Tip: Visibility Sensor Calibration
By Ron Smith, Operations Manager
Weather measurement systems provide critical information to pilots and airport
personnel. To ensure you are getting accurate readings, make sure you follow
the step-by-step procedure for calibrating AWI's visibility sensors.
Start by taking good care of the calibrator paddle. If it gets scratched,
stained, dirty, or is exposed for a long time to high humidity, its optical
characteristics will be affected. The "CAL I.D. No." on the label
tells the sensor what to expect, and the optical characteristics of a mistreated
paddle will be changed so that the number isn't valid any more. Your calibration
will be off. So don't just throw the paddle in the corner of the mop closet.
When calibrating the visibility sensor, start with clean windows. Even if
they appear clean, clean them anyway. In the shop with test instruments connected,
we immediately see a change in performance when the windows have been recently
cleaned.
The alignment of the antlers is generally north-and-south, it doesn't need
to be precise. Nevertheless, we've actually been asked to use a transit to
align them precisely. The purpose of the north-south alignment of the antlers
is only to avoid having the rising or setting sun shine directly into a detector
and blind it.
Part of the calibration procedure involves rotating the antlers, and this
is where we see some confusion. Rotating the antlers ensures that the sun
doesn't shine on the face of the paddle that the detectors are "looking"
at as it sits in the center of the antlers. What happens is that a tiny bit
of light from the emitters manages to filter through the paddle and the detectors
try to measure it. If the sun is shining on the detector side of the paddle,
the detectors are in a position similar to you trying to look at a candle
flame when it's held up to the sun -- poor signal-to-noise ratio, in technical
terms.
So the first phase of the calibration is with the detectors "looking"
North, and no paddle in place. The second phase is with the paddle installed
and the antlers rotated. While the manual doesn't point this out, you don't
have to turn the antlers exactly 180 degrees. Instead, turn them so the beam
is aligned with the sun -- that is, the sun is directly behind the two emitters.
In the evening, during some times throughout the year, a 180 degree rotation
of the antlers will make the sun shine edge-on to the paddle, perhaps even
a bit on the detector side of it.
Finally, for the third phase, have the paddle out and the antlers back in
their original position. Cover the emitters blocking any light. You can accomplish
this by folding up leather gloves and pushing them up under the hoods of the
emitters.
Two final notes on calibration:
- Don't bother trying to calibrate if the horizontal visibility isn't at
least seven miles. You'll end up calibrating in an error.
- Remember all the rotating of the antlers discussed previously? If you're
doing this after dark or under a heavy overcast where you can't make out
the sun, there's no need to rotate the antlers!
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