[time-nuts] 5065A cell flooding

[corby d dawson]

Hi,

First take a look at the leapsecond.com website. Click on various lab
reports and then rb-cells.

You will see pictures of rubidium cells from various instruments.

Take a look at the 5065A cell. 

You will see a long narrow neck coming out of the top. This neck exits
the microwave cavity and the tip of it is RTVd into the cold end of the
TED (thermoelectric device). Actually just a very tiny thermoelectric
cooler like the ones you plug into your cars cigarette lighter.

Now the normal state of affairs is that when initially built the rubidium
in the cell is all in the tip of the neck.

When the unit is warmed up not all the rubidium exits into the cell as
the tip is held slightly cooler by around 250ma thru the cooler.

When the unit is turned off the tip cools slightly faster than the cell
and the rubidium is collected back into the neck.

So as long as the unit is operating or not turned off for years at a time
you are OK.

Now say that the unit is in storage for years. Very slowly at ambient
temperatures ALL the rubidium will effuse out into the cell and coat the
walls of the tube with a thin coating of rubidium. When you turn the unit
on and warm it up there is way too much rubidium in the cell and it
absorbs almost all the light at the rubidium line frequency. 

This is cell flooding. In this situation the TED and or turning the unit
back off will not draw enough rubidium back into the tip for proper
operation.

Performing the procedure in the book passes 1 amp thru the TED cooling it
much more than normally. This causes the rubidium to be drawn back into
the neck at a faster rate. It will usually take 6 days to fully
"de-flood" the cell. If your signal does not improve after the 6days then
cell flooding was not the problem.

I usually find it safer to use a 5VDC supply thru a 1 ohm 10 watt
resistor to supply the current rather than a constant current supply at
such a small voltage. You do not want to burn out the TED!

Hope this helps!

Best regards,

Corby Dawson