32 cHAPTER 4: Operation
Model 425 Gaussmeter
4.5.1 DC Measurement
Mode
To measure static or slowly changing fields, use the DC measurement mode. In DC
measurement mode, the display shows the measured DC field and the polarity (sign)
followed by the appropriate field units. The resolution of DC readings is 3¾ digits
when the filter function is turned off and 4¾ digits when the filter is turned on
(section 4.5.1.1). The monitor output will provide a DC voltage proportional to the
measured DC field, where ±3.5 V equals ±full scale for the selected range.
Menu navigation:
DC/RMS (toggle)
Default: DC
Interface command: RDGMODE
4.5.1.1 Filter
The filter is a moving average filter of 16 readings and settles in approximately 0.5 s. It
is a moving average filter so it does not change the reading update rate. If the filter is
turned on, all reading values are filtered including the relative reading, and all fea-
tures will use the filtered reading including maximum reading and alarms. The filter
has no effect on the monitor output.
The filter function is used to quiet the display and make it more readable when the
probe is exposed to a noisy field. The filter also acts to quiet noise within the instru-
ment, making an additional digit of usable resolution available, to 4¾ digits, with the
filter on. Care should be taken when using the filter on changing fields because it may
level off peaks and slow the response of the instrument.
In AC mode, the filter function is always active to help quiet the reading. Although the
filter is active in AC, the resolution remains 3¾ digits.
Menu navigation:
In DC mode: DC/RMS (press and hold for 3 s)Q Filter (on, off)
Default: on
Interface command: RDGMODE
4.5.1.2 DC Operation Zero Probe
The zero probe function cancels out the zero offset of the probe or small DC magnetic
fields. It is normally used in conjunction with the zero gauss chamber, but may also be
used with an unshielded probe (registering Earth’s local magnetic field). The zero
probe function should be used any time a probe is changed, when operating in condi-
tions such as significant temperature change, and periodically during normal use. To
cancel large magnetic fields, use the relative mode (section 4.5.6).
During the zero probe process, do not move the probe until the zero probe process is
complete and the display returns to the normal display. If during the zero probe pro-
cess, the measured offset is larger than expected, the Model 425 will display a mes-
sage stating “The Measured Probe Offset Larger Than Expected”. The Model 425 will
continue to operate with the offset correction, but it will be up to the user to investi-
gate the nature of the offset. If through the process of elimination you are not able to
discover the reason for the offset, the error message may be an indication of a dam-
aged probe.
The “Measured Probe Offset Larger Than Expected” message can also appear if the probe
is exposed to a large magnetic field during the zero probe process. If that is the intended
operation, zero the probe in the zero gauss chamber and then use the relative mode to
cancel the large magnetic field.
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