Each grain of spawn is a starting point. Mycelium grows out of it in every direction, digests the compost and joins threads from other grains. The compost changes from dark brown to light brown as the white web spreads, and ICAR-DMR treats that colour change as the sign the run is complete.
The fungus also makes heat. ICAR-DMR holds the room at 23 plus or minus 1 °C, which gives a bed temperature of 24 to 25 °C, one to two degrees higher. The number that matters is therefore the compost temperature, not the air.