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
Basal body temperature typically lasts for at least three days and can survive as long as a week, making prediction of ovulation several days in advance necessary for avoiding pregnancy. There is limited evidence about the effectiveness of fertility awareness family planning methods, some of which use basal body temperature as one component. About 24% of women who use any type of fertility awareness program become pregnant during the first year, compared to about 85% of sexually active women who are not trying to prevent a pregnancy. The World Health Organization ranked fertility awareness methods, taken as a whole, as an "effective" method