c. Failure of components. An open resistor, a shorted capacitor, the
windings on a coil open or shorted together, a transistor opens or shorts, a
burned out tube - the failure of any of these will cause a standard to
change value or malfunction. Sometimes it is obvious; sometimes it is only
detected by a cross-check or recalibration.
d. Changes in environment. Some components in standards are sealed and
contain special additive material for a drying effect; this is to keep out
moisture. Some equipment is not protected and care must be taken to keep it
dry. The secondary transfer teams are subject to varying weather conditions
and temperature changes. During the day with sunshine and all the equipment
operating, the relative humidity inside the van may exceed that of the
surrounding air. Then when the van interior cools at night condensation can
easily occur. A good tight van unfortunately will retain its humidity but
not its temperature.
This is especially true after rains, snows and
unusually warm days with high humidity. During situations like this, even
though it may be slightly uncomfortable, you may want to operate the air
conditioner to dry out the van interior or when operating in a very high
humidity area, you may have to leave some standards turned on over extended
periods of time to prevent dampness from affecting certain electronic
components.
If these components get wet or damp, and high voltage is
applied, the situation will cause arcing and, in some cases, will cause
damage to the equipment and injury to personnel.
Another example is when
you are operating in a cold, low humidity area you may be troubled with
static electricity.
Static electricity will cause fluctuations during
sensitive measurements and the calibration technician must be alert to
detect this situation. Performing a cross-check where static electricity is
present may give a false indication that a standard is malfunctioning. Make
sure your equipment is well grounded and dry.
e. Handling and misuse of standards.
Most calibration standards are
delicate instruments and must be handled with care.
Rough handling of
standards can cause components to change values and drift beyond their
assigned tolerance. This is another reason why cross-checks are performed
at periodic intervals.
The calibration specialist cannot prevent the
climate from changing or a component from aging and failing. He knows this
will happen over a period of time; that is why he cross-checks. More often
than need be, equipment failure can be traced to neglect by the calibration
specialist.
Applying too much voltage or current, making improper
connections, shorting an output, performing no maintenance, and overfusing
are a few ways the specialist may cause a standard to fail.
4.
BEFORE, DURING, AND AFTER CALIBRATION CROSS-CHECK PROCEDURES.
a. The secondary transfer standards are calibrated by the secondary
reference facility upon completion of a calibration cycle. The team chief
is responsible to assure that his equipment is cross-checked prior
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