The hot-filament ionization gauge is widely used for measuring the level of vacuum. The electrons are emitted from a hot cathode and accelerated towards the anode. In that process the electron may ionize a gas molecule. That gas molecule is pulled towards the collector and ion current is measured with an ammeter.
The Penning Ionization Gauge, also known as cold cathode gauge is used to measure the level of vacuum. High voltage between the anode and the cathode causes gas discharge and the resulting ionic current is measured with an ammeter. The measured amperes are then converted into pressure units such as Pascals or Torrs.
The thermocouple gauge is a device used to measure low vacuum. A filament is heated up by passing a current through it. When gas molecules interact with the filament, heat is carried away. Therefore higher pressure in the chamber means that more heat is taken away. The temperature of the filament can be measured with a thermocouple where the generated voltage depends on the temperature. Therefore in this system the voltage of the thermocouple is measured and converted into pressure units like millibars or pascals.
The Pirani Gauge is used to measure low vacuum. In the system tungsten filaments are heated up by passing current through them. As gas molecules interact with the filament, heat is carried away and electrical resistance changes. Therefore if the measured current or voltage can be converted into pressure units.