Adjustment and calibration DKD, PTB national standards

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg  

After switching the modulator from the anode potential over to the ion collector potential, the modulated pressure reading p is lower than the p, reading because a portion of the ions now reaches the modulator. Hence  

Definition of terms Since these terms are often confused in daily usage, a clear definition of them will first be provided  

Adjustment or tuning refers to the correct setting of an instrument. For example, setting 0 and 100 % in THERMOVACs or setting the mass spectrometer to mass 4 in the helium leak detector. 

Calibration inspection refers to comparison with a standard in accordance with certain statutory regulations by specially authorized personnel (Bureau of Standards). If the outcome of this regular inspection is positive, an operating permit for the next operation period (e.g. three years) is made visible for outsiders by means of a sticker or lead seal. If the outcome is negative, the instrument is withdrawn from operation. 

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