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.
