Measurement of internal pressure of vacuum tubes by micro-discharge and emission current

The traditional methods for measuring internal pressure P of vacuum tubes of breakers are the magnetron and power frequency voltage withstand test. A magnet coil is needed by magnetron and only very bad tubes can be picked out by voltage withstand test. In this paper, a new method—inception voltage Vd of micro-discharge / inception voltage Ve of emission current, i.e. Vd/Ve method, is presented. It is found that by means of the relationship Vd/Ve∼P, internal pressure P can be measured accurately although vacuum gap d vary from 0.2mm∼0.3mm. By this new method, internal pressure of vacuum tubes can be measured without using magnetic coil, but with following process: firstly, when contacts is under closed situation, by screwing the nut to stretch compression spring, vacuum gap 0.2mm∼0.3mm is obtained, secondly power frequency voltage is applied to the gap and gas adsorption layers are polished by micro-discharge current, finally the Vd and Ve of the vacuum tube is measured. Theoretical study show that the bigger the Vd/Ve is, the smaller the P is. So by measuring the value of Vd/Ve, the value of P can be obtained. The relationships of Vd,Ve,Vd/Ve with P were measured in laboratory, the test results show that the measuring ranges of P can be 10⁰Pa ∼10⁻³Pa by this new method.