The balloons available that can circumnavigate the world are limited. The regular party balloons are unlikely to succeed. The balloon needs to be at least a reasonable volume to lift a payload to a desired altitude and not so big that the pressures at float burst the balloon. Most amateurs choose one of these two balloons:
The SBS-13 from Scientific Balloon Solutions. The cost is around $175 delivered. The company provides instructions in their literature that helps a lot. Experience with these balloons shows a good “free lift” is between 6 and 7 grams. The volume of this balloon is about .5 cubic meters if you wish to run the predictions of float1g. You can just give it 6 grams of free lift and let it go.
The clear chinese balloons bought from AliExpress are around $1.50 each but the quality is such that you need to prestretch the balloons and to hold them at pressure for some time to see if pinhole leaks show up or if the seams fail. AliExpress has lots of balloons and most do not work. One link that is the correct balloon is https://www.aliexpress.com/item/32803586452.html?spm=a2g0s.90423184.108.40.206bf4c4dL9KsTW Avoid any that say pvc or latex.
There are lots of different ways to work though the process of preparation and a lot can be learned by taking the time to read the postings on picoballoon.io. The most successful ham to use these is VE3KCL who has had multiple circumnaviations.
The balloons come flat with a diameter of 36”. It has an inlet neck and sometimes a sealed tab at the top. Here is a photo of one balloon.
The balloon needs to be inflated to stretch it and turn it into a normal shape. If not stretched, the balloon will fly but the altitude will be too low for a long flight. You may also have a defect in the film that will cause rapid failure.
Stretching the balloon is a simple matter but requires an aquarium air pump, a pressure gauge, some kind of pressure control and some space. Here is a photo of my test facility.
The items needed are:
an aquarium air pump.
A pressure gauge for hvac use.
A bubbler tube for pressure control
Aquarium air tubing and fittings
plastic or glass tubes for each balloon connection
They are all available on eBay.
The balloons need to pressurized to at least .5 psi. They tend to fail at around .6 psi or slightly over but will be fully stretched at around .5 or .55 psi. Holding the balloons at a pressure for a week or two is reasonable as pinhole leaks do show up after a while. If there is a leak, the bubbler will generally stop bubbling and the pressure will have dropped. The leaker can be found by closing off one balloon at a time and watching to see if the pressure rises. When the pressure starts to rise, that was the leaker. The leaks will be small and can generally be found by bringing your face close to the balloon and feeling the gentle breeze. Put a new balloon in it’s place and wait. You will probably want two balloons so test three to save yourself time. It is not a bad idea to burst a few balloons on the ground to see how much pressure your batch might stand. It varies.
The balloon volume generally will reach .17 cubic meters. If you want to validate this, measure the diameter across the balloon and the height of the balloon. Assume the balloon shape is an oblate spheroid and calculate the volume. You will need this number to estimate where it will float and what the superpressure will be. The measurements are simple if you get two boxes and a yard stick. Here is how I do it:
The volume formula is V=4Pi/3*a^2*b
The balloons will still be a bit stretchy and if you really want to get into the weeds, you can deflate the balloon and then reinflate slowly, measuring at different pressures to see the volume to expect. Playing with the float1g spreadsheet for volume at pressure will probably get you closer to what you will see.
If you would like to really get into the weeds and determine volume, take a camera and photograph the inflated balloon from the side so the seam is straight across. Print the photograph and draw a line along the seam. Find the middle and draw a perpindiclar line . Draw perpendicular lines at, say, 10 or so places to find the height of the balloon. The dimensions on the photo are scaled dimensions of the balloon since you measured it’s height and diameter already. Make the adjustment and calculate the volume as concentric annular volumes. Add them together to get the total. Don’t bother with the formula for the mylar balloon, that is great mathematics but restricted to a film that does not stretch.
Once you know the volume of the balloon, you will need the weight of your balloon and the weight of the payload plus antenna. You are now ready to estimate the float altitude. There is a marvelous spreadsheet at https://ukhas.org.uk/projects:splat. Get the latest version. At present that is float1g. That link explains all the math.
This predictor assumes ideal gas behavior and has a lookup table for standard atmospheric conditions. Download the spreadsheet and try it out.
Put in the volume of the balloon or balloons, the weight of the balloons, the weight of the payload including the antenna, and the desired free lift. The spreadsheet will do the math. It will show the lift from the balloon alone to use for fill and it will also predict the altitude and superpressure to expect. There are other numbers generated but they have a very subtle influence and can be studied later.
At this time, you should know about what pressure will burst the balloon and you can see if you are safely below that pressure. I would assume failure at .5 psi just to have a number but since we have no way of testing balloons at actual temperature and pressure, choose what you like for the limit. VE3KCL sends balloons up expecting a pressure of .5 to .6 psi. He has been very successful so it is worth considering higher pressures. After you have launched a few balloons, you might want to see how close to the burst pressure you can get.
Extra gas will not get you any significant extra altitude. What you get is a longer possible life. Gas will diffuse through the plastic and eventually the balloon will come down. There is a possibility that the balloon might stretch a bit and gain altitude as well. As the gas diffuses out, you might see a slight increase in altitude as it will weigh less. It is most likely that some weather condition will bring the balloon down before that happens but a guy can hope.
Filling the balloon is the next step. I suggest hydrogen as it has better lift and costs less. It has the risk of combustion so safety should be part of the filling process. No smoking or playing with matches. The amount of gas will not completely inflate the balloon. It will be a floppy bag. The spreadsheet will give you the lift your balloon should have on the ground with no payload attached. Make sure you have gotten all the air out of the balloon and then add hydrogen until it has the lift you are looking for. The hydrogen will lift the balloon so you don’t have to think about the balloon weight, only the lift. The lift can be measured with a low cost gram scale. We use a weight with a clip on it and zero the scale before attaching the balloon. Make sure your scale will show negative weights or do the math by subtracting the weight you use from the reading. If you like, roll up the neck of the balloon and wait until the morning. Check the lift in the morning, add whatever extra gas you need to fill and seal the neck three times with an impulse sealer.
The balloon will lose gas by diffusion through the film at a rate around .5 grams per 24 hours. The rate of loss by diffusion will go down dramatically at altitude. On the ground, the inside pressure of pure hydrogen is atmospheric and the outside hydrogen pressure is 0. When at altitude, the outside pressure is very low and so is the inside pressure. And with the dramatically lower temperature at float, diffusion is negligible.
The room where the balloon is filled with gas should be calm. Breezes will move the balloon around and make it hard to read the lift numbers. The temperature should be constant so don’t run a heater or air conditioner. You need equilibrium temperature and calm so be sure you can achieve that. There is also an urge to think the gas coming out of the cylinder will be cold and it must be allowed to heat back up. For hydrogen and helium, this does not happen. They are so nearly ideal gases they stay the same temperature as the cylinder. Hydrogen actually heats a bit but not enough to matter.
Some have observed better performance at high humidity. They suggest doing all the work when the humidity is higher. I have not seen that and tried storing a balloon in a garbage bag with a water spray and saw nothing. I could be wrong and keep your mind open to what the day is like.
After the balloon is filled and sealed, put it in a garbage bag or wrap it in a sheet to keep it down. Drive to the launch location, connect the line and release. The rest is up to Mother Nature and your choices.
There is a recent find of a nylon balloon that has a 32″ spherical shape. It can be found at Yokohama Balloons in Japan and can be bought in lots of 20 for about $17 each. They stretch to a spherical shape and have been shown to outperform the SBS or clear chinese balloons. As they are new and nylon is a new type of film to use, various stretching and preparation approaches are being used. It is too early for me to post how well this is working but so far at least one has circumnavigated at a better altitude than we usually see. Since nylon is hygroscopic and stretching still has not been completely understood, it is too early to say this is the one.