| 摘要 |
Two thermistors are mounted in a fluid detection probe, one thermistor having its surface exposed to the ambient fluids, and the other encapsulated in an insulator, such as RTV, that has a thermal conductivity greater than air. The thermistors are connected in a resistance bridge circuit between ground and a positive digital circuit voltage, which circuit applies power to the thermistors, heating them. The side of the thermistors toward the positive voltage is connected to the inputs of a comparator. When air is present, the encapsulated resistor cools more than the exposed one and, therefore, the encapsulated resistors resistance is higher producing a higher voltage at its comparator input than the exposed resistor. In the presence of a liquid, the RTV insulates its thermistor from the cooling effect of the liquid, and its comparator input voltage is therefore lower than the voltage of the exposed thermistor comparator input. Thus, the output of the comparator switches upon the environment of the thermistors changing from liquid to air, and its output is indicative of the fluid state of the probe environment. There is also a polar-non-polar liquid detector. The outputs of the comparator and the polar-non-polar detector are connected to a logic circuit, the output of which indicates the presence of air, water, or hydrocarbon.
|
