A critical calculation that you need to know is how much helium is required to lift the balloon, and how high will it go? There are some rules of thumb you should keep in mind for calculating amount of gas you need.
- The lift force you need is 1.5 times the weight of the payload for a good flight. So a 500gm payload needs about 1.5Kg Force from the Helium. Too less lift, means that the balloon will go up and not burst for a long time. So it might travel quite far from the launch site, which is not desirable. It should be noted that the weight of the payload includes the weight of the balloon.
- 1 cubic feet of gas provides 29 g lift force. So in our previous example, if we need 1.5Kg force to lift a 500g payload, we need 1500/29 ~ 51 cu.ft of gas.
Helium gas can be quite expensive. Budgeting this is an important factor that affects the cost of the flight. So you probably should give some thought to how big a balloon you want to send up and how much Helium can you afford. There is a video from Stratostar that i
used to calculate how much Helium needs to be pumped into the balloon