A device for the electric monitoring of the level of a liquid contained in a container, comprising at least one temperaturedependent resistance sensor immersed in the liquid the level of which is to be monitored, a constant current source being connectable to the resistance sensor under the control of a program control device controlling a device for detecting and storing the initial voltage drop on the resistance sensor at an initial time and the measurement voltage at a specified time after connection of the constant current source, and a device for evaluating the voltages by subtracting the measurement voltage from the initial voltage and under the circumstances inverting the difference to form a display voltage which corresponds to the level of the liquid. Several of these resistance sensors are arranged at different levels within the container, the sensors being connectable one after the other, controlled by the program control device, to the constant current source and to the device for evaluating the voltage drop on them. A fixed-value storage provides a supply of fixed-value voltages one of which respectively is selectably dependent on the respective resistance sensor active at the time, and an adder is appliable in order to form the display voltage, with the evaluated voltage which is analog to the depth of immersion of the active resistance sensor and with the fixed-value voltage which corresponds to the height of that resistance sensor.
In a device for electrically monitoring the level of a liquid present in a container, a plurality of temperature-dependent resistance probes (n-1, n, n+1) are arranged at different heights in the container, they being adapted to be connected one after the other to a source of constant current (5) under the control of a program control device. For the evaluation of the voltage at one resistance probe (for example n+1) connected at the time to the source of constant current (5), a differentiator (6) is provided as well as a comparator (7), which is arranged behind the former and is and acted on by a threshold voltage and is connected to circuitry for forming a display signal. The circuitry comprises counter (4, 19) which counts the number of resistance probes (n-1, n, n+1) whose voltage differential is above the threshold voltage after connection to the source of constant current (5).
A measuring rod carrying a plurality of temperature sensors distributed over its height is placed in such a way that at least its lower part is embedded in the product of which the height is to be measured. Signals produced by the sensors and representing the temperatures in the locations of the various sensors on the rod are picked up and the variation in time of the picked-up signals is monitored in order to identify the lowest situated sensor which delivers a signal of which the variation is higher than a predetermined threshold.
A digital sample and hold circuit for memorizing the initial voltage across an electrical resistance probe which is partly immersed in the liquid in a tank.
Provided is a fuel level detector for use in a fuel tank of an automobile or the like which can accurately detect a fuel level with a reduced power consumption exhibiting an improved reliability regardless of the ambient temperatures and the temperatures of liquid fuel in order to overcome the problems involved in a thermistor method and a metal wire method. A spontaneously heated sensing resistor having a high temperature resistance coefficient is formed on a substrate so as to detect a change in the resistance value due to cooling by heat of vaporization at a portion of the sensing resistor which is dipped in fuel as a differential output voltage. In order to prevent errors in the differential output voltage due to the correction of the ambient temperature in the fuel tank, a temperature compensating resistor is provided on the substrate.
In a method for the electrothermal measurement of level which is compensated for ambient temperature and in which a resistance probe is heated with a constant current in each case during a heating time, a comparison is effected between a first variable formed from at least one probe voltage at the start of the heating time and a second variable formed from at least one probe voltage at the end of the heating time. The forming of at least one of the two variables is effected in accordance with a time function. In order to reduce the influence of disturbing effects and, in particular, voltage noise pulses, the first variable is formed by integration of the probe voltages from the start of the heating time up to the middle of the heating time. The second variable is then formed by integration of the probe voltages from the middle of the heating time until its end. The two integrals are then subtracted from one another to produce a measurement value.
