The invention relates to a circuit arrangement for the electrothermal measurement of level with a resistance sensor heated by a constant current, having an evaluation circuit which evaluates a measurement voltage which is dependent on the state of immersion of the resistance sensor, into which circuit a compensation voltage can also be fed for compensation of the temperature.
For the electrothermal measurement of level there is used, as is known, an elongated conductor having the highest possible temperature coefficient, arranged within a container approximately perpendicular to the surface of the liquid whose level is to be measured. In the present case we are concerned, in particular, with the measurement of the level in automobile gas tanks.
For the known electrothermal measurement of level, the elongated conductor, namely the resistance sensor, is continuously heated with a current which is as constant as possible. The measured resistance of the resistance sensor depends in this case on the removal of heat via the so-called thermal resistance, which in its turn is dependent on the level. The measured resistance is determined as the drop in measurement voltage which takes place on the resistance sensor.
In general, the voltage measurement, however, is dependent not only on the level but also on other factors which interfere with the accuracy of the measurement. Aside from variations in the coefficient of resistance of the individual measurement sensors and in the current heating the sensor, which current should be as constant as possible, the accuracy of the measurement is substantially affected by the surrounding temperature to which the measurement sensor is exposed.
In order to eliminate the disturbing influence of the surrounding temperature as far as possible, various compensation circuits are known, all of which make use of at least one additional compensation resistance.
In one such known circuit arrangement (Federal Republic of Germany No. OS 31 34 912), the compensation resistance dips into the liquid whose level is to be measured. The compensation resistance is also connected to the source of constant current which feeds the measurement resistor or measurement sensor. The measurement signal derived from the measurement sensor and the compensation signal produced on the compensation resistor are processed in the evaluation circuit, their difference being formed. The current through the measurement resistor is controlled as a function of the temperature measured with it. The compensation resistor and the measurement resistor are connected in series to the source of constant current. In this circuit arrangement, however, a special lead to the compensation resistor within the container or gas tank of the car must be provided with a connection. The temperature coefficient of the compensation resistor must be adapted accurately to the measurement resistor of the measurement sensor.
These disadvantages are present, partly to an even greater extent, in another known thermoelectric level measurement device (Federal Republic of Germany No. OS 31 15 776), in which a first resistance sensor is surrounded by the material in the container, a second resistance sensor is partly surrounded by the material in the container and partly by a free space located above it, and a third resistance sensor is surrounded only by the free space. These three resistance sensors are connected electrically in series and connected as a series circuit to a source of supply voltage. A current regulator is arranged in series with the three resistance sensors. A control input of the current regulator is connected to the evaluation circuit. A series connection of three comparison resistors is connected in parallel to the series connection of the three resistance sensors. The voltage difference between a junction point of two resistance sensors and a junction point of two comparison resistors, which difference controls the current regulator, is determined by an operational amplifier. In this circuit arrangement, therefore, a total of three resistance sensors and three comparison sensors must be adapted to each other and at least three if not four wires must be extended out of the container the level in which is to be measured.
In another known circuit arrangement for the measurement of level (VDO Paper, SAE Technical Paper Series 830106, Haub et al), the resistance sensor is connected, in series with a compensation resistor, to one input of an inverting amplifier for compensation of the temperature. The compensation resistor should be preferably arranged in the vicinity of the resistance sensor and be traversed by a current which is as small as possible, so as to avoid heating. However, if the compensation resistor is arranged in the container in the vicinity of the resistance sensor, it is necessary, in this case also, for an additional connection to be brought out. Furthermore, it has been found difficult in practice to find a compensation resistor having the same temperature coefficient as the measurement sensor.
The object of the present invention, therefore, starting from a circuit arrangement for the electrothermal measurement of level of the aforementioned type, is to improve this circuit in such a manner as to reduce or avoid difficulties in the adaptation or selection of resistance sensor and compensation resistor. In this connection the simplest possible wiring path is desired, particularly in the region of the container, i.e. the number of required lead wires should be as small as possible.