Greyline Instruments manufactures ultrasonic level transmitters and controllers, Open Channel and Area-Velocity flow monitors and Doppler flow meters. They use non-contacting sensors that mount above the liquid being measured, or strap onto the outside of a pipe. Applications include "difficult liquids" that would foul or damage regular sensors including chemical tank inventory, pump control in sumps and pump stations, wastewater, slurries and sludge flow. Greyline instruments are popular in industry, water and wastewater treatment plants.