AWOS Site Preparation Tips: Handling Common Challenges
By Ron Smith, Operations Manager
After shipping as many AWOS systems as we have, we see that certain installation
issues deserve a closer look. This list focuses on those issues that come
up often enough to justify mentioning. They are in no particular order.
The visibility sensor mast is a 2-1/2 inch pipe, with a coupling just
above the concrete. The coupling has to be tight. It's been a recurring issue
that the pipe gets screwed in only snugly, but not tightly. Most of the weight
of the sensor will be at the top of the mast and it'll rock back and forth
if the coupling isn't tight. The mast should be grounded using the aluminum
clamp that's shipped with the AWOS. Often, we arrive to find a terminal crimped
to the end of the ground wire and fastened to the mast with a self-drilling
screw. That doesn't work. Drill and tap a hole to accept a 3/8" NC bolt
a couple of inches above the coupling, and secure the clamp to the mast. The
ground cable will run through the clamp and up into the electronics cabinet
which will be installed later, so leave about 3 feet of ground cable above
the clamp. The mast should stand eight feet above grade.
The tower first of all, should be vertical. This should be obvious,
but sometimes we've arrived to find the tower leaning significantly. Each
section of the tower bolts to the next with galvanized bolts that were provided
with the tower. If they're missing, contact AWI. Don't substitute plated bolts
and don't drill the holes out for larger bolts. The bolts must be tight; it's
a freestanding tower without guy wires, and when a technician climbs to the
top it's disconcerting to discover that the tower is loose and able to rock.
The tower doesn't have to be oriented in any particular way, you needn't worry
about aligning a tower face in any particular direction. There is a ground
clamp provided by AWI for the base of the tower; put it on the tower with
one of the bolts that fasten the tower together. Like the visibility mast,
a crimped-on terminal fastened to the tower with a self-drilling screw isn't
acceptable. Any conduits that cross from the circuit breaker panel to the
tower should go down to the surface to cross. Don't form a trip hazard.
The ground for the AWOS sensor site is normally a 3/4" rod ten
feet long. Drive it all the way in to the ground; don't give up and saw off
the excess if you hit rocks. The AWOS needs a serious ground. Don't leave
a mushroomed head eight or ten inches above the ground, it's a trip hazard
and a mushroomed head will cut open someone's ankle. If it's impossible to
drive a rod ten feet into the ground, you may have to bury a large copper
plate, or lay a heavy bare cable in a trench and back fill it with 'Coke breeze'.
There are other alternatives, as well.
The obstruction lights on the tower can be a problem. The conduit
can go up the outside of the tower or up the inside (the inside is better,
but it's not critical). Please run the conduit up the center of one of the
tower faces, not up one of the corners. If the conduit is secured to a tower
leg, it badly limits our options when we're installing the AWOS sensors and
equipment. The bottom of the light fixture at the top should just about rest
on the top rung of the tower. If the light fixture stands above the top of
the tower, it can interfere with the wind as it flows over the wind sensors.
Lastly, the way that the conduit is secured to the tower makes a difference.
Steel City clamps work best, plain old U-bolts are the worst. If you used
baling wire to hold it all together while you put the clamps on, remove the
baling wire afterwards. You may find a scraped and cut technician to be very
hostile. A photocell is optional; install one if the customer wants it.
The antenna mount on the tower is required only if the AWOS uses UHF
radios for the data communications to the computer indoors. If you ran a data
cable between the tower and the computer location, no antenna mount is necessary
on the tower. Like the obstruction light conduit, Steel City clamps work best
and plain U-bolts are only one notch above duct tape.
The signal and power conduits from the tower foundation and the circuit
breaker panel to the sensor foundations should be rigid. EMT is more subject
to corrosion and the fittings on the ends tend not to stay tight. At the sensor
foundations, you can use cast boxes or condulets screwed onto the ends. Put
strain reliefs in the top ends of them. Leave pull ropes in the signal conduits.
The top ends of the conduits should stand only eight or ten inches above the
sensor foundations.
The electrical power to the sensor site is less complicated than it
appears. Every part of the AWOS runs on 120 Volts. Most often, 240 Volts is
provided to the breaker panel at the sensor site and the various sensors are
powered from each leg. We're often asked how much power the AWOS draws. There
isn't a straight answer, but generally the maximum load will be around 60
Amps. It's important that the Voltage not fall below about 108 Volts under
load, or microprocessors begin resetting. One pitfall is when the power is
run through buck-and-boost transformers. In more than one case, we have seen
the power taken from a 3-phase source in the vault, run though matching buck-and-boost
transformers, and exited at the sensor site with an unloaded 108 Volts. If
the transformers have different taps, you can bring it back up to where it
should be.
The concrete foundations shouldn't be flush with the ground, but should
stand a few inches above grade. Be sure that the surface is crowned a bit
so that water doesn't stand on it.
The antenna mounts(s) at the CDP location should be as far as possible
from the UNICOM antenna, without exceeding the length of the antenna cables.
You can leave the antennas themselves for the AWI technician to install, but
install the antenna mounts and run the antenna cables. AWI normally ships
50 foot cables for the CDP location, but they can be up to 100 feet long if
necessary. Contact AWI for longer cables if you need more than 50 feet. The
AWOS antennas should eventually be about four feet above the roof. If you
are installing a data cable to the sensor site, you'll only be preparing one
antenna mast.
The lightning rod at the top of the tower should be clamped tightly
and electrically bonded to the tower. Scrape the paint from the tower where
the rod attaches, and use either bronze grounding clamps or steel city clamps
to mount it. Don't use U-bolts.
Surrounding vegetation within 150 feet of the tower should be cut
down to ten inches or removed. This is an FAA requirement.
Clean up the site when you're finished, don't leave construction debris
or litter.
|
