Oehler 35P User manual

,
I
OPERATING INSTRUCTIONS
!'\l\-I:\' -!o \1\-
3'2. l'l-OS·~'8'-.
OS-3Z12- ,
0':1-3171- - '
G5...QJ41-£
0;-3\131- 11
05- oo 15-S
Model 35P Proof Chronograph
Oehler Research, Inc.
Pos t Office Box 91:!5 , Austin , TX 78766
Phone 512 . 327 . 6900 sa l es@oeh l er-researc h. com

CONTENTS
Condensed Instructions ........................................................1
Proof Channel Background. .. ................................................2
Skyscreen JII Background........................... .. ........................3
Program Switches............................ .. ... .. ..............................4
Metric Velocities...... .. ............................................... .. ..........5
Skyscreen Mounting........................ .. ....................................6
Actual Chronographing........................................................8
Printer Output ....................................................................10
Summary............................................................................. 11
Banery ..................... .. .................. .. ..................................... 12
Edit Mode ........................................................................... 13
Printer Paper...................................................................... 14
In Case ofTrouble ..............................................................15
Glints ..............................................................
.. ........ .. ... .. ... 17
Orange Diffusers Can't Always Help .................................18
Subsonic Velocities .............................................................18
Bows ...................................................................................
19
Shotguns ................................... .. ........................ .. ..............19
Indoor Shooting.......
.. .........................................................19
Downrange Velocities............................................. .... .... .. ..20
Muzzle Velocities
from Instrumental .......... .... .. .................. 20
Wounded Skyscreens............ .. .............................................20
Standard Deviation and Load Development ......................21
Carrying Case ...................................................................
.25
Specifications......................................................................26
Warranty.............................................................................29
Oehler Research, Inc.
Post Office Box 9135
Austin, Texas 78766
Telephone 512/327·6900
FAX 512/327·6903
[email protected]
Copyright 1991, 2010
PROOF CHANNELTM, PROOFCHRONOGRAPH 1M and GLINT PROOF""
are trademarks of Oehler Research, Incorporated.
Take your Model 35P to the range, set it up, and shoot. If everything doesn't
work perfectly, you can then read the instructions. Some read only this. first
page, but the rest
of the book will help you get better results. We try to cram
forty·plus years of chronograph experience and the essence of thousands of
conversations with users into this little book. Thank you for buying our
product; we look forward to hearing from you.
CONDENSED INSTRUCTIONS
-+ Mount the skyscreens on the rail. Use the dimples to get the correct
spacing. Use thumbscrew with ntiddle screen.
-+ Remove bolt and place rifle on rest. Aim at downrange target.
-+ Place the fIrst stand approximately 8 feet from muzzle with the second
stand approximately
4' farther downrange. Place skyscreen/rail
assembly on stands with the long bolts fItting into tops
of stands.
-+ Locate your head so that you look through the skyscreen window as
you look into the muzzle and see a centered bore. Adjust height and
position stands until all are centered on boresight line.
-+ Verify gun is sighted at target and thru skyscreen windows. Any shot
hitting the target will pass through the skyscreen windows. Leave the
fore end rest in position and you can change guns.
-+ Plug skyscreen cables into the M35. Plugging in the start cable turns
the system on. Push reset button until you see
--- 0.
-+ Fire through skyscreens. The shot number and then primary velocity
will be displayed.
It prints proof velocity, shot number, and primary
velocity.
-+ If the PROOF CHANNEL detects an error, the display flashes and an
asterisk is printed alongside the velocity. You can use the
OMIT
button 10 erase any velocity thaI you suspect is incorrect. Fire
additional shots as desired.
-+ Push the SUMMARY button. Printer will show high, low, extreme
spread, average or mean, and standard deviation for printary channel.
-+ Fire more shots to add to the same group, or push the RESET button to
start a new group.
Page J

PROOFCHANNELBACKGROUND
Professional users have long recognized the convenience of skyscreens.
These same users have also recognized limitations in the reliability
of
skyscreens. With skyscreens you are at the mercy of the light conditions
existing at the range. Skyscreens work well under most conditions, but under
certain rare light conditions there will be errors.
If you haven't yet found.
these conditions, you will. Errors are seldom, and users will forgive the
system
if the chronograph flags those shots that may contain errors. The
proofchannel in the Model 35P wams you of any measurement error.
If you set your screen spacing accurately, the proof velocities should be very
near the primary velocities. The
proof velocities are measured from start
screen to the middle screen; the primary velocities are measured from start
screen to stop screen. Ifthere's a significant difference between primary and
proof velocities, the M35 waves a flag.
If the spacing between screens is
incorrect, expect larger (but fairly consistent) differences on each shot. For
example,
if you move the proofscreen more than a quarter inch from the
exact midpoint, you will see the flashing display.
The proofchannel shows
the importance
of accurate and adequate screen spacing. The proofchannel
can't measure screen spacing for you, but it sees your errors.
The size of the
differences on each shot is comparable to the size
of typical errors in your
velocity measurements.
The proofchannel tells you when to trust the
chronograph velocity reading, and when the system is fooled by light
conditions.
We could have made the proof channel so that the differences would be much
smaller. For instance, ifyou use only two screens to drive two
chronographs; they will read the same on each shot and prove nothing. An
error at either screen causes an identical error in velocity for each
chronograph. You can use two pairs
of screens with both start screens
adjacent and both stop scrcens adjacent. You get maximum spacing on both
pair, but conditions will likely affect both pair in the same way and still prove
little. The configuration
of the M35 was chosen so that an error at anyone
or two skyscreens causes a difference in velocity readings. This difference is
your warning
of a measurement error.
Page 2
SKYSCREENBACKGROUND
We 've shipped Skyscreen ill units with Oehler chronographs since 1984.
Thousands
of shooters have been pleased. The Skyscreen ill was the first
handloader skyscreen to use lenses, and the units have been refined to near
perfection.
We imprOVed the shielding of the cable so you' ll see fewer false
triggers from static electricity and electrical interference.
We reinforced the
rugged single-bolt mounting system so you can't pop the mounting bolt from
the case.
For years other manufacturers ridiculed our use
of diffusers; now our most
vocal critics advocate diffusers and sell their imitations. We've gone a step
farther and have improved ours for better performance.
The new diffuser's
special blaze orange material gives the highest sensitivity with early
and late
light.
The translucent material tames the brightest noonday sun while the
new wider diffuser and the new lens hood protects the lens from the glare of
mid-day sun.
The diffusers won't flutter in the wind and they easily absorb the blast of the
biggest guns. They will not survive direct bullet hits. Still, the side rails flex
to protect your skyscreens
if your mounting rail takes a tumble. All parts,
including the skyscreen cases, are molded
of tough polycarbonate. There's
no metal to rust
or kink, and there's no glue to faiL Best of aU, the window is
larger so they are easier to shoot through without fear
of damage.
The 4' rail provided with your system reflects our industrial experience. We
know that longer is better. A one-foot spacing between screens in inherently
twice as accurate as is a six-inch spacing. A two-foot spacing is twice
as
good as the one-foot spacing. The four-foot spacing is twice as good as the
two-foot spacing. We stop there because we've found that that four-foot rail
is as long as we can conveniently haul to the range. The rail includes
dimples to hold three skyscreens at precise spacing. As you tighten the
114"-
20 mounting screw of each skyscreen into its dimple you get exact screen
spacing. Dropping the bolt tails into the tops
of the stands may appear crude,
but it works well and you need no tools at the range.
The two folding stands
are more convenient and reliable than is a single camera tripod.
Page 3

PROGRAM SWITCHES
The programming switches for the Model 35P are normally set for use
with the four-foot rail.
The settings of the program switches control the M35. You can sele<;t
PRINTER
ON or OFF, EDIT ON or EDIT OFF, and the screen spacing. The
switches are inside next to the battery. Remove the battery compartment
cover to see the switches. A pointed wooden stick
is the best tool for
changing the switch settings, and a ballpoint pen is second choice.
The first five switch positions (numbered I through 5) must match the
distance between the widest screens, measured center to center.
The chart
below shows the switch settings for each spacing.
Ift
2ft
3ft 4 ft Sft
6ft 7ft 8ft
SWI0N OFF ON OFF ON
OFF ON OFF
SW2 OFF
ON ON OFF OFF
ON ON OFF
SW3 OFF OFF OFF
ON
ON ON ON OFF
SW4 OFF OFF OFF OFF OFF OFF OFF
ON
SWS OFF
OFF OFF OFF OFF OFF OFF OFF
8ft 9ft 10 ft 11ft 12 ft 13ft
14 ft 15 ft
SWI OFF ON OFF ON OFF ON
OFF ON
SW2 OFF OFF ON ON OFF OFF ON
ON
SW3 OFF OFF OFF OFF ON
ON ON ON
SW4 ON
ON
ON ON ON ON ON
ON
SWS
OFF OFF OFF OFF OFF OFF OFF OFF
Turning Switch 5 ON multiples the screen spacing setting by 10, or causes
the displayed velocity to be multiplied
by 10. For example, if a precision air
rifle gives velocities near 570 fps, turning Switch 5 ON causes a readout
of
5714 instead of the expected 571. The 5714 reading corresponds to 571.4
feet per second. This switch is most useful for work in metric units where
many velocities are under 999 meters
per second and you want extra
resolution.
Page 4
Switch 6 controls the edit mode. If Switch 6 is ON, the edit or replay
function is available. In the edit mode, the unit will store the results
of up to
20 shots. You can replay shots during or after the test and individual shots
can
be omitted from the summary. With Switch 6 OFF, you can fire and
summarize up to 255 shots. We ship the unit with the edit mode OFF.
Switch 7 is normally ON if you have a printer. You can turn the printer OFF
to coax a few more shots from a weak battery or if you are out of paper. .
METRIC VELOCITrES
For readout in metric units, the screen spacing should be set at an increment
measured exactly in meters.
One meter is convenient, hut two meters will
give better accuracy.
Use any rigid metal tubing with a diameter slightly
less than 18 mm.
Cut the tubing to 1.019 or 2.019 meters and mount the
skyscreens flush with each end. This gives a spacing
of 1.000 or 2.000
meters between centers. Set the distance switch as follows:
1m 2m
3m
SW1 ON OFF
ON
SW2 OFF
ON ON
SW3 OFF OFF OFF
SW4 OFF OFF OFF
SWS OFF OFF
OFF.
Switch 5 can be turned ON if aU velocities will be less than 999
meters/second. This will cause readout in decimeters/second.
Page 5

SKYSCREEN MOUNTING
Use the 4' rail with a light stand
under each end. Mount start and
stop skyscreens using a 3"
threaded stud with attached wing-
nut as the mounting screw.
The
long tail of each stud slips into the
hollow center post
of a stand.
Make sure that the screws fit into
the dimples
of the rail. Tighten
each thumbscrew and stud firmly
by hand.
We prefer a 4' screen spacing for
general use.
If you must have
longer
or shorter rail, use half-inch
EMT (thin-wall electrical conduit)
cut an inch longer than the desired
screen spacing. You can use
longer screen spacing for improved
accuracy,
or a shorter rail for
convenience.
If you choose
longer screen spacing, we suggest
one that fits inside your vehicle for
transport to the range. Regardless
of the length of the rail, the third screen
mounts midway between the two primary screens.
We discourage the use
of two-piece rails. They usually sag at the coupling,
and the uncertain sag cancels the benefits
of the longer spacing.
Page 6
It is convenient to "bundle" the cables from the skyscreens. . Gather all three
cables together near where the cable exits from the start skyscreen. Place a
wrap
of tape (electrical tape, masking tape, or whatever is handy) around the
cables at this point. Repeat the tape wrap every foot until you get to the
otherend.
The shortest cable end is obviously from the STOP screen; plug
it in first. Plug the middle length cable into the MrD jack. Finally plug the
longest
end into the START jack. Even if you remove the skyscreens from
the rail, ju st coil the bundled cables 'lasso" fashion and you'll have little
trouble with tangles.
You must mount a diffuser
over
each skyscreen. Slip a black
side-rail into your skyscreen,
hook one end
of an orange
diffuser into the standing side-
rail, hook a second side-rail
onto
the diffuser, and then slip the last
end into the skyscreen.
Page 7

ACTUALCHRONOGRAPBING
After you mount the skyscreens
for the ftrst time, actual chrono-
graphing is simple. Improper
alignment not only leads to
missed velocities, but often leads
to bullet holes
in the skyscreens.
Place the assembled skyscreens
approximately eight to ten feet
in
front of the muzzle. The
triangular light diffusers should
fonn a triangle-within-a-triangle
sight picture as you look down
the barrel at the target from your
nonnal shooting position.
To align the skyscreens, sandbag a bolt action rifle aimed at your down-range
target. Remove the bolt, and boresight back to the gun from the skyscreens.
While boresighting, adjust the first (START) skyscreen until you look
through the center
of the window. Adjust height and move stand as required.
Repeat with the STOP skyscreen. Verify the alignment by looking at the
target from your normal shooting position. After the skyscreens are aligned,
just shoot at the target. You can change guns (bolt actions, autos, lever
actions, scopes
or iron sights, or even handguns) just so you use thc same
fore-end rest and shoot
at the same target.
You can use a similar procedure with a tripod.
If you ftrst align the start
skyscreen, and then move the stop skyscreen into alignment. you'
ll ftnd that
the start skyscreen has moved in the opposite direction. That's why we
prefer a stand under each end
of the 4' rail.
Pag eS
Please don 'I place armor plate in front of the skyscreens.
Shooting next to armor placed near your face is hazardous
and downright foolish. It is far better to blow a hole in a
soft skyscreen
than to deflect a rouod or to bounce it back in
your face.
Connect the cables from the skyscreens to the input jacks
of the M35P.
Connect the far screen to the STOPjack, the middle screen to the
MID jack,
and the near screen to the STARTjack. Make sure the plugs are pushed
all the way inlo the jacks. If you can see a band of metal between the plug
body and the jack, plug it in farther. Plugging in the start screen switches
on
the M35. The printer will feed a few blank lines of paper when the start
screen is plugged in. Place the M35 at a convenient location well behind the
muzzle to protect it from muzzle blast. Ideally, the M35 should be placed as
far back as your shooting shoulder.
The display reads ---0 to show that it
is ready to begin a new group. If the
display doesn't read ---0, push the RESET button. Fire through the center
of
the skyscreen windows. The unit will display the primary velocity and will
print the proofvelocity, shot number, and primary velocity.
If no printer is
in use, the unit flashes the round number 0001 and then displays the primary
velocity.
The unit resets itselfand is ready for the next shot when the
printing stops and a velocity is displayed. The velocity remains on the
display until you fire the next shot
or omit the velocity from the summary
with the
OMIT pushbutton. If the velocity is omitted, it is erased from the
memory and the display shows ----.
If you are using the third skyscreen, the system compares the primary
velocity and the proof velocity.
If the difference between velocities is larger
than expected, then the display will blink the velocity reading and the
printout will include an asterisk
*. Even with no printer, the system still
measures both velocities and compares them. The display will still be steady
if there is no significant difference between the primary and proof velocities,
and the display will flash
or blink if there is a significant difference. The
error could be caused by inaccurate screen spacing (more than 0.25 inch error
in mounting distance,
or a bent rail), shooting a BB or pellet too high in the
screen window, muzzle blast ahead
of a subsonic bullet, inadequate light,
abnormally bright bullets,
or the "glints" caused by excessive light reflecting
from the bottom halves
of the bullets. .
Page 9

PRINTER OUTPUT
The M35P was designed for use with the built-in printer. The printer is the
only means
of displaying the velocity measured in the proofchannel, and it is
convenient not to write down numbers after each shot. Chronographs with
printers are nothing new
to Oehler; we've been making them for industrial
users for over forty years. We have maintained one absolute requirement;
the printed record must always reflect a complete record
of the test. Editing
removes individual shots from the next statistical summary, but the data
remains
in the record. The scientist crosses out data in his records; he
doesn't erase it. The printed output from the M35 saves all the velocities.
You will have the data later if you need it. Just because you don't expect a
particular velocity doesn't mean it is wrong.
Shown below is a typical output tape from the printer:
3272-01-3272
3234-02-3235
3256-03-3256
3259-04-3257
3302-05-3259 *
X
3234-06-3236
Page 10
05-3272 +
05-3235 -
05-0037 T
05-3251 M
05-0015 $
~ ProojVelocity.
Shot Number.
Primary Velocity
f- Asteriskdenotes possible error
~ Preceding shot omittedfrom summary
~ Highest Velocity
+- Lowest Velocity
~ Extreme Velocity Spread
+--- Mean or Average Velocity
+--- Standard Deviation
SUMMARY
After you have fired a test lot of ammo (up to 20 rounds in the edit mode or
255 rounds with the edit off), the M35 will summarize the test. With the
printer in use, one push
of the SUMMARY button causes the entire summary
to print.
The printed summary shows the high velocity (+) the low velocity (-
), the extreme velocity spread (T) , the mean or average velocity (M), and the
standard deviation ($). The printed round number in the summary refers to
the number
of valid rounds considered in the summary. The summary ends
with a display
of ---- showing that more rounds can be fired and added to the
data for a later summary.
If the printer is not in use, pushing the SUMMARY button briefly displays
Sh ot and then the number of rounds considered in the summary. The next
push
of the SUMMARY button gives m and the high velocity, ... LO and
the low velocity,
... ES and the extreme velocity spread, ... A and the
average velocity, . . . Sd and the standard deviation, and fmaUy
---- is
displayed showing that another round may be fued.
If you fire large lots of ammo (more than 10 rounds), it is prudent to stop and
record the summary at intervals during the test.
If the test is interrupted, you
will at least have partial statistics.
Only the primary velocities are summarized. The accuracy of the primary
channel is normally twice that
of the proof channel, and the summary of the
primary velocities is most signillcant.
Page 11

BATIERY
The nine-volt alkaline battery used in the Model 35P is the same size
commonly used in radios and smoke detectors. The word alkaline
is very
important. Many
junk batteries are labeled with Heavy Duty, Industrial
Strength, Super Power, or other impressive names.
If the battery isn't
alkaline, it probably won't work the printer.
If you use a rechargeable "nine-
volt" nicad battery, note that many are actually only 7.2 volts. Operation
of
the printer will be marginal.
A fresh alkaline battery typically powers a M35P for
25 hours including 1000
lines of printer use. The battery will typically power the M35 for 50 hours of
operation without the printer. If a printer is in use, the state of the battery is
obvious. With a fresh battery, the printer will print a line in approximately
two seconds. With the battery almost exhausted, the printer slows to three or
four seconds per line. The slow print speed does not affect the accuracy
of
the velocity measurement. The velocities shown are accurate until the printer
just stops. Note: Just because you bought a new battery does not mean that
it is a good battery, usually, but not always.
The battery compartment
of the M35 holds two batteries, one acti ve and one
spare. Contrary to custom, Oehler ships the M35 with a hattery installed.
Consider this first hattety to be the first
of many spares, and always keep a
spare battery in the M35. Always use the spare to replace a dead battery,
and don't forget to replace the spare with a fresh battery. A spare at home
in
the smoke detector doesn't count on the range. -
The M35 has no ON/OFF power switch. Over the years we've found that the
most likely cause
of a dead battery is putting away a unit with the power
switch left on, or the power switch being turned on accidentally. We've
mi ss ed the velocities of many shots because we had our chronograph turned
off. The M35 turns on when the start skyscreen
is plugged in, and it is off
when the start skyscreen is unplugged. You aren't likely to transport or store
your unit with the screens plugged in, and the screens won't plug themselves
lD.
Pa ge 12
EDIT MODE
The edit mode provides a replay of the velocities in a group, and allows you
to omit one or more velocities from the summary. We provide both
an edit
mode and a switch to cancel the edit mode. You must select
or cancel the
edit mode before the skyscreens are plugged
in . You cannot change modes
after a test
is started.
In the edit mode you can shoot and observe velocities in the usual fashion.
At any point during or following a group,
you can begin the edit function by
pushing the RESETIEDlT button once. The display will show Edit. Begin
the velocity replay by pushing the SUMMARY/STEP button.
It will show
shot 0001 and the velocity. You can omit this shot from the summary by
pushing the
OMIT button, or step to the next shot with the STEP button.
Step through the group by pushing the SUMMARY/STEP button or the
OMIT button at each shot. When the display shows ---- you can fire more
shots, repeat the replay and edit,
or reset the unit by pushing the
RESETIEDIT button twice. The unit is ready for new group when the
display shows ---0.
Use the edit function
as an aid to calculating a summary. You can't use it to
forget undesirable data and print a clean tape. When a shot is omitted with
the edit, it is not forgotten.
It is excluded from the following summary. The
velocity remains in memory and reappears
if the group is replayed again.
For example,
if you fired two five-shot groups and recorded them as one ten-
shot group, you can still get a separate summary for each five-shot group.
Edit the group, omitting the last five shots, and you get a summary
of the first
five shots. Edit again, omitting the first five shots, and get the summary
of
the last five shots. The unit stores a maximum of 20 shots in the edit mode.
After it has recorded 20 shots, the display indicates FULL. At this point you
can edit and summarize, but you can't add any more shots.
Too complicated? Leave Switch 6 OFF, and forget about the edit mode.
Some
of us still dislike computers and can't remember how to set our digital
watch.
Page 13

PRINTER PAPER
The M35P uses standard 2.25 inch wide plain adding machine tape. The
M35P has an internal box to contain a small roll
of paper in stead of using the
external rack to hold a large roll. It's
mu ch easier to transport and use at the
range. Several portable calculators
us e these small rolls, but it is much
easier to find the standard large rolls. You can easily make four or five
small rolls from one inexpensive large roll. Simply strip five arm-spans
(twenty-five or thirty feet)
of paper tape from the big roll, roll it up tightly,
and snap a rubber band around it.
The paper will feed into the printer
easier if you square the end with scissors and cut a half-inch triangle
from each
corner. Repeat until the big roll is gone and you will have a
good supply
of paper in small rolls. If you think this is a nuisance, just look
for thermal paper when you buy your roll
of plain adding machine tape at a
local store. Plain paper is inexpensive, readily available, and the printing
does not disappear with age.
To load the paper tape into the M35, remove the wire clip at the back
of the
paper box. Plug in the start screen to tum on the M35. Feed the cut end
of
the paper tape into the printer slot at the back of the paper box and pu sh any
button to generate paper-feeds. The paper feeds best from the top
of the roll.
After the paper threads through the printer, place the roll
in the paper box and
replace the wire retainer clip.
If the printer ink roller gets dry and the print is too dim, replace the ink roller.
Many battery-operated portable calculators use the same print mechanism.
Take your ink roller along to your local office supply store and ask for a
roller
to fit a hand-held calculator. They are usually available and will often
be labeled PR-40 or similar. Ifyou can't find the roller locally, a scant drop
of stamp-pad ink applied to the foam roller will work. Lacking stamp-pad
ink, you can always try a light mist
of WD-40 on the foam roller. If all else
fails, call Oehler to order a replacement.
A full big roll of paper can be mounted externally with the included wire
bracket. To mount the bracket, start with the straightest end
of the wire
bracket inside the paper box. Twist and feed the bracket through the side
hole until the straight end forms an
ex ternal hanger for a paper roll. Hook the
crooked end
of the wire thro ugh the small hole previously used for the paper-
retainer clip. Save the paper retainer clip
in case you want to again use the
smaller rolls.
Page 14
I
I
I
\
IN CASE OF TROUBLE
If the display does not change when you shoot, it means that no start signal
was received. Pos
si ble causes are the bullet pas si ng too near the top or side
of the skyscreen window, the orange diffuser not being directly illuminated
by the sun, the skyscreen not having an unobstructed view
of the sky
overhead,
or having too little light for proper skyscreen operation. Make a
deliberate effort to shoot through the middle
of the triangular window of the
Skyscreen III. Shoot airguns and arrows approximately one-third of the way
up the window. Be sure the skyscreen cables are plugged firmly into the
jacks on the back
of the M35; if you can see a band of metal between the
plug handle and the jack, it's not plugged in good.
A display
of --4) has special significance; the chronograph is reset and no
shot has been recognized.. The similar display
of ---- shows that the M35
received a start signal, but didn't get a stop signal corresponding to a
legitimate velocity, and
is anticipating the next shot. You will get this
display
if you have the cable from the stop screen plugged into the start
input,
if the start screen receives a stray input signal before a shot is fired, if
the stop screen isn't plugged in, or if the stop screen doesn't see the bullet.
You can test individual skyscreens
by plugging only one screen into the
START input. With a good skyscreen, the display should go to --4) when
you plug it in, remain at
--4) until you shoot a BB or 22, and then change
to -----when you shoot and the skyscreen sees the bullet.
The printout
of 9999 indicates that the M35 received a start signal and then a
quick stop signal so that the velocity was higher than 9999. You are most
likely to get this printout between actual shots if there
is severe electrical
noise in the area.
If the printout shows either ----, 0000 or 9999, you don't
have to push the OMIT button. The unit knows the velocity is wrong and it's
automatically omitted from the summary.
If the M35 displays a velocity before you shoot, it was probably caused by
static electricity. Skyscreens are sensitive to the same electrical noises that
cause static on an AM radio. Typical causes are synthetic clothing
in dry
weather, nearby high-voltage power lines, large appliances with electrical
motors or solenoids turning on or off, electric fences, or radar and radio
transmitters. Muzzle blast from adjacent benches can also trigger the
skyscreens.
Pag e 15

You may be concerned by differences between the primary velocity and the
secondary velocity.
If an asterisk is printed to the right of the velocities, the
system considers the differences are large enough for concern. The allowed
velocity difference becomes larger with shorter screen spacing and higher
velOCity. If the middle screen is off-center by more than 0.25 inch, or if it
misses seeing the tip
of the bullet by 0.25 inch, you will get the asterisk
warning.
If the proofchannel velocities are consistently high, then the
middle screen
is probably mounted too near the start screen. If the proof
channel velocities are consistently low, then the middle screen
is probably
mounted too far from the start screen.
If you are tempted to use short screen spacings or to not bother with the
diffusers over the skyscreens, fire a few ten shot groups to compare long
spacing versus short spacing and diffusers on versus diffusers off. Note the
difference between primary and proof velocity on each shot. While averages
usually remain close to the same, you will see the differences get larger
as
you eliminate the diffusers and decrease screen spacing. Watching the
consistency diminish with shorter screen spacing and poor light will make
you want to use diffusers and the longest practical screen spacing. You will
be able to see some differences with an air rifle or .22 LR. The contrasts are
even more dramatic
if you use a varmint rifle.
You can quickly check the operation of your chronograph system with an
airgun. Velocities will range from approximately 225 fps for a kid's BB gun,
to approximately 600 fps for adult match air rifles, up to over 1000 fps for
some adult sporter air rifles. BBs and pellets are small, short, and slow.
They provide a severe test
of skyscreen sensitivity, so you must shoot
through the lower half
of the skyscreen window. If your system works with
an airgun, but fails with larger guns, you can suspect problems related to
muzzle blast or flash.
If your Model 35 doesn't work, call us at 512 I 327-6900, Monday
through Friday, 8 am unill5 pm_ We've cured hundreds of
chronograph problems over the phone and are anxious to help. Thanks
for reading the instructions_
Page 16
GLINTS
[fyou look at a bullet illuminated by a light from the side, you can see a
small "glint"
of light reflected from the ogive. This reflected light can cause
the bullet to appear as a momentary bright spot over the skyscreen instead
of
the expected dark spot silhouetted against the sky. The patented Oehler
GLINT PROOF skyscreens recognize either the light spot or a dark shadow
caused by the bullet. Under certain conditions, the reflected light is
approximately equal to the shadow. When the glint and shadow are
approximately equal, they cancel and skyscreen performance
is erratic.
These difficult conditions are detected
by the proofchannel; you must
change the conditions or simply wait for a better day.
The glint phenomenon occurs primarily under clear skies. Unless the sun
is
shining directly on the orange diffusers, the amount of light from a clear sky
is typically one-fourth the light from a cloudy or hazy sky. Skyscreens will
adapt to this lower light level just as does a camera with automatic exposure
control, but they work better with more light. The diffuser makes an
"artificial cloud" above the bullet and converts the direct light from the sun to
the diffuse light needed by the skyscreens.
Glint problems also show up when shooting over snow or light sand. The
bottom
of the bullet is illuminated by light reflected from the ground, and the
bullet appears to be almost
as bright as the sky. The only positive solution
for the glints
is to shade the bullet from light as it passes through the
skyscreen window. Only if no light strikes the bottom half
of the bullet are
you absolutely sure that there are no glints.
Skyscreens want to see a dark bullet silhouetted against the bright back-
ground formed by the diffuser illuminated by direct sun. If there's not
enough light on the diffuser, or
if the bullet is exceptionally bright, you can
expect problems. Beautifully polished bullets are often difficult to
chronograph.
It is sometimes effective to blacken the ogive with a black
felt-tip marker. Dull the mirror finish
of the bullet with steel wool before you
apply the black ink.
Page 17

ORANGE DIFFUSERS CAN'T ALWAYS HELP
The orange diffusers shade the lenses from noonday sun and make the hot-
spot light from the sun into a uniformly bright orange background against
which the eye can see the bullet.
If you are shooting under cloudy skies,
with heavy overcast, or in the shade, then no direct light strikes the diffuser.
What little light is available
is already diffused, and the diffuser can't make it
any brighter.
If the orange diffusers don't cast a shadow, and you have
problems,
try leaving them off . You can still use the black side-rails as
aiming guides.
Expect trouble
if you try to shoot under the shade of a tree. The sun can cast
a spotted shadow on the diffuser; the bullet may go through a bright spot and
be detected or the bullet may go through a shaded spot and be missed. With
no diffuser, the eye may see the bullet silhouetted against a spot
of sky, or the
bullet might be silhouetted only against the black bottom
of a limb.
SUBSONIC VEWCITIES
With velocities below the speed of sound, the muzzle blast wave reaches the
screen before the bullet. This muzzle blast wave
is like a lens traveling
through the air at the speed
of sound, and the resulting light diffraction can
trigger the skyscreens. (The speed
of sound is approximately 1060 plus the
air temperature in degrees Fahrenheit, or 1130 fps at room temperature .)
Premature triggering of only the start screen will cause both velocities to be
abnormally low and the difference to be high. Premature triggering
of both
start and middle screens will cause the primary velocity to read abnormally
low and the proof velocity to read the speed
of sound. Premature triggering
of all three screens (a rare case) will cause both primary and proofvelocities
to read the speed
of sound.
If you see the effects of blast, the only sure solution is to install a blast baffle
midway between muzzle and flIst screen. This baffle should be a piece
of
plywood with a small hole or vertical slot to shoot through. We use a 2x4'
piece
of plywood standing on end and supported by a single hinged leg. Cut
a vertical inch-wide shooting slot from bench level to the top
of the shield.
Occasionally blast problems can be cured by moving the skyscreens farther
from the muzzle
or by slightly changing the shooting direction.
Page 18
BOWS
The M35P with Skyscreen III detectors works well for archery. Make sure
that the screens are far enough away that the arrow leaves the string before
the point gets to the first screen. Blunt points work best, followed by round
points and target points. Needle-like field points and broad-heads can give
erratic results. The skyscreens need to see an abrupt change in light.
Because arrows are slower than bullets, you must help the skyscreens by
using a flatter nose.
SHOTGUNS
The system can be used for shotshells. It reliably measures the velocity of
the front pellets in the shot string. These velocities are typically 2% to 5%
higher than factory measured velocities. The factory systems (also made by
Oehler) use inductance sensing coils to measure the velocity
of the large
clump
of shot at the back end of the shot column. A primary screen spacing
of four feet is best for shotgun work. The first screen should be placed three
or four feet from the muzzle. You will get the most consistent and accurate
results
if you use the most open choke available.
You must wear safety goggles while shooting!
INDOO R SHOOTING
To use skyscreens indoors, you must provide a substitute for the daylight.
Use a reflector type incandescent lamp shining down on the top
of the
diffuser
of each skyscreen. You must use incandescent lamps; fluorescent
lamps will not work! Mount thc lamps just high enough to uniformly
illuminate the diffuser. Common utility reflectors with 60 to100 watt bulbs
work fine. The 90 or 150 watt PAR outdoor reflector lamps with their heavy
glass cnvelopes will better resist muzzle blast. Using the Skyscreen III units
indoors with lamps will work most
of the time. Adding lamps to our
skyscreen type system will not give you all the advantages
of our laboratory
type systems. Our laboratory systems cost more, and they perform better.
Page 19

DOWNRANGE VELOCITIES
The Model 35P with printer is appropriate for measuring downrange
velocities. To measure ballistic coefficients, use one M35P at the muzzle
and a second one downrange. Accuracy is critical, so a spacing
of at least 4
feet
is preferred and 8 feet is still better. Verify system accuracy by first
shooting with both systems near the gun to assure that the readings agree on
each shot. Then move the downrange system without disturbing screen
spacing adjustments. For the best results, use Oehler's Ballistic
Explorer
computer program to calculate the ballistic coefficients and to project
downrange performance
of your loads.
MUZZLE VELOCITIES FROM INSTRUMENTAL VELOCITIES
The velocity recorded by the chronograph corresponds to the velocity at the
midpoint
of the screens. This velocity is lower than true muzzle velocity by
the amount
of velocity lost between the muzzle and the midpoint of the
screens. Typically the midpoint
is 12 feet or 4 yards from the muzzle.
Look
in the tables provided by the ammo or bullet manufacturer to frod the
velocity lost in the first 100 yards at your approximate velocity level.
Divide the 100 yard velocity loss by 25 to find the loss
in 4 yards. Add the
4-yard loss to your chronograph reading to get muzzle velocity.
WOUNDEDSKYSCREENS
It is not feasible for us to repair wounded skyscreens. It often costs more to
repair a skyscreen than to make a new one. Naturally we will repair or
replace at no charge any defective skyscreen returned to
us. We accept full
responsibility for defective parts or workmanship, and you are responsible for
bullet holes. Oehler has available extra plastic cases, lenses, and diffusers
for those who nick their screens and want to make their own repairs.
If you
shoot the "eye"
or the cable of the screen, consider it dead.
Page 20
STANDARD DEVIATION AND LOAD DEVELOPMENT
What is standard deviation and what does it mean? Because standard
deviation and average go hand-in-hand, let's fIrst talk about averages.
If you
talk about average velocity, everyone knows what you mean. You measure
the velocity
of several shots and you average your readings. If someone asks
you the velocity
of that arnrno, you say that it 's about 2950 feet per second.
You naturally quote the average velocity, and the listener understands. You
know that some shots will be faster than average and some will be slower.
You don't even worry about the exact defInition of average; it's about the
middle.
Confusion doesn't come until someone asks ifthe velocity
is uniform. You
are comfortable with quoting the average velocity, but you know that it
doesn't tell the whole story. The average does not tell you how much the
velocity readings scatter above and below the average. How do you describe
uniformity
or assign it a number grade? The standard deviation is a n'umber
that describes uniformity. The smaller the number, the more uniform the
velocity. A standard deviation
of zero means every velocity was the same.
A standard deviation
of 28 fps means you expect two-thirds of the individual
velocities
tobe within 28 fps of the average.
Mathematicians and statisticians have talked about uniformity for years.
They may call the lack
of uniformity dispersion or variance. They may talk
about the difference between highest and lowest and call it range, extreme
variation or extreme spread. They may talk
in terms of the coefficient of
variation. They prefer to talk and think in terms of standard deviation.
Standard deviation is the best measure of uniformity, and it fIts recognized
procedures, equations and textbooks.
Modern shooters consider standard deviation as the best measure
of velocity
uniformity.
In the past, shooters used extreme spread or mean absolute
deviation as the indicator
of uniformity. This was a matter of pre-calculator
convenience. Statisticians knew that standard deviation was a better
measure
of uniformity, but nobody wants to calculate it manually. If you've
never computed standard deviations manually, be assured that the pleasure
ranks right
up there with spit-shining combat boots. You avoid it if at all
possible. With machines to do the tedious calculations, we can now all use
standard deviation as the measure
of uniformity.
Page 21

Consider what happens when you test a handload. You hear it go bang, you
feel the recoil, you see where it hits the target, and you can measure the
velocity. We normally don't measure the intensity
of the bang or the force of
the recoil; we measure only the target and the velocity. Most important is
where the bullet hits the target. If all the bullets go into the same bole, and
the average velocity
is sufficient, you don't worry about velocity unifonnity.
If the group is larger than you want, you grasp at anything that will give you
a clue
of what went wrong.
The secret for making smaller groups
is unifonnity. Other things being
equal, the more uniform you can make the ammo, the more likely it will
shoot to the same hole. Uniform velocities are simply another indicator
of
uniform ammo. Uniform velocities do not guarantee small groups, nor do
large variations guarantee large groups. There are no guarantees, but you
can at least put the odds on your side. When you have uniform velocities
you can assume that you have a proper primer for the powder, that you have
a reasonable powder for the case and bullet, you did a good job measuring
the power, and that your cases were
of uniform capacity. Uniform velocities
tell you very little about bullet quality, the bedding
of the action and barrel,
or
if the gun vibrations induced by the firing just happen to fall in a sweet
spot When you have erratic velocities and small groups, your bedding is
probably good and you have a good average velocity for that powderlbullet
.combination, but be suspicious
of your primer choice and firing pin. [f you
get both erratic velocities and large groups, go ahead and make significant
changes in bullet, powder type, or gun; you probably aren't close to any
perfect combination.
The common limitation on the formal use
of standard deviation and other
statistical procedures
in shooting is the number of shots required.
Statisticians call it sample size. Invariably statisticians ask for more shots
than shooters want to fire. Shooters want to shoot five-shot groups, and
statisticians want to see at least twenty-shot samples. Firing more shots into
a group will give you a better statistical measure
of both the accuracy and the
standard deviation.
Page 22
Trying to measure the velocity uniformity of your ammo by chronographing
only five shots
is like measuring the accuracy with one five-shot group. One
group is an indication, but you can't trust it to repeat. Likewise, one standard
deviation number should be considered only as an indication
of unifonnity.
Although standard deviation
is the best available measure of velocity
uniformity, it
is not good enough to be considered the only measure of ammo
quality. Use standard deviation numbers as indicators
of unifonnity, but use
them along with other indicators
of load performance.
Do you always need to usc a large sample size
as you are developing your
loads? When I shoot a large group, backed up by large standard deviation, r
don't waste time trying to measure just how bad the load is. The load could
eventually prove to be a good one, but the odds are against
it. I'm looking
for good loads, and abandon bad loads
as quickly as possible. With the large
standard deviation confirming the large group, I abandon the load quickly
and don't feel guilty for shooting fewer than twenty shots. It's fun to shoot a
new load that I hope is good; it's drudgery to shoot a load which I expect to
be bad.
Sample size takes care
of itself with good loads. If a load looks good, even
though you've fired only one five-shot group, you don't abandon it and you
don't immediately accept
it. You try at least two or three more groups of this
load to see
if it is golden. The original sample of five shots is now fifteen or
twenty shots. [f the load continues to look good, you load and shoot it still
more. Even a statistician would be happy with the total number
of shots
fired with your "keeper" loads.
What are reasonable values for standard deviation? What
's a good group
size or a good average velocity?
It all depends on what you're trying to do.
You should use the numbers only for comparison, and you don't compare
apples and oranges.
If you're working up an elk load for a .375, comparisons
to the performance
of your ,45 ACP or varmint rifle are irrelevant. You
don't compare the average velocity
of your .375 to the average velocity of
your ,45 ACP. You don't compare groups from your .375 to the groups from
your varmint rifle. Likewise, you don't compare standard deviations
between your .375 and the other guns. The only comparisons that matter are
those you make between the .375 loads you keep and the .375 loads that you
abandon.
Page 23

What do you do with the group sizes, average velocities, and standard
deviations reported
by another shooter? His results can influence which
loads you select for trial. Choosing a load that
's listed in a manual and also
worked for him beats choosing a random load. After you've tested the other
shooter's load, his results should not sway your decisio
n. What counts is
how the load performs in your gun, when compared to other loads in your
gun.
Some shooters question the use
of standard deviation in load development.
We agree that velocity unifonnity
or standard deviation should not be the
only criteria used
in selecting a load. Standard deviation must be considered
along with everything else you know about the load. Note that the measured
standard deviation includes variations in both the ammo and the chronograph.
Chronograph systems with inadequate spacings between skyscreens often
give passable readings
of average velocity, but questionable readings of
standard deviation. Whenever you use standard deviation, remember there is
an important corollary of Murphy's law. Tts regular use can replace many
mathematical theorems and complicated statistical procedures.
Page 24
Large groups usually repeat;
Large groups with large standarddeviations always repeat;
Small groups caused by luck never repeat.
CARRYING
CASE
The carrying case is Obviously a double rifle case. We have used similar
cases over the years and have found them very convenient. When you take it
to the range, you have the entire system. You might even have spare paper
and battery tucked away. We experimented with individual pockets for each
item, but found that the single padded pocket for the Model 35P along with
the large area in which to toss all incidental items was the most convenient.
Jf you are using the large paper roll, you must remove it to fit the Model 35P
into the case. We usually remove the paper roll from the hanger but kave the
paper threaded through the unit. Sometimes rethreading the paper can
be
difficult, but it is easy enough to restore a wrap or two on the paper roll.
One trick that many users have found convenient is to leave the skyscreens
on the rail. Remove the diffusers, then loosen start and middle screens and
slide them against the stop skyscreen. Loosely coil the cables and place the
rail with screens in the case.
It will make assembly of the screens and rail
much easier next time you use
il.
Add the folded stands and the diffuser pieces. The foam in the top of the
case will keep everything together when the case
is closed.
Page 25

SPECIFICATIONS
SKYSCREEN CHARACTERISTICS:
Shooting Area
Active Area
Beam Width
BeamThickness
SCREEN SPACING:
103 sq in
84 sq in
30 degrees
0.1 inch
(6 inches above unit)
Minimum spacing
of 1 foot is allowed; minimum spacing of 2 feet is
recommended. Spacing
of 4 feet is suggested for high velocity rifle s.
Spacing of primary screens can be set from 1 to 15 feet and from 10 to 150
feet in increments of 10 feet; proofscreen is always set midway between
pnmary screens.
PRINTER:
The optional built-in printer prints with real ink on standatd 2.25 inch plain
paper. It prints primary velocity, round number,
and proof velocity as each
round
is flred. It prints statistical summary of primary velocity.
TEMPERA
TORE OPERATING RANGE:
00 to 500 Centigrade
32
0
to 1220 Fahrenheit.
Systems typically operate at temperatures down to OaF or _18
0C if the
battery can supply reliable power.
BATTERY:
A single nine-volt NEDA 1604 alkaline battery
is supplied. A fresh battery
will operate the system for up
to 25 hours including 1000 print lines. Internal
battery compartment includes space for
spate battery.
Page 26
EDIT MODE:
An edit mode allows for instant replay of up to twenty shots. During replay
you can omit questioned shots from the summary.
CRYSTAL OSCILLATOR FREQUENCY:
4.0 MHz oscillator for 0.25 microsecond time resolution.
ACCURACY:
The expected error on anyone shot depends on both velocity and screen
spacing. Typical errors
ate shown in the table. The table assumes good light
conditions,
datk bullets, exact screen spacing, and shooting through the
center
of the Skyscreen III window. Actual errors under your shooting
conditions will be reliably indicated by the PROOFCHANNEL.
IFf
2Ff
4Ff 8FT
1000 FPS 5 fps 3 fps I fps I fps
2000FPS 10 fps 5 fp s
3fps
2 fps
3000 FPS
16 fps 8 fps 4 fps 2
fps
4000 FPS
21 fps 10 fps 5 fps
3fps
You will typically see the above errors as differences between the primary
and proofchannels
of a Model 35P as you shoot under 'normal' conditions.
PROOF CHANNEL:
The system alerts you with a flashing display and printed asterisk if there is a
significant difference between the two velocity readings. Differences which
trigger the alert are shown for typical velocities
and screen spacings.
1 Ff
2Ff 4Ff 8FT
1000 FPS 42 fps 21 fps
10 fps S fps
2000 FPS
83 fps 42 fp s
21 fps 10 fps
3000 FPS
125 fp s 63 fps 31 fps 16 fps
4000 FPS 166 fp s 83fps
42 fps 21 fps
Page 27

Notes:
Page 28
WARRANTY
H your Oehler chronograph or skyscreen
doesn't work, we'll
f"rx it or replace it.
If you aren't satisfied, we'll refund your money.
We have honored this simple warranty for over forty years.
Our written warranty has been limited to three years, but
we've repaired many units at no charge even though some
were
fi fteen or twenty years old. Our customers are
reasonable;
if they shoot a screen, smash a chronograph,
or something just wears out, they expect to pay for repairs
and we fix
it at cost.
Page 29