HP 3478A, Техническое руководство

Страница: 37 / 150

TM 11-6625-3071-14
3478A
lions, listed in Table 1-1 and Table 4-1, are the
performance standards or limits against which the 3478A
can be tested. Table 1-1 also lists some supplemental
characteristics of the 3478A and should be considered
as additional information.
4-34. Any changes in the specifications due to
manufacturing changes, design, or traceability to the
National Bureau of Standards will be covered in a
manual change supplement or revised manual. The
specifications listed here supercede any previously
published.
4-35. Specification Breakdown
4-36. The 3478A’s specifications are grouped according
to instrument function (AC Volts, DC Volts, etc.). Within
each group are either one, two, or three main sets of
specifications: the 24 hour, 90 day, and 1 year limits.
The limits to which the 3478A conforms depends on
when the instrument was last calibrated or to which it
was certified, and the instrument function. The 24 hour
limits should only be used if the instrument was
calibrated within the last 24 hours, otherwise, the 90 day
or 1 year limits apply.
4-37. Each set of specifications includes an accuracy
specification for each voltage, ohms, and current range.
They are specified as a percentage of the reading and
an add-on of a certain number of counts. For example,
the 24 hour full scale DC Volts Function accuracy on the
30V Range (in the 5 Digit Display Mode) is:
±
.005% of r 3 counts
giving a full scale accuracy of +.005% or (5 counts) plus
3 counts (or .003%0), which is a total of 8 counts (or +
.008%). (This is only true at full scale and changes at
1/10 scale, see next paragraph.) If for example the 4
Digit Display Mode is selected, the percentage is the
same (.005%o), but the number of counts is different and
changes the total percentage to + .015% (.005% plus 1
count in the 4 Digit Mode is .01%o which makes the total
equal .015%).
4-38. The number of counts also changes the accuracy
of the 3478A at 1/10 scale. For example, the percentage
(same Function, Range, and Digit Mode) at 1/10 scale is
still .005%o at 1/10 scale. However, the number of
counts (3) is .03% at 1/10 scale. This gives a total of
.035%, rather than .008% at full scale.
4-39. Temperature Coefficient (Reference
Temperature)
4-40. The temperature in which the 3478A was last
calibrated is called its Reference Temperature. To
maintain the 3478A’s specified accuracy, the multimeter
must be operated within the specified range of that
temperature (shown in Table 1-1). If the 3478A is tested
or operated outside the temperature range, the
Temperature Coefficient (listed in Table 1-1) must be
added to the instrument’s specifications. It is
recommended to calibrate the 3478A in the temperature
in which it is to be tested and operated.
4-41. TEST CONSIDERATION
4-42. General
4-43. Because the 3478A is able to make highly
accurate dc measurements, certain requirements have
to be met. For example, the Digital Voltmeter that is used
to test and calibrate the 3478A should be good enough
so that its errors do not introduce any significant
uncertainties in the Performance Test and Calibration. A
voltmeter which is 10 times better than the 3478A nearly
eliminates the uncertainties. Since voltmeters (or even
standards) with these accuracies are not readily
available, use the recommended Digital Voltmeter. Make
sure the voltmeter has been calibrated to its 24 hour
specifications and used within 24 hours after calibration,
before testing and calibrating the 3478A.
4-44. Ambiguous Region
4-45. Since the available test equipment is not an order
of magnitude better than the 3478A, it is important to be
aware of the uncertainties or "ambiguities". An example
is in the next paragraph.
4-46. A hypothetical case is to check the 3478A’s 3V full
scale dc accuracy with a certain standard. The 3478A’s
90 day specification is -.01% with the standard’s
accuracy ideally .001% (ten times better). If the 3478A
reading is "30.0033" (.011% high), the multimeter may or
may not meet its 90 day limit (dependent on the
standard’s accuracy). If the standard is .001% high
(30.0003V), the 3478A’s actual reading is "30.0030"
which is in the 90 day limit. If the standard output is right
on, the 3478A reading is "30.0033" (its reading is high
and out of tolerance). Although in both instances the
standard is within its limits, it may show the 3478A to be
in or out of its limits. This creates an Ambiguous Region,
shown in Figure 4-1. The region gets bigger when the
3478A limits are tighter and/or the standard’s
specifications are less accurate. The best test is when
you know your standard’s actual limits.
Figure 4-1. Ambiguous Regions
4-4