This detector consists of a sensor and an amplifier; it utilizes the physical principle of ultrasonic energy attenuation or refraction as it passes through different liquids or interfaces to detect oil-water boundaries. As shown in Figure 1, the sensor is mounted horizontally on the side, with its detection gap fully submerged in the liquid. When the liquid medium passing through the sensor's detection gap is water, the degree of ultrasonic attenuation is constant; the gain of the detection module's amplifier can be adjusted to allow the ultrasonic waves to pass unimpeded through the sensor gap, resulting in no signal output from the amplifier. Conversely, when an oil medium—which exhibits a higher degree of attenuation—enters the sensor's detection gap, the propagation of the ultrasonic waves is blocked, and the amplifier outputs a detection signal. Thus, by appropriately setting the gain of the detection module, different liquids can be detected and distinguished. As shown in Figure 2, when two immiscible liquids exhibit similar ultrasonic attenuation characteristics, the sensor is mounted laterally at an angle of 10° relative to the horizontal plane; this configuration leverages the phenomena of reflection and refraction that occur when ultrasonic waves pass through the interface between two different liquids to distinguish the liquid boundary. When the sensor's detection gap is submerged entirely within a single liquid, the ultrasonic waves pass through the gap without obstruction; however, when two different liquids are present within the detection gap, the reflection and refraction effects occurring at the liquid interface prevent almost all ultrasonic waves from reaching the sensor's receiving end. Consequently, the liquid interface is detected, and the amplifier outputs a detection signal. It is worth noting that this same condition—and resulting signal output—will also occur if air enters the sensor's detection gap.