In Reply:—The proportioning system is designed to allow maintenance, service, and calibration in the field. Two set screws are used to secure each knob, collar, and sprocket of the assembly. This helps to eliminate the possibility of these components becoming loose during normal operation. Ohmeda recommends that this proportioning system be serviced annually as part of the planned maintenance program for the Ohmeda Modulus II Plus anesthesia system. In addition, the proportioning system should be calibrated and tested at least every 4 months.

This condition also should be evident to the medical professional, because the gas flows are easily viewed on the flowmeters of the anesthesia system. In addition, once the oxygen knob becomes loose on the shaft. It takes approximately ten complete turns before the sprocket would be stopped by the flange. Having just completed a preoperative checkout procedure, this large number of turns to obtain an oxygen flow of 1 l/min should have indicated this inappropriate condition.

Raymond T. Riddle; Manager, Regulatory Affairs; Medical Systems Division; Ohmeda Inc.; Ohmeda Drive; P.O. Box 7550; Madison, Wisconsin 53707–7550.

(Accepted for publication October 26, 1994.)