how does one purge with C02?

I read an excerpt from Russian Rivers, Vinnie about dry hopping. He says to keep a blanket of C02 above the beer when dry hopping.

What would I need to get to do this. I don't have any kegging equipment so id have to buy the gas and whatever else.

Also, how is the purging actually done

Dry hopping is done in the final stages of fermentation. Hops put in the cold beer. At that point, probably 99% of the fermentation has been completed and there is very little CO2 being produced. Short of having a source of CO2, I'm not sure how it would be done. Maybe find a used CO2 type fire extinguisher and have it filled at the extinguisher company. Should be dirt cheap. I know I have a few sitting around from my old boating days. You'd need some sort of regulator or, at least, a way to attach a vinyl hose over the nozzle and just crack the valve on the cylinder.. just enough to bleed the gas into the FV (or secondary if you use one) CO2 is heavier than O2.. so it sinks to the top of the beer. Cover and you are good.

But, if you haven't really disturbed your beer in the FV.. you should be good to go w/o the gas.. as there should already be a blanket of gas on the brew.

Just my two cents

All you need to purge is a co2 tank/regulator and a hose. You just stick the hose in the carboy (above the wort) and turn the gas on for a few seconds. CO2 is heavier than oxygen so the oxygen is pushed out the top as the CO2 occupies the space below.

Your yeast produce enough CO2, even in secondary, to provide a proper layer.

CO2 is heavier than air, so as long as you are not physically moving the CO2 layer off the wort/beer and introducing oxygen to replace it, it's still there, even after you drop in some dry hops. No need to get fancy!!