Now if you put the cap on the bottle, you can take your bicycle pump and pump up the bottle. Try it and make sure there are no air leaks. Have the cap a little loose so that there is no pressure in the bottle. Make a counterweight from another bent paperclip and a number of washers. Hook the bottle's paperclip over the beam and then slide it around and add or remove washers at the counterweight until the thing balances.
Balance is perfect when the tip of the pointer falls right along the plumbline.
Mark the bottle's position on the beam as precisely as you can. Tighten the cap and start pumping. Use a tire gauge to check every now and then, or if you are lucky, you have a bicycle pump with a pressure gauge attached. Normal air pressure is 15 psi, so if the gauge reads 15 psi, that means you have added 2 liters of extra air to your 2-liter bottle. At 45 psi, you have therefore added 6 liters of air to the bottle.