The left side nose wheel tire was damaged during pushback, when the tow bar failed. The tire was replaced and while being serviced on the airplane, the tire rim failed, fatally injuring the mechanic. Examination of the bottle used to service the tire showed two pressure scales, bars on the outside and PSI on the inside. One bar equals 14.7 PSI. The high end of the scale was 10,000 PSI and the bottle was pressurized to 4000 PSI. The tire was to be serviced to 150 PSI. A check of the regulator after the accident disclosed it was properly calibrated. According to the FAA, the tire rim was stressed to 580 PSI. Examination of the fracture surface found, 'no...manufacturing or material defects...fracture surface is consistent with a tensile overload type of failure.' According to the company, no low pressure regulators were available. They are kept under lock when not in use and the responsible person had already gone home for the day.