As beer is drawn from the barrel a void will start to be created as no air can get into the barrel. The beer will continue to give off co2 which will fill this void but if the beer is drawn quicker than it can create co2 then a vacuum will be created and the pressure inside the barrel will drop off so you may need to add co2 to fill the void and maintain the pressure. How much co2 will be required depends on lots of factors but you can expect to need to top up co2 on a 40 pint batch around 2 or 3 times.

There are 2 ways that co2 can be added and your preferred way with determine what type of valve you require.

S30 - This system uses the larger C02 bottles which have a deposit included in the initial price. You then return the empty bottles to any home brew retailed who sells them and purchase a refill for a reduced cost. The refill is basically a new bottle but your deposit is maintained and therefore the bottle is much cheaper. These bottles work with the S30 valves and you add c02 by screwing the bottle onto the valve until the valve opens then add co2 for around 1 or 2 second then unscrew and remove. Repeat the process is more pressure is required.

Pin Valve - This system uses the small 8g co2 bulbs. These are dispensed by using a plastic bulb holder and then screwing this onto the valve which dispenses all the gas in the bulb. The bulb can then be thrown away and more bulbs can be added if more pressure is required. There is no deposit on these making them ideal if you don't want to return the bottles for refills.