Abstract: Air under pressure feeds through a pin-to-box casing collar of a surface casing and into the drilling mud space inside. Drilling mud within the casing opposes air pressure in the line. When drilling mud level drops too low, air pressure in an air supply line to the collar falls below a threshold level and an alarm will sound, a chart recorder will record the event and its duration, and warning lights will flash. The chart recorder records the history of the pressure in the air supply line to indicate either an adequate drilling mud level or an inadequate level and plots these facts as a function of time. The air flow rate can quantitatively be determined by a rotometer. The rate at which air flows through the line can be adjusted.