Tracking

Once the balloon is released, you can only watch. If you are using WSPR, it is time to start your tracking software to get the wsprnet.org data and send it to APRS.fi. You can also go to http://lu7aa.org/wspr.asp and enter the data about your launch. If you are using APRS, it will show up on APRS.fi. Those websites will give you real time data and the track the balloon is following.

Knowing where the balloon will be in a day or two is very interesting. To do that, the HYSPLIT program at https://www.ready.noaa.gov/HYSPLIT_traj.php. The wonderful thing is that this program is written specifically for superpressure balloons. You need to put in the location and altitude and specify how many days prediction you want and how many hours between points. Click run and wait a bit. It will take a little time to generate the map and then you can see what is likely. You can check that against the storms shown on windy.com and have an idea about whether or not you are in trouble.

If you are interested in atmospheric data, the Univerisity of Wyoming has data that is interesting. They have isobar charts for high altitude. They can be found at http://weather.uwyo.edu/upperair/uamap.shtml. They also have a program that was written to predict the path of bursing balloons. That program can be interesting if you want to look at the pressure where the balloon is floating. It prints out the altitude vs. pressure in the area and you can use gps altitude to interpolate the pressure where the balloon is. It will also give you enough information to know the altitude of the tropopause where your balloon is. You may be below on in the tropopause so it could be interesting to study.

You can also use WSPRnet.org to see where your balloon is located. Put the call in and it will show who is receiving and where the balloon is located. It will not give you a track so you can only know the location at that time.

At times the balloon will be very fast and other times very slow. It will depend very much on what the jet stream is doing. Information about the jet stream can be found at https://www.netweather.tv/charts-and-data/jetstream. When in the jet stream, the speed can be over 200 miles per hour and when not in, it can be just a few miles per hour and even go in circles.

There are differences in the seasons. The wind is most convoluted in the summer when the sun puts more energy in the northern hemisphere. In the winter, the jet stream is closer to linear and it can carry the balloon around the world more quickly. Either way it is more interesting but there is some gratification is going around the world.

Superpressure

It is important to understand why these balloons do what they do. The latex balloons will rise until they burst. The balloon gets bigger and bigger with the expansion of the gas until the latex fails. They do not float. The superpressure balloon is not stretchy like latex. There is a little stretch but not much.

The superpressure balloon is give a certain amount of gas so it has the correct lift. The balloon will not be full. As it rises the gas expands and eventually fills the balloon. Since the plastic does not stretch (much) the density is now fixed. The balloon will rise a bit more and reach a level where the balloon overall density equals the outside air and it will stay there. The pressure inside the balloon will be higher than the outside.

There are very good articles detailing the behavior of a superpressure balloon and we will give a simplified explaination to get started. Anybody wanting to dig into the math can find great articles to detail the math.

The gases we use can be considered ideal gases. The particles of an ideal gas have no interactions with each other and are so small they never bump into each other. The nature of an ideal gas is that if you have one mole of particles, they occupy about 22.4 liters on the ground. A mole is a convient number for chemists and is 6.02 x 10^23 particles. It does not matter what the weight of the particles are. For air, the average weight is 29 grams and for hydrogen the weight is 2 grams. A mole of hydrogen will produce a lift of 27 grams. As the balloon rises, the air gets thinner and so does the hydrogen. As long as the balloon is not full, the lift remains the same.

The superpressure is an important number. The amount of free lift that the balloon has will determine what the pressure inside the balloon is at float. If the pressure is too high, it will burst. It will not be too low and the balloon will not launch. Over time, gas will diffuse though the plastic and it will rise slightly. The balloon will remain at float until the free lift goes to zero. This can be months so not much of a worry.

There is worry about the loss of gas waiting to launch a balloon. It is a reality and has to be considered. The SBS-13 balloons have a high quality multi-layer plastic that can hold the gas very well. The clear chinese balloons are less capable of holding the gas. They will lose gas at about 6 times the rate of the SBS-13. The simplest way to deal with this is to fill the balloons the night before when you have no time pressure and just roll up and clip the neck. In the morning, check again and add what you think it needs. Then do the heat seal.

The diffusion of gas at altitude is not an issue. There are too many ways to fail and it won’t be diffusion. At ground level, the pressure of the gas inside is 14.7 psi and the outside pressure of the gas is zero. At altitude the inside pressure is 2.3 and the outside still zero psi so the driving force is about 1/7th of that on the grouond but the temperature is dramatically lower so the diffusion through the palstic is lowered exponentially. The net result is, don’t worry.

The Rate of Ascent

The rate at which the balloon rises is useful to know. It allows you to predict when and where the balloon will come to float. If you know when and where you will launch, there are websites that will tell you where you will come to float and which way you will be going.

If you have an SBS balloon, they give the rate in their instructions. If you have a chinese balloon, you need to assume something. You could just use the SBS numbers and be close. If you want to get better data, it can be done.

You can use an app like VidAnalysis or some other to video the release of a balloon. It can be done indoors with a high ceiling and the appropriate weight attached to the balloon. With a video, you can pinpoint the same spot and graph the ascent. The balloon will stabilize within a meter or so and there you have a measurement. This can be done with several weights attached to the balloon to verify how ascent is affected by free lift. A great classroom project.

The rate of ascent of the balloon is essentially constant until the balloon reaches full inflation and then it will slow down until it reaches float. Since the balloon is travelling slowly, the velocity is directly proportional to the free lift. The flow around the balloon is laminar and that is different from airplanes where the drag and velocity are related to the velocity squared. Recall that the lift is constant until the balloon is fully inflated.

Once the balloon is inflated, the velocity will decrease linearly with altitude until it reaches float. This resembles the discharge of a capacitor and the time constant is the distance between full inflation and float divided by the rate of ascent. It takes about three time constants to come to float. This will not matter much since it will be in the order of 5 to 10 minutes but it is comforting to know it is done.

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.