Oehler 35P Questions

[LRHWAL]

Ray,

I will need to look through my manual, but those variations are certainly not startling to me and it seems within what Oehler believes the accuracy of the unit is. I've sometimes been hasty when setting up screens and have had the proof screen pick up an error with almost all shots until I adjusted spacing. I suggest you carefully measure the set up anyway, just in case.

I've run my unit back to back with others on several occassions and it is very telling to notice how much the readings between units vary and with the Chrony units that I've run these with (not knocking them, just that those are the only units I've tested with so far) there seems to be no trend at all. Some will read slower, some faster. Some readings will be quite close between the two units, some will vary a great deal. One unit will end with a summary of a string with an SD of 5 and the other an SD of 15! Of course the Chrony unit has screen spacing that counts against it IMHO.

I still like my proof channel and there seem to be fewer extreme readings than with the other units I've tried, but that can simply be luck. I have an Oehler 33 too and I will run them end to end sometime and report back.

I'll be away from my PC for the next week and I'm not sure when I'll get to run the test, but I'll revert some time.

I just think that a good dose of circumspection should accompany our deductions made from the data that these essentially "hobby" type units give us.

 

[Pdvdh]

LRHWAL,


Oehler Print Out:
..A.. . . . ..B.. .
2746-01-2738 = Difference 12
2711-02-2711 = Difference 0 . . . [Did you type these two velocities correctly? 2711 & 2711]
2724-03-2714 = Difference 10
2736-04-2721 = Difference 15
2747-05-2734 = Difference 13
Delta of the Differences = 15-0 = 15
ES A=36
ES B=27
(ES A - ES B) = 9

Delta of the Differences excluding shot 02 = 15-10= 5
ES A=23
ES B=24
(ES B - ES A) = 1

Here's one more:
2761-01-2749 = Difference 12
2772-02-2760 = Difference 12
2752-03-2738 = Difference 14
2750-04-2736 = Difference 14
2753-05-2743 = Difference 10
Delta of the Differences = 14-10 = 4
ES A=22
ES B=24
(ES B - ES A) = 2

Once again, I am assuming and these numbers might be normal. If they are normal, I do not see how they are of any use to me. Maybe someone can explain this?

Ray

Here's the explanation:

The delta of the differences in your first data set is 15. This is high in my experience. One clue that you may have recorded one or more bad sets of data in this first data set is that the ES off chrono A is 9 fps higher than the ES off chrono B. You either transcribed errant velocities for shot 2 in your first data set, or else the chronographs recorded compromised data on the 2nd shot in this data set. I would reject the 2711 fps data from both chrono A and chrono B and not include them in any analysis of this first data set. The delta of the differences of the remaining four shots (shots 1, and shots 3-5) is 3. A delta of 3 demonstrates very good correlation of recorded velocity for the other four shots. Collecting the dual set of data using your proof channel quickly allowed identification of bogus data for shot 2. All the other shots fired over the dual channels of your chronograph recorded a velocity difference of from 10 to 15 fps. When you see a 0 fps difference on shot two, it's apparent that one or both channels of your chrono burped out bad data. The difference in ES will always be less than or equal to the delta of the differences in velocities.

The delta of the differences in your second data set is 4. This demonstrates good, reliable chronograph operation and valid data. Take notice that the difference between the ES from chrono A and chrono B is 2 fps versus the delta of 4. The difference in ES between the two chronos/channels has to be less than (or worst case equal to) the delta of the differences between the chrono A/chrono B data sets.

I've labeled the first column of recorded velocities A in order to define that data column as belonging to chronograph A or channel A. The second column is labeled B to indicate that this data column came from chronograph B or channel B, for purposes of explaining if "chronograph A" data is consistently 10 to 15fps faster than "chronograph B", then you're golden. If you want to sell your 35P, I'm ready to purchase.

There is no way of absolutely knowing which chronograph (A or B) (primary or proof channel) is recording the velocity closest to the exact true velocity of the bullets passing over your skyscreens. If you want to adjust the skyscreen spacing of one set of skyscreens in order to obtain closer matched data sets for each shot, your could do that, but you have no way of knowing whether Chrono A data or Chrono B data is the more accurate data.

I would carefully adjust the separation of each set of skyscreens as closely as possible to the setting you've entered into your 35P. I think you said 4' spacing. I don't own a 35P, but if both sets of skyscreens are precisely positioned at a 4' spacing, then after reviewing and validating the quality of the data (in this case tossing out shot 2 in the first data set), I would simply take the average the two velocities for each bullet and accept the average velocity as the best-you-can-do velocity.

The advantage of running dual chronographs and collecting two velocities for each shot fired are: 1) identification and rejection of bad data, 2) ES and SD can be determined with a relatively high degree of precision [with about the same precision as the delta of the differences of velocity between the two chronographs - in your case within ~5fps], 3) the average of two velocities is likely to provide a more accurate velocity than a single recorded velocity.

If you had 10 sets of Oehler 35Ps and you shot over all 10 concurrently and took the average of those 10 velocities - well that's about the best you could hope for in IDing the true velocity. Unless you shot over 20 sets of 35Ps. Or 30. Or 40.

With your 35P, the difference in velocity between chrono A and B is only 10-15 fps anyhow. You could shoot a very, very long distance, before a 10 fps error in MV entered into a ballistics program would be responsible for the missing of a deer-sized game animal because of the bullet going over or under the animal.

Like I said, I don't own an Oehler 35P. But 'Jon A' (User Name) owns two sets of 35Ps and he consistently reports a single-digit delta of the differences in velocities recorded on his. I provided the links in a prior post for your use.

 

[SHRTSHTR]

phorwath,

Wow, I must admit that I did not have a clue how to use my 35P! I had to read thru the data you sent a couple times but the light bulb finally got a little brighter. lightbulb I made a copy of your explanation and it will be kept with my Oehler for reference from now on.

The 2711 velocities on A&B were correct, it was not a typo. I remember thinking that the 2711 velocities were exact and the other velocities in the shot string were off a little. I now see the error in my ways.

Your explanation of delta differences is excellent! I will use it from now on. I plan on making a proof channel 6ft. instead of a 4ft. rail I have now. In the manual, they indicate the further the spacing...the more accurate the unit will be. I will also make a gauge for checking my spacing between the the screen while in the field.

I recieved a lot of good info from this thread and want to Thank Everyone for there suggestions! I am one step closer to refining my LRH. gun)

By the way, I think I will keep my Oehler 35P for awhile longer.

Ray

 

[Pdvdh]

Ray,

Hope I was able to help and it does sound like it did. I find the second chrono invaluable for validating the quality of my chrono data, prior to evaluating ES/SD from the velocity data. It's not uncommon to get some bad recorded velocities off a chrono and if a guy isn't able to identify the blooper(s) data then it's possible to dismiss a potentially low ES/SD load and move on in the search for greener grass.

I'm envious of your 35P, and it sounds like it's a good one. I use an Oehler 33 and a PACT Pro. Of the two, the Oehler 33 is the more reliable. In good lighting conditions I generally have very few problems. In fading light the Oehler 33 keeps on trucking while the PACT starts to puke out on me. Just yesterday I received some bogus data off the PACT and a few 'no readings' as well. The lighting conditions were a bit less than favorable.

I use a 6' spacing with my Oehler and a 5' 6" spacing with the PACT skyscreens. You're correct in that the larger the spacing, the less compromised the data should be due to slight errors in the exact distance of skyscreen separation.

A six (6) foot skyscreen rail is about the longest I find still convenient for transportation purposes.

Enjoy that Oehler!

 

[436]

I also have an Oehler 35P; you're to close... You also might rub a "little" WD-40 on your pugs which go into terminal... just a little on your finger(s) then to the pugs... this will help the electronic in keeping the connection input strong.
436

 

[SHRTSHTR]

436,

Thanks for the tips, very much appreciated.

phorwath,

Identifying potential bloopers and using the delta differences are the biggest things I learned from your explanation. i was only paying attention to velocity and what I got for ES. While velocity and ES is my main goal, having confidence in my equipment and understanding what it is telling is priceless.

My 35P has also givin up the ghost in low light condition. Only had it happen once about 15 minutes after sundown. I was running a little late.

I have a lathe & Mill in my garage. I plan on making a two piece threaded rail out of 1/2 aluminum. My plan is to drill three holes in the rail equally spaced and secure them with wing nuts. This will be placed on two sturdy tripods. I think this will be much more effecient than what I have now.