Monitoring of gas composition for processes in the mTorr range is not possible with a conventional residual gas analyzer (RGA) unless a pressure reduction pumping system is used. A miniature sensor, having dimensions compatible with the mean-free path of gas molecules can operate directly at mTorr pressures while being less expensive and more compact than a conventional apparatus. This article describes a quadrupole RGA which is scaled down by a factor of 7 to operate at pressures up to 10 mTorr. This sensor requires different construction methods and drive electronics than a traditional RGA. A unique ion source with high output has been developed. The small sensor has resolution characteristics very similar to standard size models but sensitivity is reduced in proportion-to the ratio of cross-sectional area for each quadrupole r(0). Response is linear to the mTorr range and a correction method for gas scattering, based on pressure measured by a total pressure gauge included in the ion source design, extends the linear response to 10 mTorr.