Pressure is force per unit area. We use bar. Atmospheric pressure at sea level is about 1.013 bar.

Gauges usually read gauge pressure (bar g), which is pressure above atmosphere. Absolute pressure (bar a) is gauge pressure plus atmospheric pressure. Refrigerant tables use absolute pressure.

A pressure cooker is the best example. The weight holds the pressure at about 1 bar above atmosphere. Water then boils at about 120 °C instead of 100 °C, so food cooks faster.

At high altitude, the opposite happens. In Leh (about 3,500 m), air pressure is lower and water boils at about 88 °C (typical). Dal takes longer to cook.

"Saturation" means liquid and vapour are together in balance. At a given pressure, there is only one saturation temperature. If you know one, you know the other. This is the most useful fact in refrigeration.

Below the saturation temperature the liquid is "subcooled". Above it the vapour is "superheated". Technicians measure both to check a system.

Worked example — gauge to absolute pressure

A gauge on a system in Kolkata reads 7 bar g. What is the absolute pressure?

• P_abs = 7 + 1.013 = 8.01 bar a.
• Use 8.0 bar a when reading a refrigerant pressure–temperature table.

Remember

• Higher pressure raises the boiling point; lower pressure lowers it.
• At saturation, pressure fixes temperature and temperature fixes pressure.
• bar a = bar g + about 1.013 (at sea level).
• Subcooled liquid is below saturation; superheated vapour is above it.