  Recent helium shortages and helium price increases have lead to an increased
emphasis being placed on conserving helium. The need to conserve helium must be
balanced with need to maintain the high levels of purity necessary to prevent
operational problems caused by contamination. Helium losses and contamination
control are especially important for test stands that have cryogenic
distribution systems operating continuously with frequent changeover of
cryogenic temperature components that are being tested. This paper describes a
mathematical model to estimate the quantity of helium lost and the purity of
the helium after the pump and backfill procedure is complete. The process to
determine the optimal time during pump down to cut off pumping and start
backfilling is described. There is a tradeoff between trying to achieve the
lowest possible pressure during pumping and the quantity of air leaking into
the volume while pumping is occurring. An additional benefit of careful
selection of pump and backfill parameters in conjunction with real-time
pressure monitoring can reduce the labor and time required to complete a
successful pump and backfill procedure. This paper is intended to be a tool for
engineers to review their pump and backfill procedures and measured data to
optimize helium losses, system purity, and labor required.
