DMR converts this into air changes. From venting to the end of the second flush, about six air changes per hour are recommended. From the third flush, four per hour hold oxygen near 16 percent.
The damper position follows: 30 percent fresh air and 70 percent recirculation for the first two flushes, then 20 percent fresh and 80 percent recirculated. A milky crop needs a minimum of three to four air changes per hour.