Fill stations are designed to fill your constant air (C.A.) bottles (3 1/2 oz., 7 oz., 10 oz. and 20 oz. bottles) with liquid carbon dioxide (CO2). Please note that the CO2 is liquid; the bottles are designed to hold the designated amount in liquid form.
If you do not use a bulk tank with a siphon tube, you will probably transfer gas CO2, and this will change the weight of the tank. The bottles are designed to accommodate approximately 65% of their volume in liquid CO2. The remaining space is unfilled, allowing for the expansion of the CO2 as temperature convert into gas.