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one device to another. The Zeeman frequency is set at the factory and should be
39949.0. The C field current should be between 12.0 and
12.2
milliamps
and
remain fairly constant. Again, the exact value will differ between standards.
The electron multiplier voltage is another very important parameter to watch.
There is
a
limit at 2553 volts.
USNO
does have
a
standard performing quite well
below the lo00 volt specification found in the 5071A owner's manual. (Its startup
voltage was about
440 volts,) Typically, our standards have
a
startup voltage
between 1150
and 1500.
The voltage will decrease slightly and then begin to rise
slowly during the first two years of operation. The total change will usually range
from 150 to 200 volts. The voltage should continue to increase slowly after this
initial startup period. The absolute value is characteristic of
the
device. The
standard should be performing well if the value is changing slowly and
CONSISTENTLY
during the entire life time of the CBT.
The signal gain should be constant at
1 4 . 4
percent as long as the standard is below
its maximum electron multiplier
voltage.
When the electron multiplier voltage
reaches its
limit, the signal gain will increase to keep the overall gain constant.
The cesium-beam oven voltage depends on ambient temperature. The
USNO
standards
values are consistent between 7.5 and 8.0 volts. Actual values differ
between
devices.
The
cesium-beam
tube error
must
be
small (+I- 0.1 volts) for
the
standard to
be.
operating properly.
The oscillator oven voltage typically has a value of -8.7 to
-8.8
volts during normal
operation. It should change by
no more than 0.1 volts throughout the life of the
device.
The ion pump current should have
a
low startup value (typically
near 0.0) and
r e m a i n
constant.
Some
devices, however, are working quite well with constant
values of 10,20, or higher.
(USNO
has
one device working well with
an ion
pump
current of 36.0 microamps.) This value is another one of those "personality"
characteristics of each device. A high current value can indicate a vacuum or
electrical leak, that the tube has been
off for
a
long period, or that the
tube is
contaminated. A current greater than 50 microamps will cause shutdown.
The hot wire ionizer voltage references the voltage across the ionizer ribbon. A
value of
1.0 is ideal and it should
be
between 0.9
and 1.1
volts.
The
mass spectrometer value will range f r o m 10.0 to 14.0 volts. It should remain
the same value during normal performance due to environmental changes.
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