Coolant level sensors may look minor, but they provide an independent safeguard against overheating. In EDN’s fundamentals explainer, the focus is on how these sensors detect low coolant volume before an engine or EV battery pack reaches a failure point.
The article first separates coolant level sensing from coolant temperature sensing. A temperature sensor measures how hot the fluid is when it remains submerged, while a level sensor tracks whether enough coolant is present in the expansion tank. That distinction matters because a temperature probe can be misled if coolant drains away and the sensor is left measuring air.
Several sensing principles are used in production designs. Magnetic float switches combine buoyancy with reed or Hall-effect sensing. Optical sensors rely on an infrared LEDlight-emitting diodeSemiconductor diode that emits light when current flows. and prism, using total internal reflection when the sensor is exposed to air and loss of reflection when coolant is present. Solid-state conductive and capacitive sensors instead detect immersion through electrical conduction or changes in dielectric constant, while resistive sensors measure how the probe’s resistance changes as fluid wets or leaves the surface.
The piece also notes newer approaches. Ultrasonic sensors can measure fluid level with millimeter precision, and next-generation capacitive devices add digital outputs and on-chip compensation to adapt to different coolant chemistries and vibration over time. Separate leak detection sensors are used in EV battery enclosures, where even trace moisture must be sensed quickly through resistance changes and reported to the BMS.
Design constraints remain practical and severe: coolant slosh can cause false alarms, chemistry and corrosion can shift readings, and many sensors must survive pressurized systems operating around 18–20 PSI, with some heavy-duty capacitive designs rated up to 100 PSI. For cold-start diagnostics, short-duration DCdirect currentElectric current that flows in one direction. conductivity sensors are used to avoid electrolysis during continuous operation. The article frames coolant sensing as a mix of fluid dynamics, materials durability, and electronics rather than a simple on/off switch.