Method for radiometric limit level monitoring

A method for limit level monitoring with a radiometric limit switch, having:

• a radiator, which sends radiometric radiation along a radiation path extending at the height of the limit level to be monitored, wherein the radiation path is located in a state, free of medium, when the limit level is subceeded, and in a state, covered by medium, when the limit level is exceeded;
• and
• a detector installed terminally in the radiation path, which measures discrete radiation intensities striking thereon dependent on the state of the radiation path, wherein statistical distributions of radiation intensities in the state, free of medium, and in the state, covered by medium, are given by two Poisson distributions separated from one another;
• comprising the steps of:
• measuring in an interval, in which the two states are assumed at least once, successive radiation intensities;
• plotting a distribution of the radiation intensities measured in said interval;
• identifying two separated Poisson distributions within said distribution;
• determining an upper threshold value for the radiation intensity, based on the positions of the Poisson distributions identified in said distribution, the exceeding of which upper threshold value by a radiation intensity measured following said interval means a state change into the state, free of medium, has been detected;
• and/or
• determining a lower threshold value for the radiation intensity, the subceeding of which lower threshold value by a radiation intensity measured following said interval means a state change into the state, covered by medium, has been detected, wherein:
• average values of the two Poisson distributions identified in the distribution are determined;
• based on the average value of the Poisson distribution centered in the distribution about a lower average radiation intensity, an average radiation intensity to be expected in the state, covered by the medium, is determined;
• based on the average value of the Poisson distribution centered in the distribution about a higher average radiation intensity, a radiaion intensity to be expected in the state, fee medium, is determined;
• the interval is a length of time lying before the beginning of monitoring operation by the limit switch;
• the limit switch establishes the threshold values following the interval;
• and
• then enters automatically into limit level monitoring operation in which successive radiation intensities are subsequently measured, and which, based on the meaured radiation intensities and the threshold values, the state of the radiation path is determined.