New Oehler 35P, New skyscreen rail, First Test Data

[Pdvdh]

Hi Jeff,

Review Mikecr's Post number 30, and my Post number 32 within this Thread.

It will be normal to obtain different readings if the skyscreens aren't mounted with the correct spacing. I mean that they have to be mounted with a separation distance that exactly matches the value that's been programmed into the 35P computer. When the bullets pass over the start and stop skyscreens, they trigger an electronic clock in the chronograph computer. The owner/operator of the chronograph has to program the chronograph for the actual separation distance of the two Primary skyscreens. With the Oehler 35P, the skyscreens can be mounted at various separation distances. The 35P computer must be programmed for the operator-selected distance by setting several small switches - per the owner's manual/instructions. The 35P computer comes shipped from the factory set up for a 4' Primary screen separation distance. 4' for the Primary skyscreens automatically means 2' of separation for the Proof skyscreens. You want to measure the separation distance between the skyscreens and ensure they're mounted to exactly match the 35P programmed separation distance. I use a 6' and 3' spacing distance with my 35P. The farther the separation distance, the less error that will be entered into the recorded velocities due to slight error that may exist in the mounted separation distance between the skyscreens. The drawback to very large separation distances is that it's much less convenient to pack around a skyscreen rail that's 10 feet long, than one that's 4 feet long. Oehler settled on the four foot long skyscreen rail as compromise for user convenience.

Let's see if I can illustrate how a greater separation distance between the skyscreens serves to increase the accuracy of the recorded bullet velocity with these examples:

Example 1) If the 35P computer is programmed for a 20 foot separation distance, but the Primary skyscreens are actually separated a distance of 20.03 feet, one should expect the recorded velocities to be slower than the true bullet velocity by this amount: [(20.03-20)/(20)]x(100) = 0.15%. A 0.15% error in recorded bullet velocity. If the true bullet velocity is 3000 feet/sec, this 0.15% error would cause the chronograph to record a velocity which is 99.85% of the true velocity. A velocity of 0.9985 x 3000 = 2995.5 feet/sec. An error of 4.5 feet/sec. Not a severe amount of error.

Example 2) On the other hand, if the 35P computer is programmed for a 4 foot skyscreen separation distance, but the Primary skyscreens are actually separated a distance of 4.03 feet, one should expect the recorded velocities to be slower than the true bullet velocity by this amount: [(4.03-4.0)/(4.0)]x(100) = 0.75%. Now we see a 0.75% error in recorded bullet velocity. In both this example, and the prior example, the skyscreens have been mounted too far apart by a distance of 0.03 feet. However with the 4 foot skyscreen spacing, the resultant error in recorded velocities is increased by a factor of 5. Now; a bullet with a true velocity of 3000 feet/sec will record a velocity which is 99.25% of its true velocity. The 35P will record a velocity of (0.9925) x 3000 = 2977.5 feet/sec. An error of 22.5 feet/sec.

You stated that the Primary and Proof velocities differed by ~ 20 feet/sec. If the Primary skyscreens are mounted either 0.03' closer than 4 feet or 0.03' farther than 4 feet, then the recorded velocities will be off by about 22.5 feet/sec. As they were in my Example 2) - above. 0.03 feet is equal to 0.36 inches. About 3/8".

If your friend's 35P is programmed for Primary skyscreen separation of 4', then the Proof skyscreens should be mounted exactly 2 feet apart. If the Proof skyscreens are mounted either 0.015' closer than 2' or 0.015' farther than 2', then the 35P will again record velocities which are in error by approximately 22.5 feet/sec.

Sorry if this is mind-numbing overkill. Forgive me, as I am an engineer by training and profession.

Bottom line is your friend's 35P skyscreens are mounted on his rail at distances which are either too far apart, too close together, or a combination of too close and too far - between the Primary screens and the Proof screens.

If you only have a single 35P chronograph to work with, my recommendation is to carefully mount the Primary skyscreens as exactly as possible to match the distance the 35P computer is programmed for. I measure from the leading edge of the first skyscreen to the leading edge of the 2nd skyscreen.

Next mount the middle Proof skyscreen exactly 1/2 of the distance programmed into the 35P. The Proof skyscreens are always exactly 1/2 the separation distance of the Primary skyscreens. Again measure from leading edge to leading edge of the Proof skyscreens.

After all three skyscreens are mounted securely with exactly the correct separations - shoot over the skyscreens. At this point there should be very little difference in the recorded velocities. And there's no certain way to know which velocity is closer to the true velocity, unless you also have another unit which is known to be calibrated and certified to read the exact true velocities. But - you CAN know that the Primary channel is likely to have less error than the Proof channel, because the Primary skyscreens are separated twice as far apart as the Proof skyscreens.

So - IF your Primary velocities are reading slightly faster than the Proof velocities, DECREASE the separation distance between the middle Proof skyscreen and the first Primary skyscreen a little bit until the Primary Channel and the Proof Channel are in very close agreement.

IF your Primary velocities are reading slightly slower than the Proof velocities, INCREASE the separation distance between the middle Proof skyscreen and the first Primary skyscreen a little bit until the Primary Channel and the Proof Channel are in very close agreement.

By changing the location of the Proof (middle) skyscreen on the skyscreen rail, you will be able to calibrate the Proof channel to record virtually the same velocities as the Primary channel.

With my setup, I had three separate chronographs with 7 different skyscreens. Two skyscreens each for the Oehler 33 and the PACT. Three skyscreens for the Oehler 35P. I very carefully measured and mounted the skyscreens to the distances programmed into each of the three different chronographs. When I shot some 22 LR rounds over my triplicate Chrono setup, the velocities being recorded from all three units was just about identical - except that the 35P Proof Channel was recording about 6-7 feet/sec faster than the rest. So I tweaked the middle skyscreen of the 35P farther apart until the velocity from the 35P Proof channel matched the other three recorded velocities. Because I have three units and all three recorded virtually identical velocities after carefully mounting the skyscreens, I have some added confidence that the velocities being recorded are very close to the true bullet velocity. If I had six units all running at the same time, and the six units all recorded the same velocity after setup, my confidence could be about twice as great as with the three units. And so on and so on...

Did my best to explain this. My back, wrists and fingers are failing me, so I need a break! I hope this helps explain what the problem is, and what the solution is. If you were consistently seeing a 20 feet/sec difference between the two 35P channels, then it's a sure bet that the screens aren't mounted to the exactly correct separation distances.

Am I long-winded or WHAT??? Time for a glass of wine. But do let me know if you have questions. I'll try to help - to the best of my ability.

 

[Pdvdh]

Great job Paul! It is very nice to get some data other than my own CRUDE stuff.
Rich

I don't necessarily buy into the "CRUDE stuff" part Rich. The accuracy of the bullets you've manufactured leads me to believe you're maintaining VERY tight tolerances. Which means you've got some quality equipment. But you might could use a chronograph setup like mine to dial in the finished product BCs with some velocity-based BC calculations!

 

[Broz]

Thanks Paul, Thats what I figured and I asked about spacing. He stated there were indentations in the rail to mount the screens on. If I use it again we will tweek it in and I will pass this info on to him.

Thanks

Jeff

 

[Pdvdh]

I'm not using the Oehler provided skyscreen rail because I wanted a greater separation distance, and I also wanted to mount 7 skyscreens on a single rail. So I had to fabricate a rail of my own.

The Oehler rail consists of two pieces of 1/2" EMT conduit. They do indent the rail in three locations at their factory for the mounting of the three skyscreens. My guess is the indentations are off a little bit, or else the skyscreen mounting bolts aren't properly positioned in the Oehler indentations. I never used the Oehler supplied rail even once, so I don't know how accurate the indentations were on the rail they shipped with my unit.

My homemade rail is constructed of a solid 6' 8" long piece of 5/8" diameter aluminum. So I moved the skyscreens to the proper locations, tightened down the holding bolts, and then wrapped some tape around the aluminum rail on both sides of the skyscreens so I'd be able to tell if they ever shifted location during transport. I also welded on some protective aluminum flanges to help shield the skyscreens from the lumps, bumps, and jars associated with transporting them to the field and then back home again in my vehicle.

 

[elkaholic]

I don't necessarily buy into the "CRUDE stuff" part Rich. The accuracy of the bullets you've manufactured leads me to believe you're maintaining VERY tight tolerances. Which means you've got some quality equipment. But you might could use a chronograph setup like mine to dial in the finished product BCs with some velocity-based BC calculations!

Paul......Actually, I was referring more to my testing being crude than my bullets. I would like to upgrade my testing so I guess I'll have prioritize my purchases! This shooting stuff can get addictive........Rich

 

[royinidaho]

Rich,

Have you ever tried tips on those suckers?

 

[Pdvdh]

Re: SXR 190 gr bullet

Paul......Actually, I was referring more to my testing being crude than my bullets. I would like to upgrade my testing so I guess I'll have prioritize my purchases! This shooting stuff can get addictive........Rich

Yeah,
I understood what you were saying Rich.

Roy,
Rich has installed aluminum tips on these 190 gr SXR bullets I'm trying out. Let me see if I can find and post a picture... Here's a photo of the unfired bullet, one fired into water with an impact velocity of 2500 fps, and one fired into water with an impact velocity of 1400 fps. Complete expansion at only 1400 fps. Nice!

 

[elkaholic]

Rich,

Have you ever tried tips on those suckers?

Hi Roy.....Ya, I have two different tips that I use. One is brass and one is aluminum. They come to a needle point which helps raise the b.c. a little. Last year they accounted for a moose at 720 yards, a mule deer at 680, and an elk at "powder burn". So much for LONG RANGE! So far, they have performed very well and I find them to be super accurate. I shot some 2-3" groups at 880 last year, preparing for hunting season........Rich

 

[royinidaho]

Rich,

What's your source of brass tips?

PM me to keep from messing up this thread. OK.

I need details on procurement of said tips.

Thanks

Roy

 

[Pdvdh]

Dusting off a 5 1/2 year old Thread and adding some additional input after the additional experiences using my triplicate chronograph.

Rehashing for those only reading this last Post, I run three chronographs concurrently, meaning all three chronographs record the velocity for each bullet fired. One chronograph is an Oehler 35P, one an Oehler 33, and one a PACT PC2. There are three skyscreens for the 35P, two for the 33, and two for the Pact PC2. All seven skyscreens are mounted on a single, rigid skyscreen rail. The skyscreen spacing is 6' for both Oehlers, and 4' 8" for the PACT PC2. Photos of the setup, showing all 7 skyscreens mounted on the skyscreen rail, were provided in the first Post.

I've been out twice in the past week and shot ~50 rounds of .223 Remington handloads over the chronographs. Bullets were the Hornady 60gr .224 VMax. Here's a comparison of the velocities recorded for the last 40 bullets fired:

I've been using this triplicate chronograph setup for over five years now, and have recorded hundreds and hundreds of bullet velocities. The velocities recorded and presented in this spreadsheet are representative of the quality of the data I normally obtain. The maximum difference in bullet velocity recorded between the three chronographs for these 40 bullets was 8fps. The last column is a calculation of the maximum difference in the three velocities recorded, divided by the average of the three velocities for each bullet. I consider this to be equivalent to the "percent error", assuming that the average velocity is as close to the true bullet velocity as I'm ever going to measure. In these 40 shots, the maximum error occurred with the 19th shot fired. The average velocity for shot 19 was 2828.0 fps, with a maximum difference of recorded velocity of 8fps between the three chronographs. The percent error for that bullet was 0.28%. The average percent error for all 40 shots is 0.164%.

The only time my chronographs act up with any regularity is in fading light conditions. As the light dims in the evening, the PACT PC2 is always the first chronograph failing to record velocity. The Oehlers will continue to record bullet velocity, and continue to provide very comparable data, after the PACT fails to provide any reading. As light lessens even further, the Oehlers will begin to provide inconsistent velocities. I know it's time to quit when the PACT stops working in fading light. I believe the Oehler skyscreens are better able to signal the computer than the PACT skyscreens in low light conditions, and that's why the Oehlers continue to operate in poorer light. PACT may have improved their skyscreens by now. I have no way of knowing. No way to compare their current skyscreens to my skyscreens.

Other than poor/dim/failing light conditions, I obtain very comparable data from all three units. HOWEVER, I do occasionally obtain a bad velocity from any one of the three chronographs, with about equal consistency, without any obvious explanation. Any recorded velocity varying by more than 10-15fps from the other two is suspect. Ken Oehler states that every chronograph can and will sporadically produce bad data, and my experience confirms his statement. The only way to recognize faulty data is to have a second instrument recording the same bullet. The bullet speed is the same over all the skyscreens, so if one unit records a velocity significantly different than the others, that confirms it puked out a bad velocity. That's why the Oehler 35P has a proof channel which provides a 2nd, separately recorded velocity for each bullet fired. It's their effort to allow the user to identify bad/faulty data. With my triplicate setup, I receive 4 recorded velocities for each bullet fired. Two from the Oehler 35P and one from each of the other two chronographs. I can almost ALWAYS identify bad data from any of the three chronographs, because two will almost always be in agreement. So I reject the bad velocity from the faulty recording, and use the average velocity from the other two functioning chronographs as the "true" bullet velocity.

Thought this might be of interest to some of the members... those still using light sensing chronographs...

 

[elkaholic]

Nice work Paul! I may have to dig out my Pact and see how it compares to my Magneto Speed......Rich

 

[Trickymissfit]

I have the Pact PC2, and have had for a very, very long time now. Right after buying it a shooting buddy a deal on the Ohler 35. I think he had a 33 or else another guy we shot with did (I'm getting old, and this is close to twenty years now. When I bought the Pact, it was the new kid on the block, and nobody (literally) had ever seen one. I bought the optional rail mount at the same time, and range setup was a breeze. The 35 was a pain in the butt, but I later fixed this for Doug. Then it was a ten minute job at the max. The new mount for the 35 cut the spread in half, but at the same time there was less than six or seven FPS difference between the two systems. Never, of course knowing which unit was correct. The 33 was all over the place, but think most of that was from the Ohler mount moving all over the place. Keep in mind that we ran the Pact and the 35 in a tandem setup, rather than mount everything on the same rail (we were not interested in running anykind of a comparison by the way). Each unit had it pluses and minuses, and I'd been happy with either one. Thru the years I could probably count less than ten miss reads on the Pact, and yes we saw them on the Ohlers as well. A few guys have had problems with the Pact in low light; I never have. Yet when I first got it and was learning how it works; I saw problems when shooting thru it in extreme bright like at noon. I made a slight revision with the sky screens. I put tin foil ontop of the bridge over the screens. Helped a lot. Then later removed the tin foil and replaced it with a grey / gold mylar film that computer circuit boards are shipped in. That was what it needed!

For me (anyway) aligning the light bar to the barrel has always been a PIA! Didn't matter if it was a Pact or the Ohler. I made A laser affair with a target, and it worked. Yet in very bright light I still had trouble seeing the line. I tried a string with a line level, and it was a cheap fix. No more accurate than the laser, but still got me there. I do think I could have worked a little harder with the laser, and got better results. I was after the idea of getting the bullet centered thru the screens. I honestly think this gives a better idea on what's going on. Of course this is the one serious advantage the Magneto Speed has over the others.

When I bought the Pact, they and RCBS had touted a future software system that would allow the user to simply down load all the data into a note book PC. A year and a half later I asked them where the firm ware update was at. They said there seemed to be zero interest in the shooting community, and I was the first guy to ask about it.
They missed out on that one! Even a simple SD card would have worked.
gary

 

[Pdvdh]

Regarding skyscreen alignment:
I rarely ever set up my chronograph at a shooting range. In fact I rarely shoot at a shooting range. I simply do not enjoy my time at ranges. I don't shoot for the social aspects of it. I have the ability to locate a shooting location where it's only me, myself, and I, which is just about mandatory for me to enjoy the experience.

This allows me to set the skyscreen rail further downrange from my muzzle. Where I currently shoot, the middle of my skyscreen rail is set 40 feet from my muzzle. All I do to align my skyscreen rail parallel to the bullet path is look thru my scopes at their lowest power setting and align the skyscreen rail by eyeballing it through the scope. It takes a few trips back and forth from the rifle to the skyscreen rail until I'm satisfied. The farther the skyscreens are set off the muzzle, the easier it is to get them aligned parallel with the bullet's flight path.

The other advantage of setting the skyscreens farther down range is that I can shoot into four separate targets taped to my cardboard backing at 300yds, and never have to re-adjust either the rifle position, or the position of the sky screen rail.

This makes the setup more accurate (alignment-wise), and more versatile by being able to shoot differing loads into multiple differing targets while never having to re-position anything.

One more thing about the Pact PC2. In my experience, it's more important to have the skyscreen covers installed over the Pact skyscreens than with either of my Oehlers. Last week while recording bullet velocities in the evening, I received no recorded velocity from the Pact for the first two shots fired. I then installed the skyscreen covers and obtain good recorded velocity data for the rest of that shooting event.

I should add that I've upgraded the Skycreens on my old Oehler 33 from the Skyscreen II version to the Skyscreen III version, the same skyscreens that Oehler currently provides with their Model 35P. I talked with Ken Oehler and his wife at the 2009 Shot Show and asked him if there were any options available to improve/update the original Model 33 performance. He asked me to describe which skyscreens I was using, and then told me that their new skyscreens were improved and recommended replacing the Skyscreen IIs with the current Skyscreen IIIs. Which I did. This is another reason I'm convinced that the Skyscreens are the primary reason that both of my Oehler chronographs operate in lower light conditions than the Pact. There was further discussion with Ken Oehler on skyscreens than I've provided in my posts. I came to understand that skyscreen performance was key to good/consistent/repeatable chronograph performance. A computer is a computer, and they're all fast enough to provide accurate timing calculations. The key to reliable chronograph performance is the sensitivity of the skyscreens and their ability to signal the computer accurately, precisely, consistently, time and time again in a variety of lighting conditions.

I've never tried operating my Pact PC2 using the Oehler skyscreens. If the male prongs on the Oehler Skyscreen IIIs are the same size as the prongs on the Pact, I think they would work just fine. This would probably improve the performance of my Pact PC2 in low level light. I'll have to try that the next time I shoot and see what kind of data is recorded. lightbulb

 

[Pdvdh]

I've never tried operating my Pact PC2 using the Oehler skyscreens. If the male prongs on the Oehler Skyscreen IIIs are the same size as the prongs on the Pact, I think they would work just fine. This would probably improve the performance of my Pact PC2 in low level light. I'll have to try that the next time I shoot and see what kind of data is recorded. lightbulb

Today I chronographed a few more rounds. The male plugs on the Pact PC2 skyscreen cables are the same size as the plugs on the Oehler skyscreens. Shots 1-6 were recorded with the normal skyscreen arrangement. Shots 7-9 were recorded after I swapped the Oehler 33 Skyscreen IIIs with the PACT PC2 skyscreens. Both chronographs recorded valid velocities with the skyscreens swapped.

With the Oehler skyscreens on the Pact, the velocities from the Pact increased slightly and almost exactly matched the velocities from the Oehler 35P.

With the Pact skyscreens on the Oehler 33, the velocities from the Oehler 33 dropped a little from the prior velocities, compared to the Oehler 35P velocities.

I'm going to shorten the distance between the skyscreens on the Oehler 33, and then all three units should record the same velocities for each bullet fired. The Oehler 33 has always recorded around 5fps slower than the Oehler 35P and Pact PC2. I think the Pact PC2 would then operate in lower light conditions, comparable to the Oehler chronographs.

So a guy could purchase Oehler's current Skyscreen IIIs and use them on the Pact PC2, if desired. Which doesn't surprise me because Ken Oehler told me the original PACT skyscreens were pretty much a clone of his own design.