Types of Balloons
In the case of hot air balloons, the buoyant force is provided by the heating the air inside it. They are mainly used for recreational purposes.
Gas balloons are used for carrying out scientific research. They play an indispensable role in the field of astronomy. Usually, a specially made thin film is used to form an envelope for the balloon. The balloons are inflated with lighter than air gases like hydrogen and helium, which have a density lower than the surrounding atmosphere. Hydrogen is the lightest among all gases, but it is not preferred as it is highly flammable, hence scientific ballooning uses helium.
Three types of balloons are used in the field of science:
Rubber or Neoprene balloon: As the inflated balloon starts to ascend the expansion of gas inside cause the stretching of Neoprene. This process goes on till the balloon reaches about 30 to 200 times its initial volume. At this point the neoprene cover ruptures and its mission is over. The instrument in the balloon reaches the ground with the help of a small parachute connected to it.
Zero Pressure balloon: This balloon is used to carry out research at a predetermined altitude. The balloon is made of polyethylene and has a valve fitted to it. It is partially inflated while it is on the ground. As the balloon rises, the gas expands and when it reaches the expected altitude, automatic discharge of excess gas occurs through the valve. After sunset the gas inside the balloon cools and contracts reducing the balloon's volume and this causes the balloon to descend. If this mission is to be carried out for more than one day then it is filled with a ballast, usually sand. When the balloon starts to descend after sunset a small portion of ballast is released and thus the balloon remains at altitude. During day time, the gas again expands but the balloon ascends to higher altitude as it is lighter than before. Again, it automatically releases the excess gas to settle at the preset altitude. This process goes on until it runs out of the ballast and after several days the mission is over. NASA has developed zero pressure balloons using special polyethylene film, which were launched in December 2008. The image on left is that of the Zero-pressure balloon in its test launching in Antarctic.
Super Pressure Balloon: Internal pressure of super pressure balloon is maintained higher than the atmospheric pressure so as to keep its density at an approximately constant value to control its altitude. It is preferred over the Zero-pressure balloons due to its ability to stay at altitude for extended periods of time.
The image on left is that of a Super Pressure Balloon.