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Oehler 35P User manual

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,
I
OPERATING
INSTRUCTIONS
!'\l\-I:\'
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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