B-A (vacuum) gauge

A thermal-cathode ionization gauge. In the B-A (vacuum) gauge, a heated filament emits electrons and the emitted electrons are accelerated, and these electrons ionize residual gasses, then the produced ion current is measured. The B-A gauge measures the pressure of the middle to high vacuum region. The measurable range is 0.1 to 10^-5Pa. The gauge can measure lower pressures than the Penning gauge can. Since the gauge is made of a glass tube, it is likely to be damaged. In addition, the gauge has a gas emission problem. Thus, the B-A gauge adopts, in many cases, a design where the gauge head is directly exposed to the vacuum. This is called a nude gauge (measurable pressure: ～10^-9Pa). The output current is proportional to the pressure. For a TEM, the gauge is mainly used to measure the pressure in the ultra-high vacuum specimen chamber.