At a press conference on Saturday New England Patriots coach Bill Belichick laid out the team’s official explanation for how 11 of 12 game balls used in the AFC title game got deflated.
Belichick says the deflation happened naturally as a result of two factors: 1) the way the team prepares the balls, and 2) the atmospheric conditions.
According to Belichick, those two things would cause the ball to lose 1 PSI of air pressure between the time the referees measured them two hours and 15 minutes before the game and the time the referees measured them at halftime.
Here’s how, according to the coach:
1. The Patriots, like every other team, spend a ton of time trying to get the texture of the balls right. The balls undergo an extensive “rubbing process” to remove the slipperiness and make them easier to use. Belichick says this “rubbing process” goes all the way up to the point at which the balls are handed to the officials to be measured two hours and 15 minutes before the game.
2. This “rubbing process” raises the air pressure by 1 PSI, Belichick says. So if a ball is inflated to 12.5 PSI, and then rubbed, it will measure at 13.5 PSI right after the rubbing process. Physicist Kirk Hackett explained to BI:
“When the football is rubbed and squeezed, work is being done on the ball, and it will get hotter. If you want, describe the heating as due to friction. When the rubbing stops, the ball will cool back to the environmental temperature.”
3. The Patriots told the officials to inflate the ball to 12.5 PSI (the minimum level allowed under NFL rules) before the game. So two hours and 15 minutes before the game, the balls were at 12.5 PSI, but that air pressure reading was actually temporarily elevated because the ball was still under the influence of the rubbing process.
4. When the ball cools down and acclimates to the weather conditions, Belichick says, it reaches an “equilibrium” pressure level that’s about 1.5 PSI lower than it was measured at originally. So the ball starts at 12.5 PSI and then goes down to 11 PSI when the effect of the rubbing wears off the ball adapts to the cooler temperature. He explains (via ESPN):
“We found that once the footballs were on the field over an extended period of time, in other words they were adjusted to the climatic conditions and also the fact that the footballs, which an equilibrium without the rubbing process after that had run its course and the footballs reached an equilibrium, that they were down approximately 1.5 pounds per square inch. When we brought the footballs back in after that process and retested them in a controlled environment as we have here, then those measurements rose approximately 0.5 PSI. So the net of 1.5 [PSI] back down 0.5 [PSI] is approximately 1 PSI.”
“Now, we all know that air pressure is a function of the atmospheric conditions, it’s a function of that. So, if there’s activity in the ball relative to the rubbing process, I think that explains why when we gave them to the officials and the officials put it at 12.5 [PSI] if that’s in fact what they did, that once the ball reached its equilibrium state, it probably was closer to 11.5 [PSI].”
Because of the ideal gas law, the ball’s PSI level will drop when it goes from a warm environment to a cool environment. But the thing to remember is it wasn’t actually that cold during the AFC title game. It was 52 degrees at kickoff. If the ball was inflated in a 70-degree room, that temperature difference could only account for around ~0.9 PSI of pressure loss. Belichick says the team ran extensive tests that showed the PSI dropped when the ball was exposed to the weather. But those tests weren’t necessarily done in the same unseasonably warm conditions that the AFC title game was played under.
Belichick is saying that the weather, combined with the rubbing process, made the ball deflate by 1.5 PSI once it got out on the field. After it was brought back inside to be measured the pressure rose again by 0.5 PSI, and the net difference between the pregame measurement and the halftime measurement was 1 PSI.
He concludes, “So the atmospheric conditions as well as the true equilibrium of the football is critical to the measurement.”
There are still questions here.
For one, 11 of the 12 balls that New England used were reportedly found to be at least 2 PSI below the 12.5 PSI that they were measured at before the game. If Belichick says the rubbing/weather can account for 1 PSI, how did the rest of the deflation happen?
If it’s true that the difference was at least 2 PSI, why didn’t the Colts’ balls also deflate to an illegal level? Even if the Colts had their balls inflated to 13.5 PSI before the game, they would have been under the 12.5 PSI limit if the natural factors that deflated New England’s balls were really at work. According to SI’s Peter King, the Colts balls were fine when tested at halftime.
Belichick’s explanation hasn’t been widely accepted, but it at least creates plausible deniability and drags the discussion into the muck of atmospheric pressure laws.
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