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Flowmeters: an old problem reappears

The plastic bodied oxygen flowmeters produced by Ohmeda Ltd have recently been the subject of a significant number of complaints associated with apparent fluctuation in the gas flow. What usually happened was that the flow was set to 5 litres/ minute and then, after a period of time, the reading changed to around 9 litres/minute. If the flow was readjusted to give a reading of 5 litres/minute then again after a period of time the reading changed without further adjustment of the control valve. Readings as low as 2 litres/minute were observed. In certain circumstances it was found to be impossible to set a flow of 5 litres/minute. In this case, the indicator ball rises smoothly to around 4 litres/minute and then suddenly jumps to 6 litres/minute with the reverse effect when trying to reduce the flow to 5 litreslminute. These effects were very frustrating for the staff involved, since it meant that they had to check the flowmeter reading much more frequently. They began to suspect and criticise the control valve on the flowmeter and also the stability of the pressure in the hospital piped gas system. However, investigation quickly revealed that an electrostatic charge was accumulating in the flowmeter and that this was affecting the position of the indicator ball. When this happens the delivered flow is not accurately related to the indicator ball position. This problem is acute in this particular model of flowmeter, not only because it is made from plastic material which is not antistatic, but also because of the light weight of the indicator ball. This ball weighs only 0.15 g and hence the forces generated by the electrostatic charges can have a significant effect on its position. The problem may not be too critical where the patient is using a Venturi type face mask as variations in oxygen flow will not have too much effect on the concentration of oxygen delivered. However, the effect significantly alters oxygen concentration in most other forms of oxygen therapy; in these circumstances we strongly suggest that alternative flowmeters be used.