Self-contained breathing apparatus pressure changes with ambient temperature. As temperature decreases, the pressure inside the cylinder decreases. The relationship between the temperature and the pressure of a gas is defined by the formula "PV" = "nRT". (See Universal gas constant.) What is particularly important to understand from the formula is that the temperature is in kelvins, not degrees Fahrenheit. Consider the freezing point of water at 32 °F (0 °C, 273.15 K) and compare it to 96 °F (35.6 °C or 308.71 K; normal human body temperature is 37 °C). While 96 is arithmetically three times 32, the difference in temperature from a
Operating temperature Similarly, biological systems have a viable temperature range, which might be referred to as an "operating temperature". Most devices are manufactured in several temperature grades. Broadly accepted (?) grades are: Nevertheless, each manufacturer defines its own temperature grades so designers must pay close attention to actual datasheet specifications. For example, Altera uses five temperature grades for its products: The use of such grades ensures that a device is suitable for its application, and will withstand the environmental conditions in which it is used. Normal operating temperature ranges are affected by several factors, such as the power dissipation of the device.