Vacuum System Evacuation to Base Pressure

Abstract

SummaryA study has been made to compare the times taken to evacuate a turbomolecular and scroll pumped chamber to a base, high vacuum, pressure. A small test header (26.7 litre) was evacuated via two different paths: through a fore‐line, switching paths once ‘cross‐over’ pressure was reached, and directly through a turbomolecular pump (TMP) where the TMP was not operational until cross‐over pressure was reached. When starting the TMP/switching paths at 0.1 mbar, pump‐down times were significantly slower through the TMP; the time taken to reach 0.1 mbar increased by between 2 and 8 minutes dependent upon the TMP model (from a 24% increase in time with nEXT300 to a 232% increase for the nEXT240). For pressures below 8 · 10−6 mbar there was no significant difference in required pumpdown time between the two methods. The variation between methods being less than the ±30% uncertainty in pressure gauge measurement. The increase in pumpdown times to achieve pressures down to 10”5 mbar when evacuating through the TMP was seen for all TMP models tested, however a greater difference was seen for a TMP configured with Holweck drag stages than those containing Siegbahn drag stages. The best pumpdown time for pressures down to 10−5 mbar was achieved when pumping through the TMP whilst allowing the TMP to ‘start’ operation from atmospheric pressure.