The needle valve is the most common means of regulating gas flow rate. As the valve is opened, the orifice around the needle becomes larger and flow increases. The valve cartridge itself is usually removable, so it can be replaced if it is damaged.
The valve must not be over-tightened; this will drill out the orifice and cause it to become incompetent. Some valves, such as are found on most medical anaesthetic machines, incorporate a stop to prevent the valve being over-tightened. The valve control knob is usually colour-coded. In addition, oxygen flowmeter knobs frequently have fluted edges, to distinguish them from those of other gases.
Flowmeters on modern anaesthetic machines consist of a tapered glass tube containing a bobbin or ball, which floats on the stream of moving gas.
As the gas flow rate increases, the float is carried further up the tube, thereby indicating the flow rate.
Flowmeters are specifically constructed for each gas, since the flow rate depends on both the viscosity and density of the gas.
Only the correct tube and bobbin or ball can be used to repair broken flowmeters.
Since the bobbin floats in the gas stream, flowmeters will only function correctly if the tube is vertical.
Flowmeters will not function correctly if the tube is cracked.
Bobbins and balls
Bobbin flowmeters are more accurate, but more expensive, and should be read from the top of the bobbin (the shape of the bobbin is designed to optimise gas flow, and has nothing to do with being a 'pointer'). Ball float types should be read from the middle of the ball.
Bobbin flowmeter, reading 2 L/min
Ball-float flowmeter, reading 2 L/min
Inaccuracy in flowmeters
May be due to:
The tube not being vertical.
Back-pressure from, for example, a ventilator.
Static electricity causing the float to stick to the tube.
Dirt causing the float to stick to the tube.
Application of physics to flowmeters
At low flow rates, the clearance of the flowmeter is longer and narrower, thus acting as a tube. Under these circumstances, the flow is laminar and a function of the gas viscosity. (Poiseuille's law). At higher flow rates, the clearance is shorter and wider, thus acting as an orifice. Here, the flow is turbulent and a function of the gas density.
Sequence of flowmeters
If several flowmeters are mounted together and a leak develops in one flow tube, a higher proportion of the gas coming from the upstream flowmeter will tend to be lost through the leak. If the oxygen flowmeter is upstream, this will tend to deliver a hypoxic mixture to the patient.
Oxygen flowmeters should, therefore, always be mounted downstream of any other flowmeters, although there is no international agreement on this position.
Nitrous oxide/oxygen proportioning flowmeters
Owing to the potential hazards of delivering hypoxic gas mixtures to patients, a number of manufacturers developed nitrous oxide/oxygen proportioning flowmeters. In these, the separate oxygen and nitrous oxide control knobs are replaced with a single flow control and a proportioning valve that allows adjustment of the output oxygen concentration from 30% to 100%.
Other machines have retained the separate oxygen and nitrous oxide flow control knobs, but these are linked mechanically or pneumatically, so that it is impossible to deliver less than a specified oxygen concentration (typically 25% to 30% oxygen).