NARDA 8718B - Manual (Page 86)

85 Chapter 14 Probe Zero
Zeroing the Probe While In Use
The meter and probe are initially zeroed before the meter is
allowed to go into the measurement mode. If you determine that
significant drift may have occurred, it is simple to fine tune the
zero. See Checking for Zero Drift on page 82.
To zero the probe while in use...
1. Shield the probe using one of the methods described
in Shielding the Probe.
2. Press the Auto Zero key. The meter will correct for any
probe drift in about eight seconds. The meter will then
automatically return to the measurements screen.