Datron 4708 User Handbook Manual page 112

Uncertainty
and
Traceability
Appendix
2
to:
^ ^
4708
User's
Handbook
Section 7
Cumulative Tolerances
Verification Uncertainties
If
an instrument
was
correctly calibrated against the factory
Each
element
in the calibration
traceability
chain (on the next
standard
at its
uncertainty
limit,
and
then verified against a
page)
contributes
its
uncertainty
to
influence the overall
user's standard, also at
its
limit; there are
two
extremes
to the
verification tolerance limits. All uncertainties
must be
ac-
range of
traceable
results
which
could be
obtained.
If,
for
counted
for
when
calculating the
total
tolerances,
example, both
standards' traceable errors
were
equal
and
in
the
same
sense, the
instrument
would
appear
to verify as
In addition,
if
two
separate
systems
are used,
one
for calibra-
absolutely accurate.
But
if
the errors
were
in opposite sense,
tion
and
the other for verification, then the
cumulative
total
it
could appear
to
be
inaccurate
by
the
sum
of
the
two
limits
tolerance ('Validity Tolerance')
is
the
sum
of those
estab-
of
uncertainty.
lished for
each system
(as
described
earlier).
In the following numerical example, a 4708
is
verified in the
Thus on
receipt
of
the 4708, Datron's uncertainties
must be
factory
at
lOV,
on
the
lOV
DC
Range, and
with
Oppm
error
included in the
total
tolerance
limits;
but
when
verifying
against a
5ppm-high
standard
(relative to
Absolute
Accu- against the
same
Standards setup used
to calibrate the instru-
racy):
ment, they are excluded.
Two
formulae
for calculating the
total
tolerance
limits are
given
for
each
procedure, covering
the
two
types of
verifica-
tion
occasions mentioned.
Validity
Tolerance
Limit Calculations
The 4707
is
verified
by comparing
its
output with
the Validity
(Total)
Tolerance
Limits.
These
are calculated
by
summing
the appropriate uncertainties
from
the traceability chain,
and
expressing the
result in
upper and lower
absolute deviations
from
the chosen
verification value.
The 4708
checks
out
if
its
measured
output
is
between
the limits.
Where
possible, the
4708
specification
and Datron
calibra-
tion uncertainties are
given
in
a form
suitable for calculation
(see
Report Sheet RSI).
Where
this is
not
possible they can
be assembled from
the specifications
in
Section
6,
and
in
some
cases
can be
more
easily
obtained using Spec
Mode
(refer to
Appendix
1).
User's uncertainties
need
to
be assembled and
included,
expressed
in
the
form
appropriate
to
the calculations.
Whereas
Datron's uncertainties are normally included
in the
calculations
only once
(on
receipt), user's uncertainties
must
always be
included.
So
it is
sensible
to
provide a
permanent
record,
eg on Report
Sheets.
Once
the relevant uncertainties
have been
entered
on
the
Report
Sheet, the Validity Tolerance Limits can be
calcu-
lated.
The
correct calculations are identified in the individual
procedures.
It
remains
correctly calibrated,
and
could be
delivered
to
one
of
two
users:
one
user's standard
is
5ppm
higher than
Abso-
lute,
and
the other's
is
5ppm
lower.
Despite the instrument
sustaining
its
original
accuracy of
+5ppm,
and
the standards
all
being within
5ppm
of Absolute;
the
first
user
would
verify the
4708
as
having
Oppm
error,
but
the
second
would
obtain
an
error
of
+10ppm.
The
increased uncertainty
is
unavoidable
unless the
same
standard
is
used
for
each
verification.
This
is
clearly not a
practical
proposition following
delivery.
But
after
the
first
autocalibration against the
user's
standard, Datron's
calibra-
tion
uncertainty
no
longer
applies.
DCStd
4708
+5ppm
+5ppm
7-A2-1