Calibration; Introduction; General - Datron 1061 Calibration And Servicing Handbook

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.