A trigger switch is located on the handle of the Sludge Gun®. When depressed, it causes a beam of high-intensity infrared light to shine across a gap in the probe to a matching phototransistor sensor. When the LED beam encounters suspended particles in the probe gap, some of its energy is absorbed, causing the phototransistor to activate a rear-facing speaker on the butt of the gun. As the probe is lowered into the tank/pond, the instrument emits an audible tone, which varies in volume and pitch depending upon the percentage of suspended solids. Sludge blanket and overlying cloudy layer are located by observing depth markers on the cable. A thumb-adjustable sensitivity control allows the user to detect suspended solids concentrations ranging from light flocs to thick blankets. In addition, features are uniquely suited for specific applications. For example, high-power Model 10-HP excels at finding the level of biosolids in murky lagoons; Model 10-LP detects clear liquid surfaces. No calibration is required.