<P> Where more than one burner is present, the pilot can use one or more at a time depending on the desired heat output . Each burner is characterized by a metal coil of propane tubing the flame shoots through to preheat the incoming liquid propane . The burner unit may be suspended from the mouth of the envelope, or supported rigidly over the basket . The burner unit may be mounted on a gimbal to enable the pilot to aim the flame and avoid overheating the envelope fabric . A burner may have a secondary propane valve that releases propane more slowly and thereby generates a different sound . This is called a whisper burner and is used for flight over livestock to lessen the chance of spooking them . It also generates a more yellow flame and is used for night glows because it lights up the inside of the envelope better than the primary valve . </P> <P> Propane fuel tanks are usually cylindrical pressure vessels made from aluminium, stainless steel, or titanium with a valve at one end to feed the burner and to refuel . They may have a fuel gauge and a pressure gauge . Common tank sizes are 10 (38), 15 (57), and 20 (76) US gallons (liters). They may be intended for upright or horizontal use, and may be mounted inside or outside the basket . </P> <P> The pressure necessary to force the fuel through the line to the burner may be supplied by the vapor pressure of the propane itself, if warm enough, or by the introduction of an inert gas such as nitrogen . Tanks may be preheated with electrical heat tapes to produce sufficient vapor pressure for cold weather flying . Warmed tanks will usually also be wrapped in an insulating blanket to preserve heat during the setup and flight . </P> <P> A balloon may be outfitted with a variety of instruments to aid the pilot . These commonly include an altimeter, a rate of climb (vertical speed) indicator known as a variometer, envelope (air) temperature, and ambient (air) temperature . A GPS receiver can be useful to indicate ground speed (traditional aircraft air speed indicators would be useless) and direction . </P>

Where did the hot air balloon originate from