Abstract: A casing having rounded corners for ease of insertion into narrow confines defines a protective chamber and has openings permitting fluids exterior to the casing to enter the chamber. There is a fluid monitor within the chamber having a surface adapted for exposure to the fluids. There is a compartment within the casing insulated from the fluids and containing a second fluid monitor. A stainless steel cable is attached to the casing for inserting the probe between the walls of a double-walled tank. In an alternative embodiment there is a channel within the casing for receiving the distal end of an electrician's fish.

Abstract: 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.