The percentage drop in pressure using the correct absolute scale was only 4.7%. He most likely used the gage pressure in which case he found that the initial temperature of the ball would have had to have been much higher for the pressure to drop as much as the Patriots claim. Neil’s mistake on his calculation is easy to make. Another important point that I'm sure Neil and Bill did get right, you need to use the Kelvin scale for temperature, simply add 273 to the Celsius temperature. Now, using the Ideal Gas Law we can calculate what the pressure in a football should be in the event the temperature drops from 75F (24C) to 50F (10C). The atmospheric pressure is also not fixed due to various factors including weather and altitude (check out our Air Pressure at Altitude Calculator!) Anyway, to get the absolute pressure you simple add 14.7 psi to your gage reading, so the NFL minimum pressure requirement is 12.5 psig + 14.7 psi = 27.2 psia. The atmospheric pressure which surrounds all of us is about 14.7 psi. You have heard a lot about the NFL requiring footballs to be at 12.5 psi, actually that is 12.5 psi GAGE or psig, which is the pressure relative to the atmosphere while absolute pressure is relative to a perfect vacuum. So, where have many experts messed up? They are using gage pressure in the calculation and not absolute pressure. While not a steel container, a football behaves much like a fixed volume as Wilson the ball manufacturer has indicated, so for this calculation the Ideal Gas Law is appropriate to use. Long story short - the Ideal Gas Law states that for a fixed gas volume, pressure is directly proportional to the temperature of the gas, so if the temperature drops by some percentage the pressure will drop by the same percentage and vice versa.
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The ideal gas law was discovered in 1854 ( ), but apparently some renowned scientists such as Neil deGrasse Tyson and Bill Nye may need a little refresher on how to do the calculation correctly! This conclusion is based on the data from our test demonstrating that the balls lost over one psi when they went from a 75F environment to a 50F environment where they sat for 1.5 hours (the balls in question probably sat in 50F environment for more than 1.5 hours.)īased on Bill Belichick's explanation of how the game balls are prepared, we performed a final demonstration showing how a "scuffing" a ball, similar to what the Patriots probably to do prepare the balls for the game, can add thermal energy to the football which would increase the air temperature within the football. The experiment proved that changing the temperature of the air in a football impacts the air pressure of the football. The first ball dropped from 13.4 psi to 12.3 psi, and the second ball went from 12.6 psi to 11.5 psi. I found that the psi had indeed significantly dropped for both balls. I left the footballs in the thermal chamber for 1.5 hours, before checking the ball pressure again. The balls were then placed back in the thermal chamber and the temperature was then set to 10 C (50 F) - the game time temperature! I also used an infrared camera to confirm the initial temperature of each football. One ball was at 13.4 psi (the high end of the allowed range) and the other was at 12.6 psi (the low end of the allowed range - where Tom Brady likes them).
I then recorded the pressure of the balls. I placed two official NFL footballs in our calibrated thermal chamber and let the footballs reach equilibrium at 24 C (75F) - the approximate temperature when the NFL officials tested the pressure of the Pats' balls. Basically I wanted to find out whether or not temperature can actually influence the pressure (measured in psi) of an official NFL football enough to account for what was seen during that infamous Patriots Colts playoff game - so I set up the following test: Again, as an engineer I like to test theories against real-world conditions. Deflategate -How Temperature Can Influence Pressure in a FootballĪs an engineer I was intrigued by the #deflategate controversy and theories surrounding the New England Patriots.
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