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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
A tale of two chrony's
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<blockquote data-quote="LouBoyd" data-source="post: 344949" data-attributes="member: 9253"><p>All electronic chronographs have a crystal controlled time base. Those rarely go wrong and can generally be counted on to have an accuracy of 1 part in 100,000 or better. Most of the commercial chronographs have a very short measurement base. If you have a 100 fps error at 3000 fps thats a 3% difference, If the chronograph has an 18" measurement base thats only 0.54 inches bullet travel error which is shorter than the length most bullets. A difference in the lighting on the two screens can cause that much error. The sensor is trying to detect a very tiny drop in the light coming from a fan shaped area above the sensor. If one sensor is triggering on the meplat and the other is triggering on the (larger) shank of the bullet thats over a 100 fps velocity error on a typical 7mm VLD. </p><p></p><p>I can't tell you where the 100 fps error is coming from between your two units,. It's possibly the sensor spacing but more likely to be a sensitivity difference in one of the sensors. That could be from a difference in lighting (misaligned sky screen?) , contamination on a len (dust?), a difference in shot placement over the sensors(unit tilted relative to the trajectory?), or maybe just a manufacturing difference in the sensors. It's possible but very unlikely to be in the electronics. </p><p></p><p>I've used an Oehler model 35 for many years. It came from the factory with a 2 foot spacing rod for the sensors. Thats 1 foot between sensors. After a few sessions I made an 8 foot spacer and that improved my handload's measured SDs noticably. That's not to say the Oehler is a bad unit, only that you can't make reliable velocity measurements with a 1 foot measurement base unless the sensors and their alignmet are near perfect. Its also easier to make good measurement on stubby wadcutters than on long tapered VLDs.</p></blockquote><p></p>
[QUOTE="LouBoyd, post: 344949, member: 9253"] All electronic chronographs have a crystal controlled time base. Those rarely go wrong and can generally be counted on to have an accuracy of 1 part in 100,000 or better. Most of the commercial chronographs have a very short measurement base. If you have a 100 fps error at 3000 fps thats a 3% difference, If the chronograph has an 18" measurement base thats only 0.54 inches bullet travel error which is shorter than the length most bullets. A difference in the lighting on the two screens can cause that much error. The sensor is trying to detect a very tiny drop in the light coming from a fan shaped area above the sensor. If one sensor is triggering on the meplat and the other is triggering on the (larger) shank of the bullet thats over a 100 fps velocity error on a typical 7mm VLD. I can't tell you where the 100 fps error is coming from between your two units,. It's possibly the sensor spacing but more likely to be a sensitivity difference in one of the sensors. That could be from a difference in lighting (misaligned sky screen?) , contamination on a len (dust?), a difference in shot placement over the sensors(unit tilted relative to the trajectory?), or maybe just a manufacturing difference in the sensors. It's possible but very unlikely to be in the electronics. I've used an Oehler model 35 for many years. It came from the factory with a 2 foot spacing rod for the sensors. Thats 1 foot between sensors. After a few sessions I made an 8 foot spacer and that improved my handload's measured SDs noticably. That's not to say the Oehler is a bad unit, only that you can't make reliable velocity measurements with a 1 foot measurement base unless the sensors and their alignmet are near perfect. Its also easier to make good measurement on stubby wadcutters than on long tapered VLDs. [/QUOTE]
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A tale of two chrony's
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