Chronos and their angle of inclination

[Road Runner]

This info comes from a post of mine on AirGunForum which is about chronographs and their use in competitions. The chrono at some national FT shoots has been housed on a stand that points downwards at 30° and there has been some comment about whether or not this inclined angle makes any difference in its readings.

Trials done today using a Daystate CRX, Daystate 4.52's sized, 8.43 grain, 12-shot strings, red Chrony Alpha, shots: 40° down, 40° up, 40° down, 40° up and lastly horizontal. The angles were set with a 1-meter builder's level with an adjustable bubble.

40° was chosen to emphasise any effect due to gravity etc. The fastest and slowest readings were discarded from each 12-shot string to reduce variability within each sample. The downwards and upwards strings are shown together in the table of results for greater clarity. Average results:

... 40dn1 40dn2 40up1 40up2 Horiz
Mn 787.8 783.8 759.7 755.8 770.4
SE 00.66 00.75 01.75 01.03 00.99
SD 02.00 02.37 05.54 03.27 03.13
Ra 05.70 07.10 17.40 13.10 10.20
Mi 785.2 780.1 753.9 749.4 765.5
Ma790.9 787.2 771.3 762.5 775.70

ft lb 11.62 11.50 10.81 10.70 11.11

Key: Mn - mean, SE - standard error, SD - standard deviation, RA - range, Mi - minimum, Ma - maximum

The important figures are those for mean ft lb. Shooting downwards at 40°, they were 11.62 & 11.50. Shooting upwards at 40° they were 10.81 & 10.70. Shooting horizontally (only one trial) the mean velocity was 770 fps, as mentioned in a previous post of mine (equating to 11.11 ft lb).

On this evidence, I would suggest that there is a definite effect on chrono readings when the chrono is inclined at an angle.

I appreciate that my chrono may not read the same as the certified BFTA chrono/s but this is all relative. There does appear to be a principle of physics(?) to be considered. Please discuss (that sounds like an exam question!).

Maybe someone can do some better tests than mine.

Keith

[Murphy]

Another factor in the inconstancy of testing airgun power.

[Davestate]

From your results I'd suggest gravity on the hammer is playing a big part.

[supersonic]

How far away was the chrono ?

[robinghewitt]

Angling the chrono is fine so long as you angle the gun the same amount.

8 degrees out will read 1% slow because the pellet has to travel 101% of the distance to reach t'other end.

More than 8 degrees out and it goes to hell in a handbasket PDQ

[Road Runner]

How far away was the chrono ?

Approximately 3 inches - I didn't use a jig. The inconsistency is perhaps illustrated more in the uphill shots where the range/spread of results is wider.

I thought it interesting that the horizontal string fell in between the up and downward inclinations quite neatly.

Keith

[master_shriller]

I have a IR CED Millenium chrono, which I noticed the readings vary slightly depending on whether the rifle is held perfectly level or not.

So I would think if the chrono was tilted up or down it would affect the accuracy of the readings, as the pellet would be travelling different distances.


Ben

[Road Runner]

I should emphasize that 40° is quite a steep angle. It was chosen to exaggerate any difference that might occur. The BFTA chrono has been operated at 30° (I believe) this year and results are quite likely affected in this way.

Kkeith

[sniper-wolf]

Before I shot the Welsh GP I set my gun to 775fps on my Skan MK3. Got to the GP and put the gun through the BFTA chrono, the first reading was 809 the second reading was 805 and the third reading was 795. I was so annoyed that my gun was slightly over that when I got home I borrowed my mates PM1. I put a 20 shot spread through both chrono's and also through my little Chrombi (the one that clips on the end of your barrel) and none of the 60 shots were over 780fps.

So what did they do, you guessed it, the same again at Milride. I set my gun to 765 before I shot Milride, and the BFTA chrono showed 787, bit of a difference.

In my opinion the chrono should be set level, like it says in the handbook, using a spirit level.

[Road Runner]

So I would think if the chrono was tilted up or down it would affect the accuracy of the readings, as the pellet would be travelling different distances.

I'm not sure about the different distances aspect because the chrono sensors are still the same distance apart and the gun is tilted parallel to the chrono so the pellet still travels (more or less) the same distance between the sensors? I say more or less because without using a jig it is not very easy or comfortable to hold the gun at the same angle each time freehand, so to speak.

Keith

[Road Runner]

In my opinion the chrono should be set level, like it says in the handbook, using a spirit level.

Not having a Skan, I haven't seen their handbook, but there is probably a good reason for their telling you to set it that way!!!!

Keith

[Gwylan]

I was with Keith when he did his test. I've looked at the trigonometry, and an additional downhill componant of the muzzle velocity results in an increase of only about 0.25 fps. I think its the hammer, as a downward angle of 40 deg gives an increase in effective mass of 63% of the hammer. The hammer will, IMO, be more efficient at opening the valve. I'm going to try the same thing with my S400, and ProTarget. I bet the results will be different, as the hammers are not the same as Keiths. I'll try with my springer as well, bet the results will be different again. Whatever the results, I think all chronoing should be on the level, ( )..!

Gus.... PS there is a bit in this month's AGW about BFTA chronos.

[Road Runner]

I have just posted a drawing here that shows interpolated data using the data obtained today for + & -40° and the horizontal position.

I think Gus is correct - different rifles will probably show different effects. It comes down to one thing - if the manual says to set the Skan up horizontal, I imagine that is the best way to use it. It would stop all this experimentation and we could get on with some proper shooting!

Keith

[karlgixer]

i found the same sort of thing happend to me,

if i had gun straight and chrono straight it read 11.3flb.

if i angled gun down 30 digrease and chrono straight it read just under 12flb and i mean just under.???????????

[Road Runner]

i found the same sort of thing happend to me,

Thanks - nice to know I'm not the only one!

Keith