Monroe Electronics Resistivity Meter - Portable - Surface - model 272A - Manual (Page 4)

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HOW THE MODEL 272A WORKS:
Located on the insulating base of the electrode assembly are two concentric circular electrodes
of conductive elastomeric material. The dimensions of these rings are such that when the
assembly is placed on the flat surface of the material to be tested, ten squares of the material lie
between the electrodes. When power is switched on a voltage is impressed on the outer ring
and current flow in the interposed material is detected by the inner electrode thus defining the
resistivity characteristics of the material. Internal circuitry of the Model 272A interprets this
signal and provides a direct readout of the surface resistivity of the material under test in ohms
per square. (Re. ASTM D-257, para. 3.5) In the resistance to ground mode, the test voltage is
removed from the outer electrode and is applied to ground via a test lead supplied with the
instrument. Current flowing between ground and the inner sensing electrode is detected and is
converted to a direct readout of the resistance of the intervening path in ohms.
CAUTION
DO NOT TOUCH ELECTRODES
1. Electrodes will become contaminated with skin oils.
2. When in operation, electrodes may be operating at a potential of 100 volts
and are capable of delivering an annoying shock to the person touching
them. Although the current capability is limited, a distinct hazard exists in the
person’s reaction to the shock.
BATTERY OPERATION:
Model 272A was designed to operate continuously for up to 8 hours on its internal battery when
fully charged. A battery symbol will flash in the display when the battery needs recharging. To
conserve battery power, the unit was shipped from the factory with its battery shutdown timer
set for five minutes. This means that after five minutes of use (or non-use) the instrument will
shut itself off. The timer is reset simply by pressing the ON-OFF switch. The default value may
also be changed in software in the SETUP mode (described later). Optional default values are:
5 min., 10 min., 30 min. and NONE. Model 272A may also be line operated regardless of the
state of its internal battery, although errors may be introduced by noise injected through the
charging supply in this mode. To determine the degree of error, check readings in both modes.
Battery operation will always produce the most reliable reading as long as the battery is
charged.
During charging, the charge rate is reduced to a “float” rate as battery capacity is reached. The
unit may be left on charge indefinitely. The battery is charged at a much lower rate when the
unit is operating. The time-out feature is disabled while the charger is plugged in.
OPERATING ENVIRONMENT:
Proper operation of this instrument requires attention to its operating environment. Its sensitive
circuitry is affected by uncontrolled static charges, ESD events and AC electric fields. Any of
these may produce erroneous or erratic results. Generally, an ESD controlled environment is a
preferred location for measurements.