Abstract: The level of mud in the mud return line of a rotary drilling rig is determined by cutting a hole in the top of the line and securing over the hole the saddle shaped base flange of a heat insulated sound tube supporting the ultra sonic transducer of an air coupled sonar distance measuring apparatus. The apparatus continuously digitally displays the average mud level. Upon a first manual command the apparatus makes a printed record of the level, time, and date.The apparatus give an alarm signal whenever the time averaged level departs a predetermined amount from a previously set datum level. When a second manual command is exercised the alarm is discontinued and a record of level is made and the datum level is reset to the existing level. The departure measuring means is reactivated to give an alarm signal upon sufficient departure of mud level from the new datum level.A third manual command makes a record and turns off the recorder while a trip is being made and until the third command controller is reset.