DE-102010031504-B4 - Radiometric limit switch and method for level monitoring using the radiometric limit switch

Abstract

Radiometric limit switch, with - a spotlight (5), ◯ sends radiometric radiation along a radiation path running at the height of the limit level to be monitored, or which is in a free state when the limit level has been undershot, and or which is in a state covered by a medium when the limit level is exceeded, - a detector (7) inserted at the end of the beam path, which is designed to measure discrete radiation intensities (I) arriving at it, which depend on the state of the radiation path, wherein a statistical distribution of the radiation intensities (I) in the free and in the covered state is given by two separate Poisson distributions, - a measuring instrument electronics (15) connected to the detector (7), which ◯ an interval (T) starts in which a successive distribution (V(T)) of the measured radiation intensities (I) is recorded, o the distribution (V(T)) is continuously or at predetermined time intervals examined to see if it contains two separate Poisson distributions, o the interval (T) ends after two separate Poisson distributions have been identified in the distribution (V(T)) recorded up to that point, o based on the positions of the two Poisson distributions identified in the distribution (V(T)) within the distribution (V(T)) an upper threshold (S max ) and/or a lower threshold (S min ) for the radiation intensity (I) is determined, and o the in case of exceeding or falling below the threshold value (S min , S max ) by a measurement following the interval (T) Radiation intensity (I) detects a change of state to the free or covered state.

Inventors

• Hartmut Damm
• Mingzheng Jiang
• Robert Schäuble
• Simon Weidenbruch

Assignees

• Endress+Hauser SE+Co. KG

Dates

Publication Date

20260513

Application Date

20100719

Claims (9)

1. Radiometric limit switch, comprising: - a radiator (5), which emits radiometric radiation along a radiation path running at the height of the limit level to be monitored, which is in a free state when the limit level is undershot, and which is in a state covered by a medium when the limit level is exceeded, - a detector (7) inserted at the end of the beam path, which is designed to measure discrete radiation intensities (I) arriving at it, which depend on the state of the radiation path, wherein a statistical distribution of the radiation intensities (I) in the free and covered states is given by two separate Poisson distributions, - measuring electronics (15) connected to the detector (7), which starts an interval (T) in which a distribution (V(T)) of the measured radiation intensities is successively The system records the distribution (V(T)), continuously or at predetermined time intervals examines whether it contains two separate Poisson distributions, terminates the interval (T) after two separate Poisson distributions have been identified in the distribution (V(T)) recorded up to that point, determines an upper threshold (S_max ) and/or a lower threshold (S_min ) for the radiation intensity (I) based on the positions of the two Poisson distributions identified within the distribution (V(T)), and detects a change of state to the free or covered state by a radiation intensity (I) measured after the interval (T) when the threshold (S _min, S _max ) is exceeded or fallen below.
2. Border switch after Claim 1 , in which the detector (7) converts incident radiation quanta into electrical pulses (k), and the discrete radiation intensities (I) are measured in the form of pulse rates (N(t i )) which each correspond to the number (n) of electrical pulses (k) per unit of time (Δt).
3. Method for level monitoring using the radiometric level switch according to Claim 1 or 2 , comprising the following procedure steps: - radiometric radiation is emitted by means of the emitter (5) along the radiation path running at the level of the limit to be monitored, which is in the free state when the limit is undershot and in the covered state when the limit is exceeded, - the discrete radiation intensities (I) arriving at the detector (7), which depend on the state of the radiation path, are measured by means of the detector (7), whose statistical distribution in the free and covered states is given by two separate Poisson distributions, - the interval (T) is started in which the two states are occupied at least once and in which successive radiation intensities (I) are measured, - the distribution (V(T)) of the radiation intensities (I) measured in the interval (T) is recorded, - the distribution (V(T)) is continuously or at predetermined time intervals examined to determine whether it corresponds to the two separate The interval (T) contains Poisson distributions, - the interval (T) is terminated after the two separate Poisson distributions are identified within the distribution (V(T)) recorded up to that point, and - based on the positions of the Poisson distributions identified in the distribution (V(T)), the upper threshold (S max ) for the radiation intensity (I) is determined within the distribution (V(T)), if this threshold is exceeded by a radiation intensity (I) measured after the interval (T), a change of state to the free state is detected, and/or - the lower threshold (S min ) for the radiation intensity (I) is determined, if this threshold is not reached by the radiation intensity (I) measured after the interval (T), a change of state to the covered state is detected.
4. Procedure according to Claim 3 , in which - the means of the two Poisson distributions identified in the distribution (V(T)) are determined, - an expected mean radiation intensity (I b ) in the clouded state is determined based on the mean of the Poisson distribution centered around a lower mean radiation intensity in the distribution (V(T)), and - an expected mean radiation intensity (I f ) in the free state is determined based on the mean of the Poisson distribution centered around a higher mean radiation intensity in the distribution (V( T )).
5. Procedure according to Claim 3 , in which the positions of the Poisson distributions identified in the distribution (V(T)) are determined based on the corresponding mean of the respective identified Poisson distribution.
6. Procedure according to Claim 3 , in which the thresholds (S min , S max ) are determined based on the location and variance (σ f , σ b ) of the two Poisson distributions identified in the distribution (VT)).
7. Procedure according to Claim 3 , in which - the interval (T) is a period of time prior to the commencement of monitoring operation by the limit switch, - the limit switch subsequently sets the threshold values (S min , S max ) following the interval (T), and - then automatically switches to a limit level monitoring operation, -- in which successive radiation intensities (I) are measured, and -- in which the state of the radiation path is determined on the basis of the measured radiation intensities (I) and the threshold values (S max , S min ).
8. Procedure according to Claim 3 , in which - a difference exists between the Poisson distribution, which represents the statistical distribution of the radiation intensities (I) incident on the detector in the free state, and the Poisson distribution which the statistical distribution of the radiation intensities (I) incident on the detector in the covered state, a previously known blocking region (SP) exists which includes only radiation intensities (I) that cannot be assigned to either the free or the covered state, - the limit switch performs a limit level monitoring during the interval (T) based on the radiation intensities (I) measured in the interval (T), in which o an exceedance of the limit level to be monitored is detected if the respective intensity measurement (I(t i )) is below the blocking region (SP), and o a fall below the limit level to be monitored is detected if the respective intensity measurement (I(t i )) is above the blocking region (SP).
9. Procedure according to Claim 8 , in which, when recording the distribution (V(T)), only those measured radiation intensities (I) that lie outside the exclusion zone (SP) are included in the distribution (V(T)).

Description

The invention relates to a radiometric limit switch and a method for monitoring the level, comprising a radiometric limit switch with a emitter that emits radiometric radiation along a radiation path running at the level of the level to be monitored, which is in a free state when the level to be monitored is undershot, and which is in a state covered by a medium when the level to be monitored is exceeded, and a detector inserted at the end of the beam path that measures discrete radiation intensities arriving thereon, which depend on the state of the radiation path, and in which an exceedance or undershooting of the level to be monitored is monitored on the basis of the measured radiation intensities. Radiometric limit switches are typically used when conventional limit switches cannot be used due to particularly harsh conditions at the measuring point. Very often, for example, extremely high temperatures and pressures prevail at the measuring point, or there are chemically and/or mechanically very aggressive environmental influences present, which make the use of other measuring methods impossible. Radiometric level switches are used, for example, to monitor whether a predetermined fill level of a substance in a container is exceeded or fallen below. They serve, for instance, as overfill protection or as dry-run protection. For this purpose, they feature a radioactive source mounted externally on the container, which, during operation, emits radioactive radiation along a radiation path running through the container at the level of the level to be monitored. On the opposite side of the container, a detector is mounted externally and inserted at the end of the radiation path, quantitatively measuring the radiation intensity exiting the container. The exiting radiation intensity depends on the geometric arrangement and the absorption. The latter depends on the fill level of the material in the container and its density. Accordingly, the radiation intensity exhibits a minimum, dependent on the density of the contents, when the fill level is above the radiation path. Conversely, the radiation intensity exhibits a maximum when the fill level is below the radiation path. The minimum and maximum radiation intensity are now regularly determined in a balancing procedure to be carried out by the user during the commissioning of the radioactive limit switch, in which the two above-mentioned filling states are set freely and covered in the container, and the corresponding minimum and maximum radiation intensities are measured by the limit switch. Based on the minimum and maximum radiation intensities, a threshold value for the radiation intensity, usually referred to as the switching point, is defined. Exceeding this threshold corresponds to a change to the unobstructed state, and falling below it corresponds to a change to the obscured state. During subsequent operation of the limit switch, a comparison of the measured radiation intensity with the threshold value determines whether the level to be monitored has been exceeded or fallen below. A corresponding radiometric limit switch is described, for example, in the patent specification. DE 11 72 864 B1 As described. To achieve stable, chatter-free switching behavior, a switching hysteresis is often implemented. For this purpose, a lower and an upper threshold are defined based on the minimum and maximum radiation intensities. During operation, a change to the unobstructed state is only indicated when the measured radiation intensity exceeds the upper threshold, and a change to the obscured state is only indicated when the measured radiation intensity falls below the lower threshold. In the range between the two thresholds, the limit switch outputs the last determined state unchanged. Calibration procedures now regularly require the user's assistance, who must create the necessary conditions in their system for calibration and indicate the presence of each state to the limit switch so that it can measure the corresponding maximum and minimum radiation intensities and store them, assigning them to the respective state. The setting of the two filling states, free and covered, is usually associated with an impairment, interruption and/or delay of the manufacturing and/or processing procedure or an ongoing process taking place at the measuring point. It is an object of the invention to provide a method for radiometric level monitoring in which the limit switch automatically initiates the monitoring operation, in particular without prior execution of a calibration procedure in which at least one of the states must be established and the associated radiation intensity measured. The invention comprises a radiometric limit switch according to the features of independent claim 1 and a method for limit level monitoring with the radiometric limit switch according to the features of claim 3. Advantageous embodiments thereof are described in dependent claims 2 and 4 to 9. The inven