Density Sensor for SF6 and other gases The Trafag gas density sensor type 8774 was specifically designed for monitoring insulation gases. The gas density is measured with an oscillating quartz with excellent long term stability. The constant resonant frequency of a quartz oscillator under vacuum is compared with the resonant frequency of an identical quartz situated in the sample gas. The difference in the resonant frequency is proportional to the density of the sample gas. The measured density is proportional to the pulse frequency or to the mA output. With the digital variant additionally the temperature can be measured over the pulse width of the frequency signal.