Vacuum suction pressures can be applied to Ariel cylinders.  The sensitivity of changes in suction pressure should be reviewed, as in the above Low Suction Pressure paragraph.  Additionally, the mean cylinder pressure should be maintained above atmospheric pressure.  If the mean cylinder pressure is less than 5 to 10 psig, air may be pulled into the cylinder and into the gas stream through the packing cases.   If operating below 5 to 10 psig mean cylinder pressure, a purged packing can be applied, using sweet gas for the purge gas. This provides process or fuel gas to be drawn into the cylinder across the packing, rather than air. Consider installing an oxygen sensor downstream of any negative suction pressure equipment.