Effect of Temperature On Pressure Gauge Readings
The pressure gauge's indicated gas 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 estimated by using 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 degrees Fahrenheit (0 degrees Celsius, 273.15 degree Kelvin) and compare it to 96°F (35.6°C or 308.71K; normal human body temperature is 37°C). While 96 is arithmetically three times 32, the difference in temperature from a scientific point of view is not threefold. Instead of comparing 32°F to 96°F, temperatures of 273.15K and 308.71K should be compared. The actual scientifically valid change in temperature from 32 to 96 °F (0 to 36 °C) is by a factor of 1.13 (308.71K/273.15K), not 3. If an air cylinder is pressurized to 4,500 psi at 96°F and later the temperature drops to 32°F, the pressure gauge will indicate 4,000 psi (4,500/1.13). Stated differently, a drop in temperature of 10 degrees Fahrenheit (5.5 degrees Celsius) causes a pressure decrease of about 82 psi. Failure to accurately account for the effect of temperature on pressure readings can result in underfilled air bottles, which in turn could lead to a firefighter running out of air prematurely.
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