Description
FEATURES:
User-Selected Values
AH 11A capacitance standards can be ordered with any value from below 1 pF up to 115 pF. 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 1000 pF. The AH 11A can be ordered with values such as these for 10 and 100 pF. For applications requiring calibration of an AH 2500A capacitance bridge, values near 100 pF are preferred to those near 10 pF. In fact, for calibration of an AH 2500A at a single value, Andeen-Hagerling recommends using an AH 11A standard having a value of 99.99950 pF. This is 5 ppm below 100 pF. 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.
Applications of User-Selected Values
Calibration of a Range: For end applications requiring the measurement of a narrow range of values, one may desire one or more standards that fall within that range of values. Such application-specific standards can substantially increase confidence in the accuracy of measurements made within that range of values. An example might be fuel gauging where the capacitance values representing “full” and “empty” are useful calibration points since they represent the limits of the range being measured.
Calibration of a Set: For end applications requiring the measurement of a well defined set of values, the possession of a set of standards whose values equal the nominal values within the application set can substantially increase confidence in the accuracy of measurements made near values in that application set. Especially, for volume applications, this may allow using lower cost capacitance meters to achieve a given accuracy. The ability of the AH 11A to be set to any value makes such strategies possible to a degree of accuracy never before attainable.
Deviation Measurements: The ability to precisely set the value of the AH 11A allows it to serve as a reference capacitor against which an unknown capacitance is compared. This configuration allows differential capacitance measurements to be made that can be simultaneously very fast and very accurate.
Linearity Measurements: Possession of a set of only four standards in a 1, 2, 4, 5 or similar sequence allows high precision linearity calibrations to be made over one decade of values. In this example, a set having 10, 20, 40, and 50 pF standards can be used to form parallel combinations that create every decade capacitance value from 10 to 120 pF. The addition of one more value to the set such as 1, 1, 2, 4, 5 allows the set to be self-checked for internal consistency.
Monitoring
The AH 1100 frame incorporates a panel meter with a three-and-one-half digit LED display to monitor internal parameters whose stability is critical to the stability of the capacitance values. This meter can read the most important power supply voltages at the press of a button. The meter also reads the difference in the readings taken by the pair of temperature sensors in each AH 11A. For convenience, these differences are reported in units of ppm of the capacitance value. Although the temperature of the AH 1100 chassis is not critical, its temperature may also be read by the panel meter. In addition, each AH 11A standard has an associated LED on the AH 1100 front panel that will flash if control of the oven temperature is lost, even briefly.