Using The Analog Bar Graph; Nulling; Contact Bounce - Fluke 27/FM Operator's Manual

Using the Bar Graph
Diode leakage tests require that the diode junction be reverse biased when
being measured. This is accomplished by connecting the anode of the diode
to the COMMON input terminal and the cathode (ring) of the diode to the
V
ri
input terminal. Leakage at the test voltage being applied can
then be read in terms of conductance.
High-voltage stacked diode assemblies can usually be tested for forward and
reverse resistance changes using conductance. These assemblies typically
have such high forward voltage drops that the diode test or resistance modes
cannot test them.
USING THE ANALOG BAR GRAPH
The analog bar graph functions like the needle on an analog meter without
the mechanical overshoot of movements.
The bar graph is especially useful for peaking and nulling, and observing
rapidly changing inputs. Because bar graph response time is fast and
precise, it can be used to make approximate adjustments quickly. The
3200-count digital display can then be used for final adjustment.
The analog bar graph can also be used for some limited diagnostic purposes.
In situations where rapidly fluctuating signal levels make the digital
display useless, the bar graph is ideal. Like the needle on a Volt-ohm
milliammeter (VOM), the analog bar graph excels at displaying trends, or
slowly changing signals. In addition, in the Autorange mode, you can monitor
signal change through changing ranges. Many diagnostic routines using the
bar graph require practice. You will usually be looking for good or bad
signal patterns that occur over some span of time. Noisy resistance
measurements, for instance, create such patterns. Therefore, familiarity
with analog bar graph response and movement is necessary to accurately
interpret a signal pattern. Compare the bar graph response when making
measurements on a known-good unit to the bar graph response when making
measurements on a faulty unit.
Nulling
The DMM's bar graph is ideal for nulling adjustments. As an adjustment
approaches zero, fewer bar graph segments are displayed, until no bar graph
segments are displayed. The ow annunciator flickers when the input level is
within 10 counts of zero. The flickering null indication is displayed every
time the input approaches zero or swings from one polarity to the other.
Watch for the
annunciator indication, then reverse the direction of the
adjustment when the polarity sign is displayed. In one or two passes, a
near-zero input level is possible, then the digital display can be used for
exact zero adjustment.
Contact Bounce
Relay contacts may begin to bounce open when subject to vibration. Testing
for this problem is a routine troubleshooting practice for many types of
equipment. Since the bounce problem will worsen as 'the relay fatigues, early
diagnosis is important.