There are many examples of apparatus, circuits and systems employing a temperature dependent resistance element in connection with monitoring the liquid level in a container. Many of these are relatively complex, especially in their attempts to provide temperature compensation to improve the accuracy of the sensor system. One example of such a device is shown in U.S. Pat. No. 4,361,037. This employs a temperature sensitive resistance element having different resistance values in different portions of its linear dimension. This is stated as a means for compensating for the change in level of liquid due to its thermal expansion. One of the primary concerns of that patent is to compensate for the variation in heat transferred from the sensor to the liquid as a function of the initial temperature.
United Kingdom patent b 2,120,482 is another example of a complex liquid level system employing a plurality of discrete thermistors for an automobile fuel tank. A flowmeter is an indispensable part of that system. Temperature compensating resistors are not employed.
U.S. Pat. No. 4,633,491 concerns a circuit for electro thermal level measurement with ambient temperature compensation by means other than resistors in parallel with the temperature sensitive resistor.
U.S. Pat. No. 3,111,031 relates to another automobile fuel gauge liquid level measurement. It includes a thermistor to compensate for ambient temperature in some unspecified way.
U.S. Pat. No. 3,485,100 shows an external heater for heating an elongated temperature sensitive element in a liquid level measurement system. Alternatively, a series of discrete temperature sensitive resistors are heated by the separate heating element. A temperature compensating resistor is connected as one leg of a bridge circuit.
United Kingdom patent publication 2,120,482 shows several thermistors, each having a parallel connected resistor. The purpose of these resistors is to limit the voltage drop across a dry thermistor to make the circuit more linear. There is no purpose of temperature compensation for these resistors. As a matter of fact, the resistance vs. temperature characteristics of thermistors is so nonlinear and different from RTD's that it is highly unlikely that they could be temperature compensated in the manner taught herein by applicants.
Other examples of generally relevant sensor apparatus, without effective temperature compensation, are shown in U.S. Pat. Nos. 4,356,728 and 4,619,140.
The resistance of a temperature dependent resistive sensor increases as the temperature it is sensing increases so that when the sensor is energized by a constant current source, the power it dissipates also increases. This power dissipation results in additional or enhanced heating of the resistive sensor. In an uncompensated system using such a sensor, the cooling effect of the fluid increases as the sensor temperature increases due to this additional power dissipation effect and due also to higher ambient temperature. When considering the output of the uncompensated sensor, these effects (heat caused by increased power dissipation and cooling from the surrounding fluid) function in the opposite manner, but do not precisely or adequately compensate each other. Specifically, the rise in sensor resistance will be the predominant factor so sensor output will rise as temperature increases. This will cause the instrument to give an erroneous output at other than calibration temperature. These are among the factors which give rise to a number of complex attempts at accurate temperature compensation which have been the purpose of some previous patents.