[time-nuts] COTS cesium standard physics package life

[Ruslan Nabioullin]

Hi, I'm unable to fully grasp the lifetime specifications of typical 
COTS cesium standard physics packages, for both old and new designs. 
For example, the documentation of an older HP standard specifies a shelf 
life of two years if the ion pump is operated two or three times per 
year, yet at the same time the ``beam tube warranty'' period is three 
years.  Does this mean that if the physics package is fully operating 
(i.e., LOOP OPEN, OPER, or LTC), then it's guaranteed to last at least 
three years, but if it's completely shut down, except for vacuum 
maintenance sessions (using the mode CS OFF) two or three times per 
year, it will last only two years?  Also, what will the expected 
lifetime of the package be if the standard is kept as a reserve, 
specifically, continuously-powered with the mode set to CS OFF (so 
continuous vacuum maintenance), and activated or deactivated to or from, 
resp., OPER, as needed, excl. the time spent fully operating (i.e., in 
this case, the time spent in the OPER mode doesn't count)?

Thanks in advance,
Ruslan
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