Workstation pressures & cylinders

The three pressure zones of the anaesthetic machine, what lives in each, and the cylinder data that goes with them.

The three pressure systems

The machine is divided by where the pressure drops occur. Learn the zones by their boundaries and the components fall into place.

Zone	Boundaries	Components
High pressure	Cylinder → first-stage regulator. The only part exposed to full cylinder pressure	Hanging yoke, yoke block with check valve, cylinder pressure gauge, first-stage regulator
Intermediate pressure	Pipeline inlet / first-stage regulator → flowmeter valves. About 37–55 psig, nominally 50	Pipeline inlets and gauges, pipeline check valves, ventilator power inlet, oxygen supply pressure alarm, fail-safe valve, oxygen flush, second-stage regulators, flowmeter valves
Low pressure	Flowmeter tubes → common gas outlet. Only slightly above atmospheric	Flowmeter tubes, vaporisers, one-way check valve (some machines), common gas outlet

Why 45 psig from the cylinder regulator? It is set below the 50 psig pipeline pressure, so with both supplies open the machine preferentially draws from the pipeline and spares the cylinder.
Oxygen flush delivers 35–75 L/min at ~50 psig, bypassing the flowmeters AND the vaporisers. Hence its two dangers: barotrauma, and dilution of the volatile agent with risk of awareness.
Second-stage regulators reduce pressure further — to about 14–16 psig for oxygen and 26 psig for nitrous oxide — so flowmeter accuracy is preserved despite supply fluctuations.
The low-pressure system is the most vulnerable to leaks: thin-walled components, downstream of every safety device. That is exactly what the negative pressure leak test interrogates.

Gas	ISO colour	Capacity	Pressure	State in cylinder
Oxygen	White body and shoulder	625–700 L	137–155 bar (1900–2200 psig)	Gas
Nitrous oxide	Blue	1590 L	51 bar (745 psig)	Liquid + vapour
Medical air	White/black quartered shoulder	625–640 L	137 bar	Gas
Carbon dioxide	Grey	1590 L	57 bar (838 psig)	Liquid + vapour
Entonox (50:50)	Blue with white shoulder	~1800 L	137 bar	Gas
Helium	Brown	~500 L	137 bar	Gas

Larger H cylinders used as pipeline backup: oxygen ~6600 L at 2200 psig; nitrous oxide ~15 900 L at 745 psig.

Oxygen's critical temperature is −118 °C, so at room temperature it is a true gas obeying Boyle's law. A half-empty cylinder reads half pressure — the gauge tells you the contents.
Nitrous oxide's critical temperature is 36.5 °C (carbon dioxide's is 31 °C), so it sits as liquid in equilibrium with vapour. The gauge stays fixed at 745 psig until the last liquid evaporates — at which point only about 250 L remain and pressure then falls rapidly.
The only reliable way to assess N₂O content is to weigh the cylinder against its tare weight.
Cooling on use: rapid withdrawal causes latent heat loss, frosting, and a fall in vapour pressure — so a gauge read during heavy use underestimates the contents.

Filling ratio = weight of liquid ÷ weight of water the cylinder would hold. Overfilling risks a dangerous pressure rise as temperature rises.
The conventional UK/exam figures are 0.75 in temperate climates and 0.67 in tropical climates. Your source PDF states these the other way round — the 0.75-temperate / 0.67-tropical pairing is the one to quote.

Prevents misconnection at the yoke. Oxygen 2,5 · Nitrous oxide 3,5 · Air 1,5 · Carbon dioxide 1,6 · Helium/oxygen 2,4 or 4,6.
Pipelines instead use non-interchangeable DISS or Schrader quick-connect fittings — a different system for a different pressure zone.

Built from anesthesia_notes.pdf, pages 1–2.