The NFL has seen within the past three seasons two separate times when teams have been accused of using underinflated footballs. The first was the New England Patriots as they played the Indianapolis Colts in the AFC Championship game January 17, 2015 and most recently the Pittsburgh Steelers as they hosted the New York Giants in Week 13 of this season, December 4, 2016.
Several questions need to be asked. Why use underinflated footballs? What are the rules regarding the inflation of footballs? Is there anything to explain how/if footballs becoming underinflated during the game?
According to Rule 2, section 1 of the NFL rulebook:
“The ball shall be made up of an inflated (12 1/2 to 13 1/2 pounds) urethane bladder enclosed in a pebble grained, leather case (natural tan color) without corrugations of any kind.
The Referee shall be the sole judge as to whether all balls offered for play comply with these specifications. A pump is to be furnished by the home club, and the balls shall remain under the supervision of the Referee until they are delivered to the ball attendant just prior to the start of the game.”
An interesting protocol for footballs in NFL games is that each of the teams have two sets of footballs they use, one while they are offense and the other for kicking. So in one game, there are four separate sets of footballs used.
After the referees say the balls are legal pre-game, the individual teams then take over and have control over the balls that they use. That is where there is room and time for some “fishy” business, such as New England and Pittsburgh are accused of.
If a team was to intentionally under-inflate footballs, what advantage would they have? The main advantage is that the ball would be able to be squeezed more easily allowing for a better grip. This is especially important in games featuring either cold, wet or snowy conditions. For the Patriots game mentioned above, a light rain and/or drizzle was falling all game with temperatures in the mid 40s. For the Steelers game, precipitation was not falling but temperatures were chilly. The high temperature that day was 42 degrees.
Now the question becomes were the footballs used in these two games intentionally deflated to give the Patriots and the Steelers an advantage?
The two Steelers footballs tested by the New York Giants were reportedly 11.8 and 11.4 PSI (the Giants never filed an official complaint so the NFL quickly dismissed the controversy). Seven footballs that were tested by the NFL in the investigation of the Patriots had PSI measurements ranging from 10.90 to 11.80 with an average of 11.49 PSI. Could these measurements come from footballs with legal PSI levels pregame?
To figure this out, a basic law of physics called the ideal gas law (PV=nRT where P stands for Pressure (PSI), V is volume, n is the amount of gas molecules, R is a gas constant, a number that does not change, and T is temperature) needs to be applied to the footballs. This law says that when a football moves from a warm environment (such as the locker room before a game) to a colder one (outdoors), its pressure (ie. PSI) decreases due to the drop in temperature. From pregame to being used in the game, the only variables changing in this equation would be temperature and volume. These variables are directly proportional; as one goes up, the other goes up and vice versa. In this case, dropping the temperature decreases the volume.
After some complicated math and conversions, the ideal gas law says that dropping the temperature from 70 degrees to 40-45 degrees is a decrease of about 4.3% in terms of absolute temperature and thus the volume will also drop that amount. That means for the football with a 10.90 PSI (the lowest PSI of any of the Patriots footballs), the football had a PSI of 11.4 BEFORE being brought outside and played with. So, without anything else going on, this football was underinflated. But could anything else cause the footballs to lose pressure between the referee’s pregame measurement and when it was tested again after being used in the game?
The answer is yes. Normal useage in an NFL game combined with a few hours of time would account for some PSI drop (like how a balloon at a birthday party will eventually lose its helium and not be buoyant anymore). Becoming wet (the Patriots played in rain, the Steelers did not) accounts for a slight PSI drop. The warming back up of the balls to make the new measurements can also account for a small decrease in volume. Removing the needle after taking the original measurement and then taking another measurement after the balls were used would also cause some decrease in pressure. And, most importantly, the cheap pressure gauges used by the referee can become substantially incorrect over time. The measurement of PSI by gauges can change by about 0.1 psi over the course of just a few days and it is not known when these gauges were last calibrated. Even brand new gauges can exhibit a wide range of values when measuring the pressure of the same exact ball.
So, can the 1.1 PSI difference from the WORST case scenario ball between its and the lowest legal PSI of 12.5 be attributed to the several factors mentioned above? It would be almost be impossible to tell for sure but if you attribute just 0.2 PSI to each of the 5 factors mentioned in the previous paragraph then you have a 1.0 PSI difference. And it is not hard to assume that the 0.2 PSI for each of the factors is conservative. Obviously, the balls the Steelers used had a higher PSI level than this worst case scenario Patriots ball and it would be even easier to account for the low PSI levels.
What can the NFL do to stop these controversies? Having just one set of “playing” balls and one set of kicking balls that both teams use with an official in charge of these balls seems like an obvious fix. Having each team be in charge of their own balls seems like a situation that is ripe for potential problems. Major League Baseball doesn’t let each team switch balls based on what team is pitching. The umpires are in charge and a “neutral” ball is used.