The AH 11A capacitance standard
User-selected values AH 11A capacitance standards can be ordered with any value from below 1pF up to 115pF.
Furthermore, if the application for which the original value was purchased becomes obsolete, the AH 11A can be returned to the factory and set to a totally new value for a nominal cost.
Traditionally, capacitance values for transfer of higher echelon calibration values have been chosen to be several parts per million above 10, 100 and 1000pF. The AH 11A can be ordered with values such as these for 10 and 100pF.
For applications requiring calibration of an AH 2500A capacitance bridge, values near 100pF are preferred to those near 10pF.
In fact, for calibration of an AH 2500A at a single value, Andeen-Hagerling recommends using an AH 11A standard having a value of 99.99950pF. This is 5ppm below 100pF. Values slightly below 10, 100, or 1000 are preferred to those above since this allows the capacitance bridge to most conveniently display an additional digit.
Otherwise, the exact choice of values should be based on convention, personal preference, and standardization among groups that are exchanging calibration values.
