm 5 for example, by plasma-enhanced chemical vapor deposition (plasma CVD), by physical vapor deposition (PVD), by sol-gel deposition or by polymer deposition in particular, of parylenes.
The insulating layer 17 can consist of one or more superimposed layers. Several superimposed layers may be made of the same or different dielectric materials, e.g., made of the materials mentioned above. The advantage of different dielectric materials is that a material which adheres especially well to the metal of the separating diaphragm 5 and which has sufficient elasticity can be selected for the layer disposed directly upon the separating diaphragm 5, such as, for example, silicon carbide (SiC) applied by plasma-enhanced chemical vapor deposition (plasma CVD), while the other materials can be selected with regard to the desired properties of the insulating layer 17.
The insulating layer 17 preferably comprises at least one layer which forms a diffusion barrier against hydrogen. Layers made of the aforementioned dielectrics are suitable for this purpose. Hydrogen is formed, for example, in the case of corrosion of the separating diaphragm 5 caused by the medium or when a redox reaction is triggered on the separating diaphragm 5 by the medium, and can diffuse right through the metal separating diaphragm 5. Here, the diffusion barrier resists hydrogen penetrating into the interior chamber 3 of the pressure-measuring sensor and prevents the pressure-transmitting properties of the pressure-transmitting liquid in the interior of the pressure-measuring sensor from being impaired by the ingress of hydrogen.
The electrode 15 is made of metal, such as silver, and is preferably applied as a metallic coating to the insulating layer 17. Application of the electrode 15 can be carried out by, for example, physical deposition of the metal from the gas phase (PVD).
The separating diaphragm 5 has a comparatively small thickness, predetermined by the design of the pressure-measuring sensor and the pressure range in which it is to be used. Typical values for the thickness of the separating diaphragm 5 fall within the 20 μm to 150 μm range. The insulating layer 17 preferably has a layer thickness in the 0.05 μm to 50 μm range, and the electrode 15 preferably has a layer thickness in the 0.1 μm to 10 μm range.
In the event of a diaphragm rupture, the liquid medium under pressure p penetrates into the area of the capacitor. If the electrical properties of the medium differ from those of the insulating layer 17, the penetrating medium will bring about a change in the electrical properties of the capacitor.
The capacitor formed by electrode 15 and separating diaphragm 5 can be approximately described by an equivalent circuit diagram which is shown in
Both the phase shift φ and the ohmic resistance RDC of the capacitor formed by the separating diaphragm 5 and the electrode 15 are electrical properties of the capacitor, which are changed by a liquid medium that penetrates after a diaphragm rupture and which has properties that differ from the electrical properties of the insulation layer 17.
According to the present disclosure, in the event of a diaphragm rupture in which liquid medium penetrates into the area of the capacitor, at least one changing property of the capacitor formed by the separating diaphragm 5 and the electrode 15 is registered metrologically by means of a measuring circuit 19 connected to the capacitor, and continuously monitored, and a diaphragm rupture detected when the monitored property of the capacitor changes.
The extent to which a monitored property is changed by penetrating liquid medium depends upon the electrical properties of the liquid medium in comparison with the corresponding electrical properties of the insulation layer 17. Depending upon the nature of the liquid medium, the electrical conductivity or the dielectric constant of the medium play a role here, as do factors such as its polarizability, its ion mobility, and also its dissociability.
When the insulation between the separating diaphragm 5 and the electrode 15 is intact, the ohmic resistance RDC of the capacitor will be extremely high. The ohmic resistance of a capacitor, composed of a 50-μm-thick, stainless-steel separating diaphragm 5, a two-layer insulating layer 17 composed of a 10-μm-thick silicon carbide layer and a 2-μm-thick layer of diamond-like carbon (DLC), and a 0.5-μm-thick electrode 15 made of silver, thus lies, for example, in the megaohm range. If the separating diaphragm 5 is damaged, this will necessarily also affect the insulating layer 17 connected to it, which will result in the pressurized medium penetrating into the capacitor. If a medium with high conductivity, such as a strong electrolyte, forms a conductive bridge between the separating diaphragm 5 and the electrode 15, a short circuit of the capacitor will result, which is accompanied by a drastic decrease in the ohmic resistance. If this separating diaphragm 5 is exposed, for example, to a 10% aqueous solution of ferric chloride, the separating diaphragm 5 will corrode. As soon as the separating diaphragm 5 is corroded through at at least one spatially limited location, the solution will penetrate and cause a short circuit of the capacitor, due to which the ohmic resistance RDC of the capacitor will drop from the megaohm range to a few ohms.
In the case of pressure-measuring sensors used in electrically-conductive media, a measuring circuit 19 is therefore preferably provided which measures the ohmic resistance RDC of the capacitor by metrological means. The measuring circuit 19 is connected to the electrode 15 by a connecting lead 21 running through the interior chamber 3 of the pressure-measuring sensor, the pressure-transmission line 7, the pressure-measurement chamber 11, and an electrical bushing 29 provided in a rear wall 27 of the pressure-measurement chamber 11 which faces away from the pressure-measuring sensor. The measuring circuit 19 is preferably connected to the separating diaphragm 5 by an electrically-conductive connection 23, shown as a broken line in
The measuring circuit 19 can take the form of a separate unit or be part of an electronic measurement module 31 connected to the pressure sensor 13, and comprises a resistance-measuring circuit 33 for metrologically measuring the ohmic resistance RDC of the capacitor, as well as a monitoring unit 35 connected to it which continuously monitors the measured ohmic resistances RDC and detects a diaphragm rupture as soon as the ohmic resistance RDC falls below a reference value determined on the basis of the ohmic resistance RDC present when the separating diaphragm 5 is intact. If a diaphragm rupture is detected, this will be signaled by the monitoring unit 35 via a corresponding output 37. The signal can be given visually, e.g., via a display or, as is shown here, by an LED, or acoustically, e.g., by a signal tone, and/or electrically, e.g., in the form of an electrical signal indicating the damage.
In the case of pressure-measuring sensors which are used in media whose penetration into the capacitor causes only a much lower fall in the ohmic resistance RDC than is the case with highly conductive media, rather than the ohmic resistance RDC, a measured variable dependent upon the phase shift φ caused by the capacitor in an alternating current circuit will instead preferably be measured by metrological means and monitored.
Here, in the embodiment shown in
Basically, the measured variable can also be compared with a reference value determined with an intact separating diaphragm 5, and a diaphragm rupture be detected when measured variable differs from the reference value.
When the separating diaphragm 5 is intact, the capacitor formed by the separating diaphragm 5 and the electrode 15 is filled with a dielectric solid formed by the insulating layer 17. Capacitors filled with a solid create a frequency-dependent phase shift φ at high frequencies, usually in the gigahertz range, while the phase shift φ has practically no frequency dependence at lower frequencies. In contrast, a liquid contained in a capacitor creates, at a much lower frequency, a frequency dependence of the phase shift φ caused by the capacitor, which does, however, fall as the frequency increases and is virtually negligible in the frequency range in which dielectric solids cause a frequency-dependent phase shift φ. The cause of this is a greater mobility in liquids at the atomic or molecular level which, depending upon the type of liquid, has an effect upon their polarizability which, at lower frequencies, is dependent upon frequency their molecular dynamics, their ion mobility, or their dissociability, and is a significant reason for the size of the resistor R provided in the equivalent circuit diagram shown in
Accordingly, the measured variable is preferably determined at at least two different frequencies of the alternating voltage applied via the measuring circuit 39 to the capacitor formed by the separating diaphragm 5 and the electrode 15, said frequencies falling within a frequency range which lies below a cut-off frequency above which the capacitor creates a frequency-dependent phase shift φ when the separating diaphragm 5 is intact. In this variant of the present disclosure, a diaphragm rupture is detected when the measured variables measured at different frequencies exhibit a frequency dependence. This will already be the case when at least two measured variables measured at different frequencies differ.
The frequency range within which the liquid medium penetrating into the capacitor causes a frequency dependence of the phase shift φ, and thus of the measured variable dependent upon it, depends upon the properties of the medium. Accordingly, the measured variable is therefore preferably measured at frequencies which cover as large a frequency range as possible in particular, a frequency range of 100 Hz to 100 kHz. Particularly suitable for this purpose are frequencies which have at least one frequency in the range of less than one kilohertz, e.g., 500 Hz, at least one frequency in the range of a few kilohertz, e.g., 5 kHz, and at least one frequency in the range of more than 10 kHz, e.g., 20 kHz. This approach has the advantage of considerably enlarging the range of liquid media whose penetration into the capacitor is detected with the device.
In order to metrologically register the measured variable which depends upon the phase shift φ, an alternating voltage is applied to the capacitor from the measuring circuit 39. In contrast to the measurement of ohmic resistance RDC, the galvanic separation between the separating diaphragm 5 and the electrode 15 caused by the insulating layer 17 is here suspended.
A variant of the present disclosure is shown in
In precisely the same way as in the previous embodiment, here too a measured variable dependent upon a phase shift φ′ between alternating current and alternating voltage, e.g., the dielectric power loss or the equivalent tangent of the loss angle δ′, is metrologically registered and monitored by means of the measuring circuit 39. In contrast to the previous embodiment, the measuring circuit 39 is here, however, not connected to the separating diaphragm 5, but rather connected to the two electrodes 43 by connecting leads 45 running through the interior chamber 3 of the pressure-measuring sensor. The measuring circuit 39 thus registers and monitors a measured variable dependent upon the phase shift δ′ caused by the series-connected capacitors. This measured variable is dependent upon the electrical properties of the two capacitors and thus changes as soon as a medium with electrical properties different from insulating layer 17 penetrates into the area of at least one of the two capacitors following a diaphragm rupture.
This variant offers the advantage that the measurement is carried out via the two electrodes 43, and no electrical connection between the electrodes 43 and the separating diaphragm 5 is thus created by the measuring circuit 39. Here, the insulating layer 17 will still create a galvanic separation when the separating diaphragm 5 is already damaged, but no galvanic connection to one or both of the electrodes 43 has yet resulted from the penetrating medium.
Metrological registration, monitoring, and the detection and signaling of a diaphragm rupture which may have been detected will, with this variant as well, take place in the way described above in connection with the embodiment shown in
Even though the present disclosure is described here on the basis of pressure-measuring sensors, the device according to the present disclosure for diaphragm rupture detection can, of course, be used in other applications as well, in which an electrically-conductive separating diaphragm closes off an interior chamber to the outside and to which a pressurized liquid medium can be applied from the outside.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102014118616.8 | Dec 2014 | DE | national |
The present application is related to and claims the priority benefit of German Patent Application No. 10 2014 118 616.8, filed on Dec. 15, 2014 and International Patent Application No. PCT/EP2015/076909, filed on Nov. 18, 2015, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/076909 | 11/18/2015 | WO | 00 |
