For Exercises 12.11 to 12.13, see page 612; for 12.14 and 12.15, see page 614; for 12.16 to 12.18, see page 619; and for 12.19 to 12.21, see pages 625–626.
12.12 Auto thermostats.
A maker of auto air conditioners checks a sample of four thermostatic controls from each hour's production. The thermostats are set at 75°F and then placed in a chamber where the temperature is raised gradually. The temperature at which the thermostat turns on the air conditioner is recorded. The process mean should be . Past experience indicates that the response temperature of properly adjusted thermostats varies with . The mean response temperature for each hour's sample is plotted on an control chart. Calculate the center line and control limits for this chart.