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Verification
Report
4708
RS
1
Model 4708
Serial
Number
Calibration
Interval
—
SOdays—...
Specification
---
Accuracy
—
Date
Checked
by
Company/Dept
Note:
On
receipt
of
the instrument
it
is
recommended
to
check
at
the
values
shown
in
the
tables.
The
'Validity
Tolerance'
Why
is
it
necessary
to
calculate
this
tolerance?
It
is
impossible
to verify
the
specification of
an
instrument
with
absolute
certainty,
even
using the
original calibration
equipment
to
make
measurements.
All
measurements
carry
a degree
of uncertainty,
this
being
quantified
by
the
traceability of
the
measuring equipment.
The measurements
attempt
to verify that
the instrument
performs
within
its
specification;
ie.
it
operates
within
the tolerance
of
its
accumulated
uncertainties.
But as
the
measurement
itself
has
its
own
accumulated
uncertainties,
these
must be added
to
those
of
the
instrument
in
order
to
set
a
Validity
Tolerance'.
If
an
instrument performs
within
its
validity
tolerances,
all
that
can be
assumed
is
that
its
verification
is
uncertain.
For example;
if
results
agree
exactly with the instrument
settings,
the
measurements
are
still
at least
as
uncertain
as
the
traceability
(to
absolute accuracy)
of
the
measuring equipment.
The
only
certainty
is
that
if
one
result
exceeds
its
validity
tolerance,
then the instrument
has
failed to
verify.
For
a
verification
measurement
to
be
acceptable,
therefore,
each
validity
tolerance
must
express a continuous
trace of
all
the uncertain-
ties
from
the instrument
terminals, via
'Absolute Accuracy',
to the
measuring equipment
connections
to
those
terminals.
The
validity
tolerance
of
the trace
is
obtained by adding together
all
the intervening
uncertainties
at
the time the
measurement
is
made.
The
specification
sets out the
worst-case allowances
(relative
tolerances)
for
the
instru-
ment
performance. For
the
measuring equipment,
worst-case
toler-
ances must
also
be assumed. The
full
extent
of
the
accumulation
is
illustrated
below
in
the simplified
diagram;
Implementation
on
Receipt
of
Instrument
The
tables
in
this
report
document
provide
columns
to
enter both the
relevant
results of
measurements and
results of
calculations
from
the
measurements. Guidance
is
given
in
the
form
of
calculation
equations,
where
to
obtain information,
and
tables
to simplify
the
calculations.
Wherever
appropriate
and
possible, the figures
in
the
columns
are
already entered (90
day
Specification).
The
recommended
methods
of
setting
up
the
equipment,
and
measuring
the
instrument
outputs,
are
described
in
the
body
of
Section
7.
Implementation
after
User-calibration
Once
the
instrument
has been
calibrated
against the
user's standards,
then Datron's calibration-standard
uncertainties
can be
removed
from
the
validity
tolerance
calculation.
It
is
still
necessary
to
include the
worst-case
user's calibration-stan-
dard
uncertainties
for
the time period
elapsed
since the instrument
was
last verified
or
calibrated.
It
is
assumed
that
users
will
wish
to
employ
their
own
standard
report
formats
on
these occasions.
4708
RS
1-1
7-30
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