4 .5 . 2 AC M eas u re m e nt M o de s 33
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For best results, allow the instrument and probe to warm up for at least 5 min before
zeroing the probe, and at least 30 min for rated accuracy. The probe and the zero gauss
chamber should be at the same temperature.
To zero the probe in the zero gauss chamber follow this procedure.
1. Allow the temperature of the probe and chamber to equalize (a large tempera-
ture discrepancy affects the quality of the calibration).
2. Carefully place the probe tip into the chamber. Orientation of the probe is not
critical.
3. Begin the zeroing process by pressing Zero Probe and then Enter. A calibrating
message appears on the top line along with a progress bar on the bottom line.
4. Press Escape at any time during the zeroing process to cancel the sequence and
return to the normal display. Previous probe offset values will be retained.
5. The process is complete when the instrument returns to normal display.
Menu navigation:
Zero Probe
Interface command: ZPROBE
4.5.2 AC Measurement
Modes
The Model 425 offers two AC measurement bands to improve the overall AC measure-
ment performance. In narrow band mode, the instrument uses a 100 mA, 5.4 kHz
square wave excitation current. This type of excitation provides the benefit of noise
cancellation characteristics, but it limits maximum frequency to approximately
400 Hz. In wide band mode, the instrument uses a 100 mA, DC excitation current.
This type of excitation provides the greatest frequency range for RMS measurements,
up to 10 kHz, but this wide bandwidth allows more noise into the measurement. It is
recommended to use the narrow band mode unless the frequencies of interest are
greater than 400 Hz. The resolution in AC measurement mode is fixed at 3¾ digits.
Menu navigation:
DC/RMS (toggle)
Default: DC
Interface command: RDGMODE
To change between the two AC measurement bands, follow this menu navigation.
Menu navigation:
In AC mode: DC/RMS (press and hold for 3 s)Q (filter band narrow, filter band wide)
Default: filter band narrow
Interface command: RDGMODE
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