  To achieve an uncertainty of 0.25 mK in single-pressure refractive-index gas
thermometry (SPRIGT), the relative pressure variation of He-4 gas in the range
30 kPa to 90 kPa, should not exceed 4 ppm (k=1). To this end, a novel pressure
control system has been developed. It consists of two main parts: a piston
gauge to control the pressure, and a home-made gas compensation system to
supplement the micro-leak of the piston gauge. In addition, to maintain the
piston at constant height, a servo loop is used that automatically determines
in real time the amount of extra gas required. At room temperature, the
standard deviations of the stabilized pressure are 3.0 mPa at 30 kPa, 4.5 mPa
at 60 kPa and 2 mPa at 90 kPa. For the temperature region 5 K-25 K used for
SPRIGT in the present work, the relative pressure stability is better than 0.16
ppm i.e. 25 times better than required. Moreover, the same pressure
stabilization system is readily transposable to other primary gas thermometers.
