Do changes in air pressure affect pellet velocity?

by Trevor Foss and Beau Bayless
Copyright PyramydAir.com ©2008. All RightsReserved.

Note: The 2008 California Science & Engineering Fair included an airgun-related entry from Beau Bayless and Trevor Foss, eighth-grade students at All Saints' Episcopal Day School in Carmel. Their entry won first place in the Physics & Astronomy section. In a slightly different format than our usual articles, we present their project.

Objective
Our objective was to find how changing the air pressure of an air gun affects the average speed of the pellet. We thought that if you double the air pressure, the average speed will double, if you triple the air pressure, the average speed will triple, and so on.

Methods & materials
We constructed an air gun that would shoot pellets at variable pressures through a barrel with a sensor at the beginning and the end. Then we set the pressure to 100 psi and fired the air gun. We recorded the time that the pellet took to pass through the first and second sensors and used the formula Speed = Distance/Time to get the speed. We repeated those steps four more times and averaged the five speeds. We repeated that procedure, increasing the air pressure by 100 psi each time until we reached 500 psi.

Results & discussion
Our graph (above) shows the average speed for each of the gas pressures. For 100 psi, the average speed was 58 meters per second; 200 psi produced 77 mps; 300 psi produced 95 mps; 400 psi produced 105 mps; and 500 psi produced 109 mps. We concluded that our hypothesis was wrong in the saying that doubling the psi would double the average speed. However, we discovered that although increasing the pressure increased the speed as the psi increased, the increase in the average speed decreased.

Data analysis & conclusion
Our experiment investigated how the change in the pressure (pounds/square inch) of an air gun affects the average speed of a pellet. We were trying to find out the relationship between the pressure of the airgun and the average speed of the pellet. Our graph shows the average speed for each of the gas pressures. For 100 psi, the average speed was 58 meters per second, 200 psi produced 77 mps, 300 psi produced 95 mps, 400 psi produced 105 mps and 500 psi produced 109 mps.

Between 100 psi and 200 psi, the average speed increased by 18.6 mps,between 200 psi and 300 psi the average speed increased by only 17.9 mps, between 300 psi and 400 psi the average speed increased by 10.8 mps, and between 400 psi and 500 psi the average speed increased by 3.4 mps. We think we got these results because the greatest change in pressure is between 100 psi and 200 psi. And, 200 psi is twice the pressure of 100 psi, while 300 psi is only 1.5 times as great as 200 psi, 400 psi is 1.3 times as great as 300 psi and 500 psi is 1.25 times as great as 400 psi. So, the increases in pressure were also proportionally less. We concluded that our hypothesis was wrong in saying that doubling the psi would double the average speed.

Our experiment is important because it tells people how fast their airsoft guns shoot. People can use this because most airsoft gun's speed is a guess based on distance and impact. This experiment can tell people how fast their airsoft guns really shoot and if airsoft manufacturers are lying about speed to increase sales.

We think we controlled our variables quite well. We used the same pellet size and mass, the same length of the barrel and the same type of sensors.

One variable that we did not expect was that we had to use a spit wad and a pellet. Since the pellets we used were transparent, the sensors did not register the pellet going past the sensors. To solve this problem, we added a small rolled up piece of paper for the sensors to register the speed. We tried to use the exact same sized wad, but maybe some of the wad sizes could have been slightly off.

If we were going to do this experiment again, we would do 20 shots per pressure and then average them. We would have also made sure that we used the exact same size spit wad with the pellet.