[glend (Glen)]

Chrism any dry gas will do, as long as it will not transport moisture, sso not CO2. I would check the sealing rings. That should not happen.
I do use Argon gas when i reassemble the top but that is only because i have it on hand. I have used it for years to exclude moisture inside my cold finger cooled DSLRs ( which i Argon bag). With the ASI cooled cameras, there is a silicon ring inside the top which the cap screws onto and presses to create a seal. There is also a sealing ring under the the sensor cap window. When you change the dessicant pills you want to do it in a dry room if possible, but really the new pills should absorb any moisture that gets inside the cap - provided you have a good seal.
My technique, and this is just my obsessive way, is to change the pills with the camera in my argon box, that way no moisture can be sealed inside when you assemble. An Argon box is simply a small plastic storage box to hold the argon. Argon is heavier that air, so it will sit inside a box. If you buy a bottle at Bunning you will need a valve/regulor which you can get on ebay. BTW nitrogen is not heavier than air obviously. I would not recommend trying to blow gas into the cap area as you may blow dust onto the sensor, a box full of gas avoids that. Do not breathe Argon.

You don't have to use Argon and i am pretty sure they don't do that in the factory.

BTW ZWO now have a cover glass heater that attaches to the top of the camera and encircles the sensor window, it is powered by a Y cable and keeps the sensor window from fogging on the outside. It should not fog on the inside due to the pills. The heater cost $19 from the ZWO website.