[time-nuts] HP cesium beam tube life

[cdelect at juno.com]

The "shelf life" referred to is only related to tube vacuum without
pumping occasionally.

It was derived from an estimate of how badly a worst case tube would
leak.

Most tubes have an almost perfect vacuum seal and this "shelf life" is
indefinite.

Leaving a 5061A/B on and in the Cs-off mode keeps the ion pump on. 

The actual life of a continuously working tube is conservatively
estimated by the warranty lengths HP assigned to the various tube styles.

Cheers,

Corby