There is supposed to be air at the top of the vape cart. the cart is actually 1.1ml when completely full. When the cartridge is made, the oil is poured to the top of the glass tank and capped quickly to prevent leaking. Over time the oil gradually soaks into the coil area creating space at the top of the tank. This can be as much as 20% of the total amount, which can make tanks appear partly unfilled while actually containing a full amount of product.