Radiometric measuring device for carrying out measurements in explosion-prone areas

Description

[0001] The invention relates to a radiometric measuring device for carrying out measurements in potentially explosive atmospheres, comprising a scintillator which converts radioactive radiation incident on it into photons, and a detector connected to the scintillator which detects photons generated in the scintillator and reaching the detector and converts them into electrical measuring signals.

[0002] In industrial metrology, there are a variety of applications where radiometric measuring instruments must be used in locations where there is a risk of explosion. Examples include gas stations and chemical plants where explosive gases can occur.

[0003] Strict safety requirements apply to devices used in potentially explosive atmospheres. These requirements aim to prevent sparks that could potentially trigger an explosion, or to prevent a spark generated inside an enclosed space from affecting the surrounding area. This goal can be achieved in various ways. The corresponding explosion protection measures are divided into types of protection, which are regulated by relevant industry standards, such as the IEC 60079 series of standards of the International Electrotechnical Commission. A safety circuit through which measuring devices in potentially explosive atmospheres can be supplied with power is shown, for example, in the German publication DE 10 2010 040833 A1 and is marketed, for example, by the Endress + Hauser group under the product name "RMA42."

[0004] A particularly relevant type of protection for measuring instruments in practice is "Intrinsic Safety (Ex-i)." For devices designed according to the "Intrinsic Safety" type of protection, the values ​​for the existing electrical energy, which is reflected in particular in the electrical quantities current, voltage, and power, must remain below a specified limit at all times. The limits are selected so that in the event of a fault, e.g., due to a short circuit, the maximum energy released is not sufficient to generate an ignition spark or dangerous overheating. Typically, the energy is kept below the specified limits by power-limiting measures, for example, appropriate combinations of current- and voltage-limiting components. In addition, depending on the device's design, minimum safety distances or insulation measures must be provided between individual components or circuits of the device.A safety circuit by means of which measuring instruments can be operated intrinsically safely in potentially explosive atmospheres using protective resistors is shown in the patent application DE 10 2011 004 061 A1.

[0005] Where limiting the energy to intrinsically safe values ​​is not possible, additional ignition protection measures of other types of protection must be applied, e.g., ignition protection measures of the "Pressure Encapsulation (Ex d)" type of protection, the "Potty Encapsulation (Ex m)" type of protection, or the "Sand Filling (Ex q)" type of protection. This requires relatively complex mechanical measures.

[0006] Detectors used in radiometric measuring devices for detecting ionizing radiation typically require very high operating voltages, particularly voltages in the kilovolt range, for their operation. Classic examples of this are Geiger-Müller counters and scintillation detectors, in which gamma radiation incident on a scintillator is converted into light pulses in the scintillator, which are then converted into electrical signals via a high-voltage photomultiplier. A radiometric measuring device that operates in a pulsed mode for explosion protection purposes is shown in DE 10 2007 053 860 A1 and is marketed by the Endress + Hauser Group under the product name "FMG60."

[0007] To enable the use of these measuring devices in potentially explosive atmospheres, the components subject to high voltage are now regularly housed in a potting encapsulation or a flameproof enclosure. In addition, sufficient spatial separation and electrical isolation of these components from the other components of the measuring device prevent the potential for flashover of high voltages to unprotected areas, particularly to intrinsically safe signal circuits. Therefore, in addition to the measures of the "Intrinsic Safety (Ex-i)" type of protection, other types of protection, especially complex encapsulation, must always be implemented. For example, a radiometrically based measuring device encapsulated using a pressure encapsulation, is described in publication DE 10 2009 002816 A1. A fire-resistant encapsulated measuring device based on radiometric technology is shown in GB 02495197 A1.

[0008] These parallel measures are not only costly but also require a lot of space. Accordingly, these measuring devices can only be used where sufficient space is available at the measuring site.

[0009] It is an object of the invention to provide a radiometric measuring device with a detector for detecting ionizing radiation, with which measurements can be carried out in potentially explosive atmospheres, which requires little space at the measuring location, can be manufactured cost-effectively and, in particular, can be safely installed using simple means.

[0010] This object is achieved by the measuring device claimed in claim 1. Advantageous embodiments thereof are described in the subclaims.

[0011] The invention and further advantages will now be explained in more detail with reference to the figures of the drawing, in which three exemplary embodiments are shown; identical elements are provided with the same reference numerals in the figures. Fig. 1 shows a radiometric measuring device according to the invention used as a limit switch; Fig. 2 shows a radiometric measuring device with a measuring unit powered by an additional circuit; and Fig. 3 shows a radiometric measuring device with a measuring unit equipped with an RFID transponder.

[0012] Fig. 1 shows an application example for the use of a radiometric measuring device according to the invention. As an exemplary embodiment, a radiometric limit switch is shown, which monitors whether a predetermined level of a filling material 1 in a container 3 is exceeded or not reached. The container 3 is located in a - in Fig. 1 potentially explosive atmosphere designated 'Ex'. The measuring arrangement shown comprises a radioactive emitter 5 arranged on the outside of the container 3 at the level of the fill level to be monitored, which emits ionizing radiation, in particular gamma radiation, through the container 3 at the predetermined fill level. The radiation intensity emerging through the container 3 on the side of the container 3 opposite the emitter 5 depends on the absorption in the container 3, which in turn depends on the fill level in the container 3.

[0013] The measuring device comprises a measuring unit 7, which can be mounted in the potentially explosive area and operated via an intrinsically safe power supply, and a higher-level unit 11, which is connected to it via a connecting cable 9 and is arranged outside the potentially explosive area and provides the intrinsically safe power supply to the measuring unit 7. The higher-level unit 11 is, for example, a transmitter power supply unit.

[0014] The measuring unit 7 measures the radiation intensity emerging from the container 3. Based on the measured radiation intensity, an exceedance of the predetermined fill level is detected when the measured radiation intensity falls below a predefined threshold, or an undershoot of the predetermined fill level is detected when the measured radiation intensity exceeds a predefined threshold.

[0015] Alternatively, other measured variables, such as a fill level of a filling material in a container or a density of a medium being irradiated, can also be determined with the radiometric measuring device according to the invention by means of a corresponding arrangement of the radiator 5 and the measuring unit 7.

[0016] Measuring unit 7 contains only electrical components that, individually and in combination, meet the 'intrinsically safe' type of protection and are to be supplied with power during measurement operation. This means that all electrical components that are to be supplied with power, both individually and in combination, do not require more current, voltage, and power than is available via the intrinsically safe power supply.

[0017] According to the invention, only explosion protection measures that are classified as intrinsic safety are provided in measuring unit 7. Explosion protection measures of other types of protection are neither provided nor required.

[0018] The measuring unit 7 comprises a rod-shaped scintillator 13, which converts ionizing radiation incident on it into photons, which then propagate along the scintillator 13. Instead of a solid scintillator body, individual or multiple scintillation fibers, for example, combined into a bundle, can be used as the scintillator 13. The scintillator 13 neither carries current nor is subjected to voltage.

[0019] At least one semiconductor detector 15, which can be operated via an intrinsically safe power supply, is connected to the end of the scintillator 13. This detector detects photons generated in the scintillator 13 and reaching the semiconductor detector 15 and converts them into electrical measurement signals. The semiconductor detector 15 is preferably a semiconductor detector chip.

[0020] For protection against environmental influences, a casing surrounding the scintillator 13, e.g. a protective tube, and a chip enclosure surrounding the semiconductor detector 15 can be provided.

[0021] The aforementioned semiconductor detector chips are particularly suitable for intrinsically safe operation. Examples include the components sold by Philips under the designation DPC6400-22-44, the digital silicon photomultiplier sold under the designation DPC3200-22-44, the silicon photomultiplier sold by Ketek under the designation PM3350, or the silicon photomultiplier sold by SensL.

[0022] The semiconductor detector 15 comprises one or more end faces facing the scintillator 13, which are equipped with one or more light-sensitive cells. The light-sensitive cells each comprise one or more photodiodes, arranged, for example, in an array. The photodiodes are preferably avalanche photodiodes operated in Geiger mode and connected electrically in parallel. The operation of such semiconductor detectors is described, for example, in English patent application GB 2479053 A.

[0023] With regard to the intrinsically safe operation of the measuring unit 7, in particular of the semiconductor detector 15, semiconductor detector chips are used that are equipped with photodiodes that have low breakdown voltages of less than 30 V, preferably even less than 20 V, and can thus be operated with a correspondingly low operating voltage, in particular an operating voltage of less than 30 V. The semiconductor detector chips listed above from the above-mentioned manufacturers are particularly suitable for this purpose.

[0024] During measurement operation, the measuring unit 7 is supplied with power in an intrinsically safe manner by the higher-level unit 11. Preferably, the semiconductor detector 15 is designed as an intrinsically safe semiconductor detector chip and represents the only electrical component in the measuring unit 7 that requires power during measurement operation. This makes it possible to limit the use of intrinsically safe ignition protection measures in the measuring unit 7 or even to completely dispense with other measures associated with intrinsically safe ignition protection, such as insulation and / or minimum distances between components to be supplied with power.

[0025] For illustrative purposes, the following example is based on the requirements of the IEC 60079-11 standard for Ex ia protection type for gas group IIC and temperature class T4. Its power requirements are determined by an operating voltage of 28 V, an operating current of no more than 2 mA, and a corresponding maximum power consumption of 56 mW. This semiconductor detector 15 has an internal capacitance of 3.5 nF, an internal inductance of 200 µH, and a thermal resistance of 500 K / W. It is designed for operation in a temperature range of -20°C to +50°C.

[0026] As a higher-level unit 11, a measuring transducer power supply unit designed according to IEC60079-11 and complying with the type of protection Ex ia IIC is provided here as an example, which provides a maximum output voltage of 30 V, a maximum output current of 3 mA and a maximum output power of 90 mV in terms of safety technology, and allows an external concentrated capacitance of a maximum of 60 nF and an external concentrated inductance of a maximum of 100 mH.

[0027] A coaxial cable with a capacitance of 18 nF and an inductance of 100 µH is used as connecting cable 9.

[0028] Output voltage, output current and output power of the higher-level unit 11 cover the supply requirements of the semiconductor detector 15.

[0029] In addition, the sum of the capacitances of semiconductor detector 15 and connecting line 9 is smaller than the permissible connectable capacitance of the higher-level unit 11, and the sum of the inductances of semiconductor detector 15 and connecting line 9 is smaller than the permissible connectable inductance of the higher-level unit 11.

[0030] The capacitance of the semiconductor detector 15 is 3.5 nF, which is significantly below the upper capacitance limit of 66 nF resulting from the present voltage values ​​according to IEC60079-11.

[0031] Likewise, the inductance of the semiconductor detector 15 at 1 µH is significantly below the upper inductance limit of 1 mH resulting for the present current values ​​according to IEC60079-11.

[0032] If a fault occurs in which the entire maximum power of 90 mW available in the semiconductor detector 15 is converted into heat via its thermal resistance with the aforementioned conversion factor of 500 K / W, the temperature increases by 45 °C. Thus, the temperature in the semiconductor detector 15 is still below the upper limit of 130 °C specified by temperature class 4 even if the semiconductor detector 15 was used at a temperature of +50 °C corresponding to the upper range limit, and its temperature increases by 45 °C starting from this upper range limit due to the conversion of the entire power.

[0033] Measuring Unit 7 can therefore be used in potentially explosive atmospheres without requiring additional measures of other types of protection in addition to those associated with intrinsic safety. This allows Measuring Unit 7 to be manufactured very compactly and cost-effectively. Furthermore, it is easy and safe to install.

[0034] In principle, two or more semiconductor detectors 15 can be connected in parallel to each other via corresponding connecting cables 9 to the higher-level unit 11 and supplied with power via the higher-level unit 11. Since intrinsic safety is ensured by a single intrinsically safe power supply from the higher-level unit 11, no additional explosion protection measures are required. Only the rules for intrinsically safe design according to IEC 60079-11 apply.

[0035] Fig. 2 shows an alternative embodiment of a radiometric measuring device according to the invention. Due to the similarities with the Fig. 1 The following merely explains the existing differences in more detail. For the rest, please refer to the description of Fig. 1 In contrast to the Fig. 1 In the embodiment shown, the semiconductor detector 15 is supplied with power via an intrinsically safe additional circuit 17 arranged between the semiconductor detector 15 and the higher-level unit 11. In conjunction with the example described in detail above, an additional circuit 17 designed in accordance with ignition protection type Ex ia IIC T4 is particularly suitable. The additional circuit 17 is connected to the higher-level unit 11 via a connecting cable 19, e.g. a two-core installation cable, via which it is intrinsically safely supplied. In addition, it is connected to the semiconductor detector 15 via a connecting cable 21, e.g. a coaxial cable, and provides its intrinsically safe supply. In this embodiment, the maximum output voltage, the maximum output current and the maximum output power of the additional circuit 17 specify the corresponding maximum input values ​​on the semiconductor detector 15 side.These are identical in amount to the values ​​previously specified for the directly feeding higher-level unit 11.

[0036] The additional circuit 17, like the semiconductor detector 15, can be operated in potentially explosive atmospheres. In addition to the measures required for the intrinsic safety type of protection, no further measures for other types of protection are required.

[0037] Fig. 3 shows another example that is useful for understanding the present invention, but is not part of it. It differs from the examples shown in Fig. 1 und 2The exemplary embodiment shown is characterized by the fact that the measuring unit 7 is powered wirelessly, and the semiconductor detector 15 requires much less operating energy. For this purpose, the measuring unit 7 is equipped with an RFID transponder 23 connected to the semiconductor detector 15 or integrated therein. The higher-level unit 11' is designed here as an RFID reader, via which the measuring unit 7 is supplied with power wirelessly and intrinsically safely, and the measurement results of the measuring unit 7 are read out. The supply energy delivered by the RFID reader to the semiconductor detector 15 is also so low here that the "intrinsic safety" type of protection is maintained. In this embodiment, too, no further measures of other types of protection are required in the measuring unit 7 in addition to the measures assigned to the intrinsic safety type of protection. 1Filling material 3Container 5Radioactive source 7Measuring unit 9Connecting cable 11Superior unit 11'Superior unit 13Scintillator 15Semiconductor detector chip 17Additional circuit 19Connecting cable 21Connecting cable 23RFID transponder

Claims

1. A radiometric measuring device, which is configured to determine as a measured value a density or a fill level of a filling material (1) located in a container (3), wherein the container (3) is arranged in a potentially explosive atmosphere, with - a measuring unit (7) which can be used in a potentially explosive atmosphere, with ∘ a scintillator (13), which is arranged in such a way that the scintillator (13) converts radioactive radiation coming from the container (3) into photons, and ∘ a semiconductor detector (15) which is connected to the scintillator (13) and can be operated by an intrinsically safe power supply, which measures photons that are produced in the scintillator (13) and reach the semiconductor detector (15) and converts these into electrical measurement signals, ∘ a transmitter power supply unit (11) connected to the measuring unit (7), ∘ wherein only the explosion protection measures that are assigned to the intrinsic safety type of ignition protection are provided in the measuring unit (7), ∘ wherein the transmitter power supply unit (11) is arranged outside the potentially explosive atmosphere, and ∘ wherein the transmitter power supply unit (11) is configured to provide an intrinsically safe power supply to the semiconductor detector (15) in measuring mode.

2. The radiometric measuring device as claimed in claim 1, in which - the semiconductor detector (15) is the only electrical component in the measuring unit (7) that is intended to be supplied with power in measuring mode, and - only a single explosion protection measure that is assigned to the intrinsic safety type of protection is provided in the measuring unit (7), which entails the semiconductor detector (15) being a semiconductor detector chip that can be operated by an intrinsically safe power supply.

3. The radiometric measuring device as claimed in one of the preceding claims, in which - the semiconductor detector (15) is a semiconductor detector chip with one or more light-sensitive cells connected in parallel, - the light-sensitive cells have photodiodes, in particular Geiger-mode avalanche photodiodes, and - the photodiodes have low breakdown voltages of less than 30 V, in particular less than 20 V, and - the semiconductor detector (15) can be operated with a correspondingly low operating voltage, in particular an operating voltage less than or equal to 30 V.

4. The radiometric measuring device as claimed in one of the preceding claims, in which - an intrinsically safe auxiliary circuit (17) is provided between the semiconductor detector (15) and the transmitter power supply unit (11), - the auxiliary circuit (17) is supplied with intrinsically safe power via the transmitter power supply unit (11), and - the auxiliary circuit (17) provides an intrinsically safe power supply to the semiconductor detector (15).