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The pressure contained within gas cylinders is initially very high and falls as the contents are used. The function of the regulator, or reducing valve, is to convert this variable, high-pressure gas supply to a constant, low-pressure gas supply of around 50 psi.

Advantages of using a regulated, low pressure system in an anesthetic machine are:

  • It is easier and safer to manufacture the pipework in the machine using low-pressure piping and connectors, and the consequences of a gas leak are less serious.
  • It eliminates the necessity of constantly adjusting the flowmeters as the pressure in the cylinder falls.
  • It enables better control of the gas flow to be maintained: since the needle-valve and flowmeters are larger for a given flow rate, they are less prone to inaccuracy due to wear or the presence of dirt.


As gas flows out of the low-pressure chamber, the drop in pressure reduces the force generated by the diaphragm (D) against the spring (S), allowing the valve (V) to open and admit gas from the high-pressure chamber. The output pressure may be adjusted by a screw (A) that alters the force applied by the spring.

The regulated low pressure will only remain constant over a certain range of pressures in the cylinder. If the contents of the cylinder are used to the point of exhaustion, the pressure in the low-pressure system will fall. This may present a hazard if, to compensate for this, the flow control is further opened and, later, a full cylinder is connected to the system. The high pressure in the new cylinder will now generate a very high flow into the breathing circuit which may result in a rapid increase in pressure and barotrauma to the lungs.

Two-stage regulators
Two-stage regulators essentially consist of two single-stage regulators mounted in series, so that the low-pressure chamber of the first stage becomes the high-pressure chamber for the second stage. Since the pressure in the intermediate chamber is relatively constant, this arrangement provides very precise control of the pressure in the final chamber. Two-stage regulators are relatively expensive and so are only used in situations where it is important to maintain the output pressure within close tolerances.

Oxygen flow regulators
Some oxygen regulators incorporate a Bourdon-type flowmeter and flow restrictor, and are intended for delivery of oxygen directly to patients.

Flow regulator

They should not be used to supply oxygen to the anesthetic machine since the limited flow (typically 8 or 15 l/min) will be inadequate to operate the quick-flush or ventilator.

Low-pressure gas supply  

Comments on this article should be addressed to Dr Guy Watney
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