For pressure measurement below atmosphere or vacuum, different gauges are available. Manometers and bell gauges can go up to 0.1 Torr. Diaphragm gauges are usable up to a pressure of 10-3 Torr. For pressure below this value, electrical gauges like Pirani or Ionization Gauges are used. Vacuum measurement is broadly classified into Mechanical Type, Thermal Type, Ionization Type, and Radiation Vacuum Gauge.
- A standard manometric type mechanical gauge that is used down to 10-4 Torr is the McLeod Gauge.
- Thermal type vacuum gauge can be further divided into Pirani Gauge and Thermocouple Type Vacuum Gauge.
- Ionization Gauge can be divided into Hot Cathode Type and Cold Cathode Type.
- One type of radiation gauge called Alphatron is explained.
- Other type of vacuum gauge includes the Quartz Reference Vacuum Gauge.
The heat conductivity of gases is independent of its pressure, at normal pressure. But, heat conductivity starts falling as the pressure is lowered t 10 Torr and below. The reason behind this is less collision between gas molecules within the wall and also their small number in a specific volume. The energy is carried to the walls of the container due to this collision. Thus, lesser number of molecules will be available to take the heat away from the source.
At low pressures, the heat loss that occurs from a hot wire mounted in a glass or metal tube is due to the following factors.
- Conduction through the lead mines
- Conduction in the gas
Out of these, convection is comparatively negligible. A new clear wire has a small surface emissivity and is god for producing high temperature at low gas pressures. Due to oxidation and carbonization, the surface tends to deteriorate. This causes error of the device at low pressure ranges.