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. Combined mass The combined mass of an average system can be calculated as follows: {| class="wikitable" |- ! Component ! Pounds ! Kilograms ! Mass fraction |- | envelope | align="right" |250 | align="right" |113.4 | |- | 5-passenger basket | align="right" |140 | align="right" |63.5 | |- | Double burner | align="right" |50 | align="right" |22.7 | |- | 3 fuel tanks full of propane | align="right" |3 × 135 = 405 | align="right" |183.7 | |- | 5 passengers | align="right" |5 × 150 = 750 | align="right" |340.2 | |- | Subtotal | align="right" |1595 | align="right" |723.5 | |- | of heated air* | align="right" |5922 | align="right" |2686.2 | |- | Total | align="right" |(3.76 tons) 7517 | align="right" |3409.7 | |} * Using a density of for dry air heated to . Theory of operation Generating lift Increasing the air temperature inside the envelope makes it less dense than the surrounding (ambient) air. The balloon floats because of the buoyant force exerted on it. This force is the same force that acts on objects when they are in water and is described by Archimedes' principle. The amount of lift (or buoyancy) provided by a hot air balloon depends primarily upon the difference between the temperature of the air inside the envelope and the temperature of the air outside the envelope. For most envelopes made of nylon fabric, the maximal internal temperature is limited to approximately 120 °C (250 °F). The melting point of nylon is significantly greater than this maximal operating temperature — about 230 °C (450 °F) — but higher temperatures cause the strength of the nylon fabric to degrade more quickly over time. With a maximal operating temperature of 120 °C (250 °F), balloon envelopes can generally be flown for between 400 and 500 hours before the fabric needs to be replaced. Many balloon pilots operate their envelopes at temperatures significantly less than the maximum to extend envelope-fabric life. The lift generated by of dry air heated to various temperatures may be calculated as follows: {| class="wikitable" |- ! Air temperature ! Air density ! Air mass ! Lift generated |- | |