## Measuring Free Lift

The free lift is the extra lift that causes the balloon to rise. It is the lift after all assembly and ready to release. This is done when filling the balloon so the weights of the beacon, antenna and harness need to be known. The weight of the balloon itself is not necessary for this step as the gas will be lifting the balloon and it is the extra gas that is being measured. If one balloon is used, the math is simple. If two are used, do the same math but divide by 2 and do that in each balloon.

There are many small scales that can be bought. They are cheap and can be less than \$10. There is always concern about the accuracy of the scale and if the scale has changed. It is a good idea to buy a low cost set of laboratory weights to verify the operation of the scale is correct. I use the set sold on Amazon as “American Weigh Scales 14-Piece Small Calibration Weight Kit – Red”. It costs about \$13. You can check linearity and accuracy and have some peace of mind about the launch.

The usual method is to put a weight with a clip to hold the balloon. Gas is put into the balloon until it can lift itself and then a bit more. At any point, pause, role up the neck of the balloon just to keep the gas in and clip it to the weight. The scale will read a negative number which is the lift. (If it cannot display negative numbers, don’t tare and just do the math.) Do this in small increments as it is easier to add gas than to remove it.

The room needs to be still. The balloon will wave in breezes and just moving around makes enough to be seen. Allow the air to be still and that will be the free lift. The neck can be rolled up and clipped for a while as there is no pressure in the balloon and the gas will stay where you want it.

## Choosing the time and place For A Launch

The time and place to launch your picoballoon is important. Things to consider are location, surface wind, cloud cover, the path the balloon will follow during ascent and the path it will follow one at float altitude. Consider the things that can go wrong. The balloon can drag across the ground, snag a tree or power line, drift into lee turbulence from a bluff, dip into water, get into clouds and get wet, get drawn into an updraft in a cumulus or cumulonimbus cloud from below, float over a growing thunderstorm and so on. With a little thought the list would get much longer. So, plan the time and place…..

Choose an open spot. It can be a field, dock or a beach. If the day is very still, almost any place will work. At this point, you should have a pretty good idea of the float altitude and some idea of the rate of ascent. If it is an SBS-13, they mention the rate of ascent as a function of free lift. I you are using clear chinese balloons, you probably should learn by experiment. For a first guess, if you have to guess, use between 1 and 2 meters per second.

Use the windy.com to see what the surface winds at your chosen location will be. You can look days in advance for planning but look for wind of 5 miles per hour or less. You can probably launch during higher wind speed but the risks of problems increase.

Having chosen the launch location and time, go to to http://predict.habhub.org/ which will predict the trajectory during the rise to float. Input the location, the launch time, and where it asks for burst altitude, put float altitude. Put in your expected rate of ascent and for the descent rate, put in a very small number as the program exects a bursting balloon with a payload returning by parachute. If you put in a very low descent rate, the line it draws will be the direction it will follow at float.

The rate of ascent for slow balloons like we send up is pretty much constant all the way up. It is also proportional to lift so, double the lift, double the speed. It does not act like an airplane where drag is proportional to the square of speed. It is too slow and the flow is laminar. Once the balloon is fully inflated it will slow and reach float altitude. The approach to float has math like the discharge of a capacitor. The time constant is the distance between full inflation and float divided by the rate of rise. There is no need to worry about the approach to float but it will be in the last 15 30to minutes of the ascent, depending on your free lift.

The above program does not take that into account but don’t worry, it is trivial.

At this point, you have a fairly good idea of where and when the balloon will begin float. It is time to run HYSPLIT.

Put in the altitude and time. Select how far in the future for the prediction you want to see and it will tell you where the balloon will go. This program is specifically written for superpressure balloons so it is perfect. Just what the doctor ordered. It will take a little time to sort out the inputs as there is a lot to select and it too much for me to detail here.

After you know where the balloon is going to go, switch back to windy.com. Look at the cloud tops information. What you want to understand is if you will be floating into a storm or a cloud. A storm can tear things apart and a cloud will probably get it wet so it will descend. Remember it has no net lift at that point so every extra gram is a problem. There is nothing you can do about these things but evaluating before launch might save the balloon for later.

Assemble the balloon and payload. Half the dipole is above and half below for wspr. Just a 2 meter whip is below for the aprs trackers. Make sure the line from the balloon to an aprs tracker is long enough that the beacon and balloon are unlikely to bump in turbulence.

So, you have assumed a location and time. If you are using 2 meters and aprs, the balloon is easily launched by one or two people. Release the balloon upwind from the beacon and release the beacon when directly below the balloon. You don’t want it to swing down and drag on the ground. You might not want to launch in a wind faster than you are willing to run!

If you are using a tracker on 20 meters and using wspr, the antenna is significantly longer. I suggest three people but it can be done with less. Line up with the wind, release the balloon, keep the payload below the balloon until the line is straight and overhead and release. The third person keeps the line from tangling in weeds and so on.

Now the balloon is floating into the sky and all you can do is watch. Make sure you get video to share as it is easy to get engrossed in the launch and get no video. That is a good job for spectators and they are usually eager to be there.

Now watching the progress is the fun part. If you have a 2 meter aprs beacon, it will show automatically on APRS.fi. Once it is over about 10,000 feet, it will be loaded to https://tracker.habhub.org/#!mt=roadmap&mz=11&qm=1_day&f=AD1L-11&q=!RS_*;

For APRS.fi, put in the balloon call sign to see where it is. For the habhub tracker, the column on the left lets you select the balloon you wish to see and clicking on it will zoom to it’s location. Both sites give some telemetry coming from the beacon. Using APRS will give good tracking over North America and Europe. It might be picked up in China and Japan and again as it comes back to North America. There is lots of nail biteing while you wait.

If you choose to use a wspr beacon, the coverage is wonderful and you will almost always know where the balloon is. Night and very high latitudes will stop transmission but it still is just great. Usually the signal is sent on 20 meters and can go as far as 8,000 miles. You will know where it is over the ocean and over the land. Some nations make it illegal to transmit in their airspace and the beacons stop sending in that space. That would be primarily England and North Korea. There may be GPS jamming over some conflict areas in the middle east. The problem with wspr is the signal does not get picked up by aprs.fi. It is picked up by the wspr network. You can run a python script to go to wsprnet.org and download the data, convert the format and upload it to aprs.fi. That script is on Github. You can also ask for the tracking to be done by http://lu7aa.com.ar/wspr.asp?other=lu7aa and the map on his site will give you a great display.

It is also interesting to see what the jet stream is doing and where your balloon is relative to that. The jet stream can be seeh here: https://www.netweather.tv/charts-and-data/jetstream . If you would like to see what the air pressure at high altitude to see if the balloon is at the altitude/pressure you expected, go to http://weather.uwyo.edu/upperair/uamap.shtml . If you would like to study what pressure the balloon floats at, http://weather.uwyo.edu/upperair/balloon_traj.html, written for tracking balloon ascents and descents will give pressure and altitude information that can be used to interpolate the pressure from the gps altitude.

## Considerations for generating your own hydrdogen

It is not uncommon, at first, to consider producing your own hydrogen. This has been done successfully by one individual and his balloon circumnavigated the world once. In general the effort is rather big and is just generally a poor idea. If you intend to produce hydrogen with wet chemistry, make sure you know enough about chemistry and the kinetics of a reaction to control what you are doing. If you follow some of the Youtube videos where wads of aluminum foil or soda cans are used, realize that you have started a reaction over which you have no control. If you cannot stop a chemical reaction, you should not start it. If you use some of the gas generators being demonstrated on Youtube, make sure you know what you are buying or building. Some of them produce an explosive mixture of hydrogen and oxygen and things will not go well.

Producing hydrogen can be done with electrochemistry or chemical reaction. Either way, the gas you produce will be saturated with water and will need to be dried for the balloons to function properly. Remember that the balloons will be at temperatures of -50F or so and all the water vapor will be ice. Not a good thing to have in your balloon.

If you choose to produce the gas with electricity, do the math and realize that you are going to need about 5 moles of gas to function and each mole will require two Faraday Constant of electrons to product. That is 96485.3 coulombs each. A coulomb is the number of electrons in one ampere in one second. This is a pretty hefty power supply and will take a lot of time. It is, however, something you have total control over. You can probably use a low cost electrode and pvc to do these things. Seems like a lot of work for something you can buy at the local welding shop.

To just ballpark the prolem, an SBS-13 balloon will require about 10 grams of hydrogen. That is 5 moles. Each mole will require 2 Faraday Constants of charge so 10 Faraday Constants. Each Faraday constant is roughly 95,000 coulombs so 475,000 coulombs. A coulomb is the charge carried by one ampere in one second. A 100 amp supply would require 4,750 seconds or roughly 1.3 days. You will not have 100% efficiency so it could easily be twice that. This is not a trivial task.

You can choose to produce the gas with a chemical reaction. There are a variety of videos doing such a thing on Youtube. I see things like a wad of aluminum foil and muriatic acid. They toss the aluminum foil in the acid and step back. Once the reaction begins to go, it increases rapidly. That is a good reason to step back. Notice that the temperature and surface area involved in the chemical reaction is not controlled. There is a huge surface area in a wad of aluminum foil and when the reaction begins, the temperature is rising quickly. The high surface and temperature make this dangerous. I believe the Youtuber did get garbage bags to lift off. In reality you will need to also dry the gas and carefully measure and control the lift of the balloon. Things will not go well. Don’t do that. Soda cans will be problematic because they are coated and very thin. They won’t work well either.

If you insist on producing hydrogen with chemistry. Consider the aluminum water reaction. It requires a small amount of sodium hydroxide to keep the surface active as aluminum forms an oxide coat which protects it from the outside world. If you use thick pieces of aluminum, like aluminum angle from a hardware store, the surface area will be stable. If you have a heating mantle and the glassware to do this and if you have a condenser to provide reflux, you can produce hydrogen with reasonable possibility of control. And don’t forget you now need a cooling water supply for the condenser, The gas will still need to be dried for the balloon. You can see the problems here. A heating mantle with temperature control, glassware for the reaction vessel, a reflux condenser to contain the water evaporated by the heat of the reaction, a water supply, a drying column to finish the water removal, This is such a poor idea compared to buying a cylinder you should not do it.

## General Description of the preparation of a balloon

These tend not to be the electronics but from not understanding the balloons and how they work. So, the first part of this presentation is to build awareness of what the mistakes might be. If you release a balloon and you had an error is how you prepared and planned the launch, the balloon will probably be lost.

If you recall your high school chemistry, you studied the gas laws and promptly forgot them. They are important. The gases we use, hydrogen or helium, and the atmosphere are basically ideal gases. So lets talk about them without using any math.

An ideal gas means the atoms and molecules are moving around as if they are the only ones there. They are not attracted or repelled by their neighbors. The kinetic energy (speed) determines their temperature. The number of them in a volume determines their pressure.

A convient amount of particles for chemists is the mole. (A mole of electrons is 1 Farady Constant of charge.) At sea level on a regular day, that many particles occupies 22.4 liters, no matter which gas it is. That is a cube on the order of 11 inches per edge. If it is a mole of air, it weighs about 29 grams. If it is a mole of hydrogen, it weighs about 2 grams. That much hydrogen in a balloon displaces that much air so the bouyancy is 27 grams. As the balloon rises, the temperature and pressure change and they expand the same and so the lift is the same. That means the lift on the ground will be the lift until the balloon bursts or reaches float.

Lets go over the two types of balloons

Burster: This balloon has a stretchy latex balloon and as the balloon rises, the gas expands causing the balloon to stretch. It will continue to stretch until the latex fails. This balloon can carry payloads of 5 pounds or more so complex packages can be launched. The flight lasts a few hours and reaches altitudes over 100,000 feet. If you are lucky, you can find it and use it again.

Superpressure: This balloon has a plastic envelope. When launched, the envelope is not filled and as the balloon rises, the gas expands and eventually fills the balloon. Once full, the plastic cannot expand and so it’s density the lift decreases until it reaches float. It now has a pressure greater than the air, this is superpressure. This type of balloon can stay aloft for months, if you are both good at it and lucky. It carries a payload of around 15 grams and is solar powered. The are very rarely recovered but a few have been. It can circumnavigate the world. They float in the range of 40,000 feet. They are generally smaller, and will travel with the winds.

Most flights use two sources of balloons. Regular party balloons will not do much for you. The volume and the weight is too low to perform well. The commonly used ones are the SBS-13 made by Scientific Balloon Solutions and the clear 36” balloons made in China through AliExpress.

The SBS-13 balloons cost in the range of \$165, are high quality and will float a little higher than the clear Chinese balloons. Follow the instructions given by them and you are in pretty good shape. The volume is around .5 cubic meters and they weigh about 105 grams. The instructions tell how to fill, seal and assemble the flight. Experience points to a “free lift” of about 6.5 grams as a sweet spot.

The Chinese balloons cost around \$1.50 each in lots of 10, two are required for a normal payload and the quality control is poor so you must do some prestretching and testing for pinholes, film quality, and weak seals. They stretch to a volume of around .17 cubic meters and weigh about 35 grams. Because of the weight to volume ratio of these, they float a little lower but high enough to circumavigate the earth multiple times.

Qualatex also makes a 36” balloons that works but is less commonly flown.

They both SBS and clear chinese balloons comfortably carry around 15 grams of payload.

It is very common to fail with either of these choices. The best and lightest electronics are useless if the balloons fails so don’t overlook this part of the hobby. The common failure mode can be from too much gas, leaks causing gas loss, insufficient altitude to clear weather, floating into clouds during ascent, a launch that never clears the power lines and so on.

Clear chinese balloons are the most cost effective but require the most skill.

The first step is that of doing the prestrech and pressure testing. The balloons can burst between .5 psi to maybe .65 psi. They need to be pressurized to at least .5 psi to pretretch the balloon. They need to be held at pressure for a while to allow pinholes to appear if they are going to and to allow the plastic to stabilize so it does not contract much.

The equipment needed is an aquarium air pump, tubing, and fittings, a pressure gauge and a water column of some sort for pressure regulation.

(insert photo of test setup)

The aquarium pump provides all the air pressure needed, the balloons can be taped to either the tubing or a stiff tubing of some sort. The pressure regulation is done by a water column and adjusting the depth of the tube in the water. A leak is noticed when the bubbles stop and closing lines off one at a time will find it rather easily.

To prepare for launch, remove the air. One of the air pumps can be turned into a cheap vacuum pump and used or you can just roll up the plastic or put it under a blanket to slowly press the air out.

To prepare for a launch you need a hydrogen supply. The simplest way is to just get a cylinder from a welding supply store and buy a hydrogen regulator. A few have generated their own hydrogen. I am a chemical engineer and I looked into that. You should never start a chemical reaction you cannot stop and it is not a simple as you would think from the idiots on YouTube. Doing it well is expensive and difficult. I have only seen that done once but you can buy a hydrogen generator and do it with electrolysis.

Next you need a small scale to measure in grams. Put hydrogen into the balloon until is begins to rise. At this point, the gas has carried the bag and the extra lift will carry the payload and provide the extra lift called “free lift” The free lift is the critical number. Too little and the balloon will rise slowly and be vulnerable to gusts, moisture, etc. Too much and it will burst. Experience will teach you how much free lift but something like 5 grams per balloon seems to be good. So, zero the scale with a weight and a clip as the tare. Clip the balloon to the scale and read the lift as a negative number. The room should be free of drafts. When you reach your chosen amount, roll the neck and clip it.

You must know how much gas to seal inside the balloon. That is where the gas laws and lots of math comes in. Fortunately, the whole job has been written as a spreadsheet so you only need to know the inputs and outputs. The spreadsheet has the name float1g. The most current version can be obtained from (england website)

(show float1g and mention the inputs and outputs.)

Hydrogen will diffuse through the plastic film. In a day, a chinese balloon will lose in the range of .5 grams of lift. You can seal the balloon once it is filled or wait to just before launch and adjust the gas prior to sealing. Gas diffusion at float is not a problem as the pressure and temperature are much lower. The SBS balloons have something like 1/6 the diffusion loss so they can be sealed upon filling for launch the next morning. Sealing is done with a simple band sealer. Two or three seals are a good idea.

Once sealed, the balloons can be placed in a garbage bag for transport, wrapped in a sheet or towel and put in the back of your car. The whole package is assembled at the launch location. It is itmportant to have low wind and to not launch into clouds.

## Altitude, superpressure and Volume Calculations

It is important to know the volume of a balloon in order to run the calculations in the spreadsheet float1g. That spreadsheet does all the math for you and you need to know the payload weight, the balloon weight, the free lift and the volume of the balloon.

The weight of the balloon and the payload are pretty simple, just weigh them with the scale you bought for free lift. The volume of the balloon is another. It can be simply assumed to be what others have found and things will be close enough. The chinese balloons are .17 cubic meters each and the SBS-13 according the SBS is .5 cubic meters. You can just use those numbers.

For the chinese balloons, you might want to measure things as you stretch the balloon. That way you can be sure you know what is going on. The measurements can be simple. Tape a metric stick to the floor, put a box on one end at zero and tape it to the floor to stay put. Place the balloon next to the box, use another box to hold the sides agains the cardboard on both sides and then read the meter stick. Do this for the distance across the seam and across the faces. A good approximation is to us the formula for an oblate spheroid.

Vol= 4Pi*a^2*b

Where a is the radius between the seams and b is the radius across the faces.

There is a formula for mylar balloons that is an interesting formula and the math was quite a task to put together. It has the restriction that the film is inelastic which ours is not so it will not work.

If you really want to get into the weeds and verify how good the formula is, inflate a balloon to the pressure you will tolerate. Take a photo so the seam forms a straight line. Print the photo as large as you can. Make sure you measure the balloon. Draw a line across the seam, find the center, divide the line into convient segments and, knowing the scale, find the dimensions of each line. Since the balloon has radial symmetry, calculate the volume of your annular rings, add them up and compare it to the volume of the oblate spheroid. I think they will be pretty close.

The math used to calculate the float altitude and superpressure is in the spreadsheet float1g. The complete expaination can be found at https://ukhas.org.uk/projects:splat. It includes a massive table of atmospheric parameters for a standard atmosphere. We never have a standard atmosphere but it is the best we can do and does a very good job.

## On the use of hydrogen and helium

There are a few surprising things about hydrogen and helium and a few obvious ones. This assumes you will use a cylinder from a welding supply store. The hose for filling can be anything. I use the acetylene line and torch head of an old welding set as it has valves and is handy. The threads on the tank and regulator for hydrogen are left handed as any combustible gas is. That makes the acetylene line a good match.

Purchase a regulator for the cylinder. It is not sensible to use the valve on the cylinder as the pressure is very high and you will have no control.

The helium available from party balloon stores is commonly a diluted mixture and will not work well. Make sure you know if you are getting pure helium. Do not mix hydrogen and helium in one balloon unless you know if the helium is pure. If it is diluted, it is most likely with air which is free. If that is the case, you may be mixing hydrogen with oxygen and that will not end well.

The behavior of these two gasses surprises most of us. We expect gases to cool when expanded and we have a very high pressure gas. This effect is the Joule Thompson effect and applies to gases being released through a valve. For hydrogen or helium, it is basically zero. There will be no change in temperature when filling so no need to wait for the balloon to come to room temperature. It will already be there.

When the balloon is ascending, a different thing happens. The gas expands but cools as the expansion takes some of the energy out of the gas. The cooling is the adiabatic lapse rate. The lapse rates of hydrogen and helium during the ascent of the balloon are greater than the lapse rate of the atmosphere. The balloon will get cooler faster than the air around it. This is not important in a clear sky but it does matter if the balloon were to go into a cloud. It will be cooler than the cloud and there will be condensation that will increase the weight of the balloon. Given that water droplets will stick to the balloon and condensation will also occur, things can go very wrong. Try to have a clear sky.

## General discussion on balloons

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.9042311.0.0.32bf4c4dL9KsTW 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

friction tape.

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.

## Considerations about weight in balloons

There is a great interest in building a beacon at the absolute minimum weight. This makes sense as the less the weight, the higher the balloon will float and the more likely it will be above the weather. If the beacon is made too fragile, then the likelyhood of failure increases. It is a judgement call about the weights of each and every component.

Consider the weights involved in flying an SBS-13 balloon.

Weight of the balloon 105 grams
Weight of the beacon, antenna, solar panels, and harness 15 grams
Weight of hydrogen to provide 6.5 grams of lift 10 grams
Total 130 grams

The electronics will weight in the range of 5 or 6 grams and maybe even a little less. Reducing the weight by a half gram by using smaller antenna wire will have very little impact on the float altitude and no bearing on the life of the balloon. The float spreadsheet predicts about 100 feet lower for a half gram. If the wire is broken during launch or fails in flight, the flight is lost.

If the balloon is to be inflated with helium, the total gas weight will be about 20.5 grams. The balloon now weight about 10 grams more. The equilibrium float will go down about 1,600 feet.

The weight can be reduced by trimming excess plastic, shortening the harness, finding the lightest foam to use for support, lightest solar panels, and so on. All of this adds up and is important to consider. The total weight and float altitude of the clear chinese balloons is a little different but the considerations are the same.

## Preparation of the clear chinese balloons

The clear balloons from AliExpress are the most economical way to fly superpressure balloons. They cost in the range of \$1.50 each and two are required. They have survived at least 9 complete circumnavigations of the earth. They float a little lower than the SBS-13 so they are a bit more likely to run into weather and fail. The most important issue is the quality control at the manufacturer is not quite good enough.

The balloons can be inflated with gas and used as is but you run the risk of having a defect in the plastic that you do not see. If that is the case, the balloon may begin to leak during ascent or it may develop a pinhole leak in a day or two. You can increase your probability of success by prestretching and endurance testing.

The balloons are able to tolerate about .5 psi differential pressure. If the balloons are inflated to .5 psi and held there for a couple days, the failures of the seams or the development of pinhole leaks will be detected and the good balloons found. It is not uncommon to lose 20% or more of the balloons but at that price it makes sense.

The equipment needed to do that is simple. An aquarium air pump, a pressure guage for HVAC types of work and a glass cylinder or jar able to hold water about 18 to 24 inches deep (to be used for pressure control). For removing the air, a second air pump of the type suggested can be modified to be a small vacuum pump and used to pull out the air.

The air pumps at about \$10 at amazon can be found under the title Tetra Whisper Easy to Use Air Pump for Aquariums

Instructions on how to convert it to a vacuum pump are here: https://www.instructables.com/Reverse-aquarium-pump-to-vacuum-pump/

There are probably quite a few pumps that can do this and this is just what I found.

A pressure guage is also needed. I found this one for about \$30 on eBay: Digital Manometer Differential Pressure Meter Gauge 2 Pipes Air Flow Test ±2.999

The air pump feeds into aquarium air line, a tee routes one line to the cylinder filled with water, the next tee connects the pressure guage and then the line continues to the balloons with one tee per balloon until the last one. When the air pump is plugged in and air begins to flow, pinch the line after the water cylinder line and adjust the depth of the line so the pressure guage shows the pressure you like and just let it run. When all the balloons are filled and stretched, the excess air will bubble out in the water. Just let it run and see which balloons fail. A failure will either be a pop or the bubbler will stop. If the bubbles stop, pinch off the line to one balloon at a time and watch to see if the pressure is increasing. That will find the failed one and it can be replaced.

When you are satisfied that the balloons will last, switch the air pump with the vacuum pump, pinch off or close the line to the water column and wait. The air will be gone in a while.