Abstract: An ultrasonic level detector includes a transducer for generating and receiving bursts of sonic energy at a surface to locate the position of the surface. In response to receiving a burst of energy reflected from the surface, the transducer generates an electrical signal, which is then supplied to a variable gain amplifier. After amplification, the electrical signal is supplied to a comparator and a peak detector. The comparator generates a timing signal upon the electrical signal exceeding a threshold. A window generator circuit generates a receive window that controls whether the electrical signal is supplied to the comparator and the magnitude of the threshold on the comparator. The peak detector determines the maximum amplitude of the electrical signal, which may be used to vary the amplifier gain and the number of pulses included in an excitation signal which drives the transducer. Upon receipt of the timing signal, a microprocessor determines the distance between the transducer and the detected surface.

Abstract: A point level sensor senses capacitance of a material in a vessel to determine level. The capacitance is input to a comparator which is part of an oscillator circuit developing a square wave signal having a period corresponding to capacitance. A microprocessor includes an internal counter which determines the period of four pulses to determine an average period. The microprocessor drives relay circuits providing discrete outputs according to specific precalibrated levels to be sensed. The point sensor includes a self-test mode which operates once per second to test for shorting of the probe, expected frequency response and an open probe.

Abstract: A data entry terminal for storing a plurality of user generated answers is described which includes a removable volatile read/write memory module. The memory module is used to store formatting information used to configure the data entry terminal as appropriate for a particular set of questions. The memory module can for example store a set of correct answers as well as a set of user answers for each of the questions of the test, and it can store data used to disable certain functions of the data entry terminal, such as the ability of a user to revise an initially entered answer. The data entry terminal operates to record user answers in the portable memory module, which can then be removed from the data entry terminal and applied as an input to an external computer for analyzing user responses. The portable memory module includes a battery for powering a volatile semiconductor memory included in the module.