The standard type of ionization gauges is Penning gauages. The operation principle of penning gauge is similar to that of ion vacuum pump. The gas molecules in vacuum enter the gauge and move between two parallel electrical plates or tube-based anode and cathode biased at a DC voltage of 2,000 V. There is a powerful magnetic field across the anode and cathode; therefore, the electrons, originated from gas ionization, spiral toward the cathode, resulting in more collisions with other gas molecules and thus higher ionization current. This current can be detected, calibrated, and read either in an anolog or digital format in terms of vacuum. This gauge is employed to measure high and ultra-high vacuum levels up to 10^-8 torr, generated by diffusion pumps, turbomolecular pumps and ion getter pumps.

For instance, Figure 3179 shows the gauge tube is connected to the vacuum chamber of an electron microscope.