The type of gas and pressure used to push the beer from the keg can differ depending on the type of beer and length of draught line. Depending on the beer, several blends of gas may be necessary to maintain proper carbonation and beer dispense rate, both of which are necessary to guarantee a good pour. Typically, CO2 (carbon dioxide) and N2 (nitrogen) are combined in different quantities and subjected to pressure while in contact with beer to allow the beer inside the keg to stay carbonated and fresh.
Each beer has a specific gravity value which should be approximately matched by the gas blend and pressure. When the content of CO2 is too high, a risk of over-carbonation occurs over time which causes foam at the tap. When content of CO2 is too low, under-carbonation occurs over time which leads to flat beer. When increasing or decreasing the pressure, flow rate at the faucet is also effected.

A qualified draught service technician can troubleshoot your system to determine which mixture and pressure is right for your beer profile.