Chronograph Weirdness

[catorres1]

So wondering if anyone has seen this before.

I have an Oehler 35P. I have chrono'd loads for my 260 consistently at around 2725, give or take.

Friday, I took it out and tried to chrono the ELDX loads I am making for my 300. Got a lot of errors that I attributed to the lateness of the day and bad lighting.

So I came back today and reshot the loads and for kicks, shot some of my old 260 loads.

Well, oddly, all the loads came back running quite a bit slower...like 60 to 80 fps slower. Even the same loads I shot on friday.

Temps were the same in all conditions, as I have it noted in my log. Cases, primer, powder, bullets, rifles, distance to chrono, everything...all the same.

I talked with Oehler, and will stop by probably tomorrow, they suggested just light conditions could cause this, differing times of year etc.

Not real happy if that turns out to be the case. I really want to know the velocity so I can figure come ups etc.

Anyone else experienced this?

On a good note, the new 300 looks to shoot the ELDX's pretty well, does not seem picky at all and, at least accuracy wise, will do the trick for me, at least the initial results appear to indicate that.

 

[ShtrRdy]

I've experience something similar to what you're describing. It was with a different brand Chrono. I figured it had something to do with the lighting.

 

[catorres1]

Thanks for the reply! I am going to Oehler tomorrow. They seemed very nice on the phone. Hopefully, it's a machine problem. That we can fix. Light, not so much.

 

[FearNoWind]

... I talked with Oehler, and will stop by probably tomorrow, they suggested just light conditions could cause this, differing times of year etc.

I have a Oehler gathering dust in the shop. Similar experiences convinced me I needed a Magnetospeed. Check out their Sporter model. You won't be sorry.

MagnetoSpeed Sporter | MagnetoSpeed

 

[catorres1]

Did you have these same problems with a 35p?

I did talk to a friend today and he may have an extra magnetospeed that he can pass on, he's going to look.

If that's the case, that would be great....easy setup looks a treat!

 

[Greyfox]

I have had issues similar to yours with my 35P and other optical chronographs. It was usually lighting conditions that caused the problem. I solved the problem by chronographing under constant lighting conditions only.... overcast. I now use a Magnetospeed, and just recently acquired a Lab Radar unit.

 

[catorres1]

I may have to go the magnetospeed way. It's hard enough for me to have to get out there with the chrono as it is, let alone waiting for the same conditions.

Hate to spend the money there, I am more of a 'find a load and stick with it' kind of guy, but lately, it seems like I am always having to change for some reason or another!

 

[TOM H]

I may have to go the magnetospeed way. It's hard enough for me to have to get out there with the chrono as it is, let alone waiting for the same conditions.

Hate to spend the money there, I am more of a 'find a load and stick with it' kind of guy, but lately, it seems like I am always having to change for some reason or another!

I have 35P also and I've only had problem couple times with light and they did mention that in their manual. One thing you can do is not plug in proof screen see if that helps and I run mine both ways.

 

[Engineering101]

catorres1

I am a circuit design engineer by trade and have designed and built my own chrono using a 100 MHz time base for more accuracy (10ns resolution). I did the software myself so it does only what I want it to do etc. In the process of that I learned a few things about chronos. As applies to your situation I would first suspect alignment as a chrono that is off 10 degrees in pitch and yaw will read a 30-06 about 100 fps slow. Perfect alignment will give perfect results but a chrono will never read fast from an alignment error. I use a boresight laser and see through targets mounted on the chrono to align mine and I never have issues. It is always repeatable. Good luck.

 

[catorres1]

catorres1

I am a circuit design engineer by trade and have designed and built my own chrono using a 100 MHz time base for more accuracy (10ns resolution). I did the software myself so it does only what I want it to do etc. In the process of that I learned a few things about chronos. As applies to your situation I would first suspect alignment as a chrono that is off 10 degrees in pitch and yaw will read a 30-06 about 100 fps slow. Perfect alignment will give perfect results but a chrono will never read fast from an alignment error. I use a boresight laser and see through targets mounted on the chrono to align mine and I never have issues. It is always repeatable. Good luck.

Hmm, okay you may be on to something here! I was having a hard time getting everything as level as I liked, and it was super windy, so at one point, the chrono "pitched" I guess, so I had to stop and reset it. So maybe, just maybe, that is what caused the problem. Maybe stuff was off slightly.

I went to Oehler today. I will say that they are absolutely great people! They took my machine apart and checked and tested everything. No problems at all. They tested the sensors. Two seemed to be a little low in their sensitivity, so they replaced all three sensors.

But they told me that honestly, they did not think they fixed anything, as the sensor will 'fire' or not, but that would not cause a low reading, but to give it a try and then call them back.

So what method do you use to check pitch and yaw? I did not quite understand...do you mount targets on each screen and then laser through them to assure alignment?

 

[Pdvdh]

One straightforward way is to increase the separation distance between your skyscreens. This will make it easier to ensure the skyscreens are parallel to your bullet flight path. I set mine on a 6' 9" long rigid skyscreen rail, which allows for a 6' separation distance.

It's much easier to align the skyscreens when they're mounted 6' apart, than 2 or 4' apart. Of course it becomes less convenient to transport and store the unit. It's a compromise. 12' spacing would be even easier to align, and would decrease the error in the readings. But you'd need a trailer to haul it around.

You can find some pics of my setup at this Thread:
http://www.longrangehunting.com/forums/f19/new-oehler-35p-new-skyscreen-rail-first-test-data-60778/

 

[Engineering101]

catorres1

Yeah, I basically do what you said. I mount a see through plastic target (cut from a product container I bought) on each skyscreen where the bullet is supposed to go. The plastic target is marked with a grid. Both targets are identical. If you point your rifle at the paper target downrange (you need a rest and rear bag to hold the rifle still) and then insert the bore sight laser and then align the chrono in pitch and yaw so the laser hits both targets in the same spot you should be good. Even then I still use a final visual verification looking through the scope on low power to verify that the uprights are all visible and lined up with each other as they should be.

By the way, pitch is the direction of movement you get on a rollercoaster as you top out and head down and yaw is the direction you turn your head when looking left and right before crossing the street. The third axis is roll which has no effect on chrono accuracy.

 

[Rckymtnshooter]

I chased around chrono readings and realized that shooting in different types of light was my problem. I bought some infrared sky screens and built a box with a lid. Once I was controlling the lighting my readings became consistent.

 

[375fan]

Have a PACT ii that I've used for years. Would get strange readings depending on lighting conditions. Finally figured it out. Bought a Magnetospeed about 3 years ago, love it. Don't have to try and wait till conditions are always the same, best part, not having to deal with lining up barrel, screens and target.