Cars scuttle along the undulating surface of our planet at the bottom of an ocean of air twenty miles deep. They troll that ocean for its perpetually recirculating oxygen to burn their fuel and continue their scuttling travel. Sometimes their engines need a lot of air, when accelerating or climbing a hill. Sometimes they need less, at cruise; sometimes much less, when idling or decelerating. And each car changes the amount of air it swallows almost constantly. What’s more, the available oxygen in that ocean of air changes, too, with the density of the air itself, with the weather as the temperature and barometric pressure changes. These variables make measuring intake air a difficult problem. We’ll look at a Nissan diagram, but each carmaker using a mass airflow sensor uses it in a similar way.