The working principle of Pirani gauge is:

Pirani gauge:

• It is based on a hot metal wire suspended in a tube that is exposed to gas pressure media.
• The Pirani gauge measures the vaccum pressure dependent thermal conductivity from the heated wire to the surrounding gas.
• The heated Pirani sensor filament is typically made of a thin (< 20 μm) Tungsten, Nickel or Platinum wire
• As gas molecules collide with the Filament wire, heat is transported from the hot wire.
• The heat loss is a function of the gas pressure and at low pressure the low gas density and long mean free path between gas molecules provides low thermal conductivity.
• At high pressure the high gas density and short mean free path between molecules will result in high thermal conductivity.