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 proof channel 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
[f you 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 sky screens 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 proof channel; 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
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