Diluent cap value means a default value of percent CO2 or O2 which may be used to calculate the hourly NOX emission rate, when the measured hourly average percent CO2 is below the default value or when the measured hourly average percent O2 is above the default value. The diluent cap values for boilers are 5.0 percent CO2 and 14.0 percent O2. For combustion turbines, the diluent cap values are 1.0 percent CO2 and 19.0 percent O2.
40 CFR § 72.2
The terms used in this part, in parts 73, 74, 75, 76, 77 and 78 of this chapter shall have the meanings set forth in the Act, including sections 302 and 402 of the Act, and in this section as follows: