Section IV Model 34SSA
± (0.003%
+
0.001%)
=
±
0.004% of reading
Similarly, at one tenth of full scale (O.I V) the least signifi-
cant digit (10
microvolt)
is equal to 0.01% of reading so the
accuracy
specification is:
± (0.003%
+
0.01%)
*
i
0.013%
of
reading
These specifications do not include the temperature coeffi-
cient that must be added if the instrument is operated out-
side of the 22°C to 24°C range.
4-17.
DC ACCURACY TEST
CONSIDERATIONS.
4-18.
Because of the high dc accuracy of the 34SSA, a
precision dc calibration standard is required to
verify that
it
meets its dc accuracy specifications. To thoroughly test
the performance on all ranges, the standard must be
capable
of
delivering outputs within the range of 0.10000 V dc to
1000.000 V dc. The accuracy of the standard must be
such
that its errors do not introduce significant
uncertainties
in
the 34S5A
test
readings. Ideally, the
accuracy
of the
stand-
ard should be ten times better than the 34S5A specifica-
tions being tested
—
a ten to one error reduction nearly
eliminates measurement uncertainties caused
by
the stand-
ard. To test accuracy specifications on the order of
±
O.00S% of reading, however,
a
standard with a specified
accuracy
of
t
0.0005% (S ppm) would be required. Since
this type of accuracy,
over
the range needed to completely
test the accuracy of the 34SSA, is generally not
available
outside of a standards laboratory, some compromises may
be required. If you
have
access to primary in-house (NBS
certified) standards or
have
calibrated transfer standards
that are capable
of
delivering the required output voltages,
we recommend that you use them. If you do not have
access
to
such
facilities you may. depending on your specif-
ic
accuracy requirements,
choose to do
one
of the follow-
ing:
a. Use a dc
calibration standard that is four or five
times more accurate than the 34SSA specifications to
be tested. (A discussion
of
the potential uncertainties is
given
in
following
paragraphs.)
b. Use a highly stable calibrated
standard and add the
correction factors (given
on
the calibration chart) to the
34SSA test readings.
c.
Send the 3455A to an -hp-
Service
Center
or some
other NBS-certified standards facility for
calibration.
4-19. Several
of today’s
commercially available
dc
calibra-
tion standards
provide
the output
voltage
range and
resolu-
tion
needed
to test the
performance
of
the 34S5A but they
are not, in general,
an
order
of
magnitude more accurate
than the 34SSA.
When
using such standards it is important
to be aware of the uncertainties or “ambiguities” that may
be encountered. These potential
ambiguities are described
in the following paragraphs.
4-20.
First, consider the case where a digital voltmeter
(DVM) is to be tested for
a
full-scale
accuracy of
±
0.01%
of reading on its 1-volt range. Ute DVM
is cormected to a
dc calibration standard u^ose specified accuracy
is
±
0.001%
of setting and with the standard set
to
-t-
1.00000 V,
the DVM
reads
+0.99992
V which is 0.008% low. The
dc
standard's specified
accuracy
is
ten times better than the
specification being tested and at 1 V its maximum error
contribution
to the
DVM
reading is 10
microvolt
or
0.001%. If the standard is 0.001% low the actual DVM
error is
•
0.007%; if it is 0.001% high, the actual DVM error
is
-
0.009%.
In
either
case the
DVM
is within its specifica-
tion
and, since
this measurement
is not a calibration but is
only
intended
to
verify
that the
DVM meets
its specifica-
tion, the standard’s
error can be ignored.
4-21. But what
if
the
DVM
reading is
+ 0.999908 V? Here,
the
DVM appears
to be in
tolerance (0.0092% low) but
the
margin
is
only 0.0008% which is
less
than
the 0.001% maxi-
mum
allowable error contribution of the standard. If the
standard’s output is 0.001%
low,
the
actual DVM error is
-
0.0082% rather than
•
0.0092% so the
DVM
is within its
specification. If. on the other hand, the standard's
output
is
0.001% high, the actual
DVM
error is
-
0.102% and the
DVM
is
slightly out of tolerance.
Chances
are good
that
the
DVM
is within its specification
but
the only
way
to
tell for
sure is to
use
a more
accurate standard. As the example
points out, there are regions of ambiguity
even when
the
standard is ten times
more accurate than the instrument
being tested.
With
a
ten-to-one error reduction,
however,
these
regions
are
relatively narrow. In this
case,
the
DVM
could
be
out of tolerance but if so, its
maximum out-of-
tolerance
error is only
-
0.0002%. As long as the
DVM
reading
is within specified tolerances, the maximum
DVM
error that can exist is
± 0.011%
which
is
the
sum of the
maximum DVM error and the maximum allowable error
of
the standard. A potential
deviation
of
±
0.001% from the
DVM
specifications could, in
many cases,
be
acceptable.
Also, if the
standard has been recently calibrated and is
known
to be
well within its specification, readings in the
narrow ambiguous regions may reflect marginal DVM per-
formance or indicate the need for adjustment.
4-22. Now
suppose the dc standard's specified accuracy is
± 0.0025%
—
only four times better than
the
1
0.01%
DVM
accuracy specification.
If the
DVM
reading is
+ 0.999890
volt, it appears
that
the DVM is 0.011% low. However,
if
the dc standard is 0.002% low (well within its specification)
the
DVM
is only 0.009% low and is in tolerance. Con-
versely,
if the
DVM
reading is
+
1.00081 V the DVM
appears to be 0.0081% high and well within its specifica-
tion. But if the standard is
0.0023% low,
the actual
DVM
error is + 0.014%
and the
DVM
is out of tolerance.
4-23.
Figure
4-1
shows how the
error tolerances of the
standard combine with those
of the
DVM
to
produce
the
positive
and
negative ambiguous regions
used in the pre-
ceding examples. From Figure 4-1, the following observa-
tions can be made:
a.
If the DVM reading is
in
tolerance
by a
percentage
that is greater than the
maximum
allowable error of the
standard, the DVM is definitely within its specification.
4-2
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