Saramc,
This topic has come up several time in the past.
I honestly do not know where people get the idea that using inert gas is a great way to store wine in a container that has a lot of headspace. Whoever gave you this idea was dead wrong!
1) Just because you squirt in some argon, it does not mean that you have evacuated ALL of the air in the container. Unless you are working under clinical conditions, you will always have residual air in the container.
2) Gasses mix. It is not like oil and water! Inert gasses only reduces the amount of air exposure. Inert gas does not form any kind of protective barrier over your wine.
3) All gasses expand and contract with temperature changes. Temperature changes along with barameteric changes have the effect of having your container "Breathe". The headspace gas (over time) will escape to the outside through the path of least resistance. This could either be through the point of contact of the stopper and you container, or through a fermentation trap (if used). The oposite is also true, outside air will get into your container as well. In short, over time, your argon will go "bye-bye".
Inert gas should only be used when you either have a tight pressure seal (a stopper does not qualify) or only as a measure for the very short term (say no more than 2 weeks).
I know of a lot of batches that became oxidized simply because the winemaker was lured into a false sense of security. I have seen folks almost reduced to tears saying "I can't understand it, I used argon and it STILL became oxidized".
My advise is this, Keep the container full and save the money you would spend on inert gas and its equipment. Spend the mony on grapess instead! By keeping the container full, there is very little expansion and contraction that will occur, thus just a miniscule amount of "Breathing" occurs and your wine will be much, much, safer.
If topping up your container is not an option, just check out this thread...
http://www.winemakingtalk.com/forum/f3/carboy-air-space-bladder-trial-version-18124/