We report measurements of the mechanical Q of a 32.7 kHz quartz tuning fork as a function of pressure for helium and argon at T = 300 K and for helium in the temperature range 7.0-0.7 K. In the low pressure ballistic regime, the damping due to the surrounding gas is inversely proportional to P, while for higher pressures, a hydrodynamic treatment accounts for most of the variation of Q with P. We have combined the ballistic and hydrodynamic models together with calculations of the thermal transpiration correction to correlate the tuning fork Q at low temperature with the pressure measured with a room temperature pressure gauge. The fork was found to be useful as an in situ pressure gauge for pressures above ～0.1 mTorr. A dissipation peak and frequency drop associated with the superfluid transition in the adsorbed helium film is also observed for T < 1.4 K.
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