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Softening the Vacuum with He


\begin{shaded}
\begin{itemize}
\item {\bf How Long:} $\sim$24 hours.
\item {\bf Thermal Shock:} Moderate.
\end{itemize}\end{shaded}
The previous section softened the vacuum to atmospheric pressure, keeping $ P_{\rm OVC}$ constant. Alternatively, you can keep the mass of gas in the OVC constant, by softening with He. This will slow the warm up and result in less of a thermal shock to the cold mass.

To do this:

  1. Wait until all parts of the fridge are warmer than 5K. To prevent condensation of He.
  2. Adjust the pressure regulator of a He gas cylinder so that the absolute pressure is just slightly higher than 1 atm (the regulator will read $ \sim$ .1 bar). And adjust the flow rate to be very low.
  3. Purge the line with He, and then attach the line to the OVC valve.
  4. Very slowly, crack the OVC valve open.
  5. Leak He into the OVC until the pressure reads $ \sim$ 20 mbar. Then close the OVC valve.
  6. Disattach the He bottle.
  7. As the cold mass warms, the gas will slowly expand. Monitor the OVC pressure to ensure that it does not rise above 500 mbar.


next up previous contents
Next: Circulating Warm Mixture Up: Speeding Up The Warm Previous: Softening The Vacuum with   Contents
Jonathan Ouellet 2014-08-26