The Ionization Gauge Family

All previously mentioned gauges require a certain level of particle density for operation. Once the level of particle density has dropped below a certain level (approximately 1018 particles/m3), it is not possible to detect transfer of momentum forces either from gas to solid wall or from gas to gas. On the other hand, it is possible to ionize gas particles and then count the ionized molecules. 

There are essentially two types of ionization gauges used in the laboratory The hot- and the cold-ion gauges. A third type, the radioactive ionization gauge, is so limited in both scope and use that it will not be discussed in this book. 

The concept of the ionization (ion) gauge is quite simple. Under a given electrical load, the available gas within the vicinity of the vacuum gauge is ionized either by heat or by a high-field (electrical) emission. Then, the ionized gas is collected and counted. From this count you can interpret what you have read as a unit(s) of vacuum and thereby infer the vacuum within the system. 

Hot-cathode gauges are considered fast pumps, but cold-cathode gauges pump 10 times faster. At 10 10 torr there are only 106 molecules per cubic centimeter. If 

---