EasyManuals Logo

DATA PRECISION 938 User Manual

DATA PRECISION 938
21 pages
To Next Page IconTo Next Page
To Next Page IconTo Next Page
To Previous Page IconTo Previous Page
To Previous Page IconTo Previous Page
Page #8 background imageLoading...
Page #8 background image
42-5124
MODEL
938
3.5
MAKING
MEASUREMENTS
WITH
MODEL
938
CAUTION
A
maximum
excitation
voltage
of
2.8
volts
(peak)
is
applied
to
the
capacitor
under
test.
Observe
the
polarity
conventions
for
polarized
capacitors.
3.5.1
General
Follow
the
sequence
below
when
making
measurements.
1.
Select
the
range
switch
for
the
maximum
expected
capacitance.
2.
Turn
power
ON.
3.
Check
the
zero
display.
Before
connecting
the
capacitor
under
test
(but
after
inserting
any
test
leads
or
test
fixtures),
rotate
the
front-panel
thumbwheel
for
a
zero
display.
Note
that
the
maximum
offset
that
may
be
introduced
with
the
thumbwheel
is
about
20pF,
and
that
a
negative
value
is
possible
with
the
thumbwheel
use.
4.
Insert
the
capacitor
leads
into
the
spring
contact
connec
tors
(or
into
the
test
fixture,
or
connect
alligator
clips
to
the
capacitor
leads.)
Do
not
hold
the
capacitor.
5.
Read
the
display.
The
value
is
direct
reading
in
the
electrical
units
(picofarads,
nanofarads,
or
microfarads)
indicated
at
the
selected
range
switch.
If
the
three
less
significant
digits
are
blanked
(indicating
an
Out-of-Range
measurement),
select
the
next
higher
range
scale.
If
the
display
indicates
one
or
more
leading
zero
s,
shift
to
the
next
lower
range
scale
to
improve
the
resolution
of
the
measurement.
3.5.2
Powering
the
Model
938
a.
The
Model
938
is
normally
powered
from
a
standard
9V
carbon-zinc
battery
of
the
transistor
radio
type.
A
Data
Precision
Battery
Eliminator,
Model
BE9,
may
be
used
to
convert
the
in
strument
to
line
operation,
and
its
use
effectively
removes
the
battery
from
the
circuit.
b.
The
Model
938
incorporates
a
battery
(or
battery
eliminator)
voltage
sense
circuit.
A
low
voltage
output
from
these
power
sources
is
indicated
in
the
display
by
LO
BAT.
Appearance
of
the
LO
BAT
indication
when
using
the
battery
eliminator
is
in
dicative
of
a
low
ac
source
voltage.
When
using
internal
battery,
a
LO
BAT
display
indicates
a
nominal
battery
output
of
less
than
7V
and
that
approximately
20%
of
the
battery
life
remains.
The
3-6
MODEL
938
OPERATION
&
RECALIBRATION
battery
should
be
replaced
(or
shift
to
use
of
a
Battery
Eliminator)
as
soon
as
possible
after
the
appearance
of
the
LO
BAT
indication.
However,
in-spec
measurements
may
still
be
made
for
several
hours
after
the
LO
BAT
appears
before
the
in
strument
becomes
inaccurate.
A
weak
or
dead
battery
should
be
removed
promptly
to
prevent
possible
damage
because
of
leakage.
3.6
RECALIBRATING
THE
MODEL
938
3.6.1
General.
The
Model
938
requires
two
adjustments
to
obtain
in-spec
perfor
mance
on
all
range
scales.
One
of
the
adjustments
is
internal
gain
trimmer
R4.
It
calibrates
the
reference
voltage
that
is
developed
by
capacitor
discharge
for
the
dual
slope
integrating
A/D
converter.
The
second
adjustment
is
a
front-panel-
accessible
zero
potentiometer
R8,
by
which
up
to
20pF
offset
may
be
introduced
to
compensate
for
interal
and
lead
stray
cpacitances.
The
design
incorporates
sufficiently
high
quality,
precise
,
and
stable
components
to
assure
overall
specified
performance
with
these
two
adjustments.
The
Model
938
is
designed
to
remain
in
calibration
for
at
least
12
months.
Recalibration,
when
required,
should
be
performed
as
described
in
the
paragraphs
that
follow.
3.6.2
Access
to
Calibrating
Adjustment
R4.
Remove
the
back
cover
of
the
multimeter
and
locate
the
access
hole
in
the
inner
shield
cover.
See
Figure
3-2,
referenced
earlier
when
installing
the
battery.
The
adjusting
potentiometer
is
mounted
on
the
pcv
board,
and
its
shaft
is
accessible
through
the
hole
that
is
identified
by
the
label
R4
printed
on
the
shield
cover.
3.6.3
Recalibrating
Procedure.
(1)
Set
up
the
meter
with
the
lead
configuration
to
be
used
in
the
recalibration.
Insert
any
test
fixture
(Kelvin
clip,
e.g.)
or
con
nect
any
test
leads
to
be
used
later,
t
Remember
that
even
very
short
leads
may
have
a
significant
effect.)
(2)
Select
the
200pF
measurement
range.
(3)
Adjust
the
front-panel
control
R8
for
000
display.
Back
off
the
thumbwheel
position
one
half
turn
from
a
negative
display
(4)
Connect
a
standard
capacitor
whose
value
is
near
full
scale
on
one
of
the
higher
ranges,
and
is
known
to
within
±0.05%.
(5)
Select
the
appropriate
range
scale.
(6)
Adjust
R4
for
a
display
equal
to
the
known
capacitance.
(7)
Remove
the
standard
capacitor.
Replace
bottom
cover.
3-7

Questions and Answers:

Question and Answer IconNeed help?

Do you have a question about the DATA PRECISION 938 and is the answer not in the manual?

DATA PRECISION 938 Specifications

General IconGeneral
BrandDATA PRECISION
Model938
CategoryMeasuring Instruments
LanguageEnglish