Another chronograph question

[ChuckS1]

This is driving me nuts.

Went out today with my Chrony Beta. Sunny, about 9:15 AM, 72 degrees and a little breeze. Sun is coming up behind me, at about my 7:00. Sun is peeking through the trees and there are intermittent shadows on the chronograph. I have it set up 15 feet from the bench, with both sets of rods installed and the diffusers. New battery installed. Chrony aligned with the target. Tripod leveling bubble and torpedo level on the chrony both say everything's level.

First round from my USFA SAA and 6.3 grains of Unique and a 454424. Velocity reading is 301.
No error codes indicating that either sensor missed the bullet. Next 9 rounds all have very low readings, about half of what I know the fps should be.

So, does the sun's angle come into play here? I see that sunlight is hitting the first sensor, but the second is in the shade. I've tried putting a piece of black craft paper on top of the diffusers to form a solid surface and that's seemed to have helped before when the sun is higher, but not today.

Is there any chance that using both sets of rods (about 14" long) with the diffusers is contributing to it? The Chrony user manual really doesn't say to use both and sort of implies to just use the 9" rods. I've always used them both and I've gotten good readings with both. Never tried just the 9" rods alone.

I would shoot later in the day to get the sun directly over head, but it's nice when the range first opens so that I have time to get everything set up, less crowded, etc. Seems to me there's a simple fix here, but not one that's intuitively obvious.

I'm thinking a cardboard box over the top and sides, or a white 5 gallon bucket with the bottom cut out (saw that on another thread) might be the answer.

Anyways, longer post than I wanted, but I'm sure I'm not the first with this problem, so any help would be appreciated. Thanks.

 

[ChuckS1]

This is driving me nuts.

Went out today with my Chrony Beta. Sunny, about 9:15 AM, 72 degrees and a little breeze. Sun is coming up behind me, at about my 7:00. Sun is peeking through the trees and there are intermittent shadows on the chronograph. I have it set up 15 feet from the bench, with both sets of rods installed and the diffusers. New battery installed. Chrony aligned with the target. Tripod leveling bubble and torpedo level on the chrony both say everything's level.

First round from my USFA SAA and 6.3 grains of Unique and a 454424. Velocity reading is 301.
No error codes indicating that either sensor missed the bullet. Next 9 rounds all have very low readings, about half of what I know the fps should be.

So, does the sun's angle come into play here? I see that sunlight is hitting the first sensor, but the second is in the shade. I've tried putting a piece of black craft paper on top of the diffusers to form a solid surface and that's seemed to have helped before when the sun is higher, but not today.

Is there any chance that using both sets of rods (about 14" long) with the diffusers is contributing to it? The Chrony user manual really doesn't say to use both and sort of implies to just use the 9" rods. I've always used them both and I've gotten good readings with both. Never tried just the 9" rods alone.

I would shoot later in the day to get the sun directly over head, but it's nice when the range first opens so that I have time to get everything set up, less crowded, etc. Seems to me there's a simple fix here, but not one that's intuitively obvious.

I'm thinking a cardboard box over the top and sides, or a white 5 gallon bucket with the bottom cut out (saw that on another thread) might be the answer.

Anyways, longer post than I wanted, but I'm sure I'm not the first with this problem, so any help would be appreciated. Thanks.

 

[Erich]

You have correctly diagnosed the problem; you need to keep the sunlight from hitting the electronic eyes directly. Those diffusers aren't halfway enough here in the arc lamp New Mexico sun. I tape up folded paper plates in front of the electric eyes to keep the sun from shining directly onto them. It looks like crap, but it works great.

 

[tdan]

Next 9 rounds all have very low readings, about half of what I know the fps should be.

Make sure that your start/stop skyscreen wires are hooked up to the unit properly. I hooked up my wires backwards on my Pact once, and I got readings exactly like yours........about half the velocity that should have appeared.

 

[Erich]

Not an issue with Chrony products.

 

[Steve C]

The chrony works by reading the break in light from the shadow of the bullet as it passes over each sensor. These sensors are very sensitive to light changes. The time vrs the distance determines the velocity. The difusers screens are there to make the lighting as uniform as possible.

Anything that interferes with the light sensor, flashes of light from lightning, mirror reflections, shadows over one sensor, passing shadows, etc. will cause the unit to read incorrectly.

 

[G-ManBart]

Originally posted by Erich:
You have correctly diagnosed the problem; you need to keep the sunlight from hitting the electronic eyes directly. Those diffusers aren't halfway enough here in the arc lamp New Mexico sun. I tape up folded paper plates in front of the electric eyes to keep the sun from shining directly onto them. It looks like crap, but it works great.

I'm going to build a chrono box one of these days, but an IPSC/USPSA Metric target upside down on top of the diffusers works pretty well to keep direct light off the sensors and the white background seems to help as well. R,

 

[Dale53]

The idea is to shade the sensors. A number of practical ways can be devised. One is to simply put a target stand (the portable type with a full size silhouette target on it) in such a way as to shade the screens.

Eric's way works just fine for him and anyone else who wants to try it.

Milk jugs are translucent and you should be able to cut screens from them (they won't block the light, just INTENSE light) to fasten to your sensor set up.

Dale53

 

[Ignatz]

The one time I went to the range to chrono I too was getting really low readings. Here my unit got switched to Meters per second instead of feet per second. I'm at the point where I only chrono on overcast days. Right before it rains seems to give me the most consistant results....Results I can believe. Luckily in Pa.,we have plenty of overcast days!