Figuring out FPS
Hey all,
I have a question and want to get all of your thoughts.
I just got a Cricket .25 but do not have a chrony. I want to get a round about figure on how many FPS she is shooting. So if I zero at 30 yrds then put a target at say 70 yrds and put the cross hairs on the target, theoretically, if I measure the distance of drop (and everything else is plugged into chairgun correctly) couldn’t I just adjust the FPS in chairgun until the POI matches and get a round about number.
In my head, this should give me a correct round about number. What do you guy think?
If this is not correct or anyone knows a better way to figure FPS without a chrony please feel free to enlighten me. I want to get the dope on my BP.
Thanks in advance
I hope everyone understands what I am asking.
All Replies
Viewing 7 replies - 1 through 7 (of 7 total)
You can download this free sound recording software and use the distance formula D=rxt to get the fps. You will need a lap top with a mic.
http://audacity.sourceforge.net/
~ GKU
if you input all the parameters correctly and exactly into chrono connect its very close to an actual chrono,
plus a loud steel target is a must for the phone to be able to hear the hit downrange.
and i always use mine at a distance of 30-50yds,had widely varying results when it was closer.
I don’t know how accurate is chrono connect and frankly I am suspicious of how good a smart phone can detect the exit time and the impact time accurately. There is a recent post by someone on general topics where his chrono connect showed 1350 fps while chrony showed 950 fps.
Good concept though.
Awny
If you have a smartphone you can download Chrono Connect. It will give you a pretty close reading and a lot less work/hassle then your original plan. Lite version is free so maybe give that a try.
I understand, and thanks for the reply. I plan on getting a chrony, just don’t have one yet. So, I guess I will play with it just to see how close it will be, then I guess I will just have to set up targets at different known ranges and shoot….then record the results until I get my dope. I enjoy doing that, I come from a pb background but these airguns are so much fun. I am taking my daughter to the range tomorrow to teach her a little about shooting and didn’t want to hog the rifle figuring out the range to mil dot, but then again it’s zero’d at 30 yrds and that is probably all she will need. Though it’s a real confidence builder for someone who has never shot before to just pick up a rifle and shoot at a very long range and actually hit what you are shooting at.
These Crickets are amazing!
Hi KGB1024
Theoretically it should work. The problem however is that the error introduced by the method is higher than the usual desired estimated accuracy for the speed. Your 70 yard shots are not going to hit the exact point. Even if you did your part well, you will be looking at 1/2 inch or more grouping at 70 yards. that constitutes the error in your drop (y) evaluation measure which will reflect in your estimate of the speed v.
Ignoring air resistance (which is fine for the purpose of this error calculation) one can show that the error in estimating the speed is approximately
delta_v = (g/distance) * v^3 * delta_y
substituting an optimistic 0.5 inch for delta_y will lead to delta_v approximately 20 fps for v of 900 fps. This mean if your method resulted in 900 fps, its really means 880-920 fps, and that’s assuming you got a 0.5″ grouping. This is quite a high uncertainty in comparison to what shooters look for.
So bottom-line is yes you can do it, but with great source of error.
Awny
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Chrono connect is very accurate if you measure the distances correct. Test ir with my chrono
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