Datron 1061 - Calibration; Introduction; General

sEcTloN
1
CALIBRATION
1.1
INTRODUCTION
1.1.1
General
The
purpose
of
calibration
ís
to
take
account
of
any
long-term
drifts
in
the
components of
the
instrument
and
to
restore
the
accuracy,
traceable to
a
known
standard.
The
períod
between
calibrations
depends
upon
the
accuracy
performance
required
from the
instrument
and for
guidance,
guaranteed
accuracies
ior
24
hours,
90 days
and
1
year
are
quoted.
The
calibration procedures
presented
in the
following
pages
should
cater
for
most
calibration
situations.
lf, how-
ever,
a special
problem
arises,
please
contact
our Customer
Service
Section.
1.1.2
The Essentials
for
Good
Calibration
Temperature
-
So
that
the instrument can
meet
its
specification
over
the
quoted
temperature range,
the temp-
erature
environment
should
be stabilised
at 23oC
+1oC.
ln
addition,
temperature
gradients
around the instrument
should
be
considered,
therefore
calibrate
the instrument
in
its
normal
operating
position
and
allow
plenty
of room
for
vent¡lat¡on.
Warm
up
-
lt is
essential
that the instrument
has
fully
temperature stabilised
if
the best
results
from
cali-
bration
are to be achieved.
Therefore,
at least
a 2 hour
warm-up
period
is
recommended
during
which
time
the
line
supply
or
the covers
should not
be removed
even
for
a
short
period.
ln addition, if the covers
have
been
removed,
make certain that
they
are correctly
fitted
and
that
the
leaf
contacts
to the
Earth and
Guard
Shields are
in
good
shape.
Calibration
Source
-
To
perform
a useful
calibration
the accuracy of the
source should
always
be
at
least
four
times that of the
instrument being
calibrated.
ln
most
cases, examples of
likely
sources
are
given
for
each
cali-
bration function,
With some
calibration
sources,
the
output
may take
several seconds
to
settle to a final value,
therefore
unless
a shorter
settling
time
is assured, a
period
of
l0
seconds
is
recommended
before
each
calibration
operation,
Guarding
-
lt
ís
preferable
to arrange
for
the
DVM
to
be calibrated
w¡th
'Local
Guard'selected.
Furthermore
to
arrange for
the 'Lo'terminal
of the
DVM
to remain
at
'earth'
throughout
and
let
the calibration
source
float.
lf a
'Remote
Guard'
connect¡on
ís
necessary
then
examples
are
shown
in
the Operating
Manual.
1.1.3
The'AUTOCAL,
process
1.1.3.1
General
The
Datron
'AUTOCAL,
process
means
that
complete
calibration
of
AC,
DC,
Ohms
and
Current
on
every range
can
be carried
out
from
the
instruments
own
front
panel.
ln the
process,
an
internal
non-volatile
memory
stores
calibration
constants
for
each
function
and
range
as deter-
mined
when
the
instrument
takes
a
series
of
l6 readings of
the applied
calibration
source.
lnternally,
each
of
the
readings
is
deviated
by
one
sixteenth
of a digit and when
an
average
is
taken,
the instrument
is
able to resolve to
better than
one least
significant
digit
displayed.
Access
to the
non-volatile
metnory is
gained
using
a key inserted
into
the
rear
panel.
When calibratíon
is
complete,
the
key is
removed,
therefore
preventing
acci-
dental
or unauthor¡sed
use
of the
calibration
rout¡ne.
1.1.3.2 Procedure
Outline
e
-
select the
'FUNCTION',
and
,RANGE'to
be cali-
brated
and cancel
any'MODE'
or'COMPUTE'
buttons.
lnsert
rhe key into
the
'CALIBRATE
ENABLE'
keyswitch
on the rear
panel
and turn to
the
'CAL'
position.
(The
'cal'
legend will be
displayed
on the
front
panel.)
lf the
instrument
is fitted
with
Option 50
IEEE
Bus,
set
the rear
panel
address switch
to 31 i.e.
all 1's.
-
Connect
the calibration source to
the
¡nput
terminals
and
operate the keys
shown in the tables
ín
the following
pages.
When
a
'CALIBRATE'
button
is operated,
its
assoc-
iated
L.E.D. indicator
will light and extinguish
when
the
calibration
operat¡on
is executed.
-
When
all calibratíon
is
complete turn
the
keyswitch
to
'RUN'and
remove
the
key.
1.1.3.3 The
Five'AUTOCAL'
Keys
'Zero'
-
This
takes
account of
offsets in
the instrument
and in
the calibratíon
source.
'Gain'
-
This
sets
a scaling
factor
for
each
range
and
function.
'lb'
-
This
nulls
the input bias
current
of
the
DC
voltage
measurement
circuits
to around
1OpA.
Therefore
it
only has a significant
effect
on
the
low
DC voltage
ranges
and high resistance
Ohms
ranges.
lt can
be
operated
as
often as required and
independently
of other
calibration
operations.
lt
will be
seen
that
successive
operations
of
'Ib'
approach
the
final nulled
value of
current
iteratively.