If due to bad reflection a substantial number of echoes are lost and the number
of invalid (incorrect) echoes between two valid ones, is smaller than k the unit
will maintain the output signal continuously. The greater the programmed value
of k, the less sensitive the unit will be to invalid echoes but the reaction time for
error indication will increase. To maintain continuous operation the
programmed range should be kept as narrow as possible (with far-end
blocking).