EMPLOYEE STATES HE RAN OUT OF ACETYINE ON WELDING TRUCK WHILE WORKING ON PROJECT. TOOK OUT EMPTY TANK AND WENT TO LIFT IN A FULL ONE, ALONE. WHILE LIFTING, FELT PAIN IN LOWER BACK. DID NOT REPORT INJURY TO SAFETY COORDINATOR UNTIL AFTER NOON ON 10/22. CAME IN TO WORK ON 10/23. LEFT EARLY BECAUSE OF BACK PAIN. DID NOT COME IN 10/24. DR DIAGNOSED LOWER BACK MUSCLE STRAIN.