As W1HLO prepares to launch its high altitude balloon (HAB), we need to determine if the weather balloon can lift our payload and how much helium is needed. We have a few 300 gram balloons in stock, so we will be using one of those. The "300 grams" refers to the weight of the balloon when deflated. HABs have a burst altitude – generally, the heavier the balloon the higher the burst altitude.
One cubic foot of helium will lift approximately 28 grams. So, if you have a payload that weighs 200 grams, adding the payload weight plus the balloon weight (200g + 300g) will give you 500g. Dividing 500g by 28g/cu. ft. (around 18 cu. ft.) will give you the cu. ft. of helium needed to keep the system in equilibrium. In other words, 18 cu. ft. of helium in this case will make the payload and balloon float in front of you and not rise! So, an extra amount of positive lift is needed by adding 100 grams into your equation. Instead of adding only the balloon weight plus the payload weight, also add in 100g so that the total weight is 600g. Then, 600g divided by 28g/cu. ft now equals ~22 cu ft. of helium needed to lift the balloon and payload.
The burst altitude varies from manufacturer to manufacturer, so be sure to check your balloon's specifications. To weigh your payload, including the parachute, tie wraps, paracord, batteries, etc., a postal scale that can measure grams will do just fine. Don't forget that you have to weigh everything that is being lifted PLUS the positive lift (100g) in order to caculate a correct amount of needed helium! Increase the positive lift to generate a quicker ascent. You can use the Balloon Performance Calculator to try different values of positive lift for determining the ascent rate and ascent time for certain baloons. Knowing the rough ascent time is beneficial if you have batteries for a device that will only last a couple of hours! As a rule of thumb, most folks divide the payload weight in half to determine the positive lift value. To recap, a good formula to use for determining the amount of helium you need is:
(payload weight + (payload weight / 2) + balloon weight) / 28 = helium needed in cu/ft