THE POOL OWNER’S DILEMMA
The thermal behavior of swimming pools is
complex, due to a number of factors that act to
cool the water, while the sun and artificial means
work to keep the water warm. Cooling forces are
at work day and night, and include evaporation,
conduction into surrounding soil, air current
effects, and nighttime longwave (infrared)
radiation into space.
Gas-fired heaters are the most common pool
warmers, but up-front costs for equipment and
installation run into the thousands of dollars.
And then there’s the operating cost and the price
of natural gas. These heaters are prodigious
polluters; for an average-sized pool, a 1°F rise
in temperature results in spewing 50lbs to 60lbs
of carbon dioxide into the atmosphere.
Roof-mounted solar heaters are a second option.
Water from the pool is pumped through solar-heated heat exchangers and returned to the pool.
Like gas-fired heaters, solar heaters are costly and
are generally unsightly. They’re also a problem in
Bubble plastic blankets are passive devices that
cover the pool. They function mainly as water
conservation devices by inhibiting evaporation,
thus slowing evaporative cooling. They also block
nighttime radiation loss.
Manufacturers claim these blankets also behave
as warmers, transmitting additional energy from
the sun into the water. But water already has
a very low albedo (ratio of incident energy to
reflected energy). It’s about . 10 for deep water,
a bit higher in white-bottomed pools, and it’s
doubtful that bubble covers improve on that.
(The low albedo of water is the reason for the
great concern about the receding of the polar ice
caps. Open water warms much more rapidly than
ice fields, which reflect more than 80% of the
Moreover, pool blankets destroy the aesthetic
appeal of a backyard pool; they’re hard to manage,
difficult to clean, and unsafe where small children
are around; and they’re often dumped after a
132 Make: Volume
MATERIALS AND TOOLS
Black polyethylene film, 6 mil commonly used by
landscapers and builders
1. Mod the soldering iron.
Polyethylene film can be easily spot-welded to
hula hoops made of the same material, by using a
slightly modified soldering iron.
Add a simple guide to your iron so that it self-guides along a spot-welding path that’s uniform
around the entire hoop. To do this, just add a bar
that extends 1½" past the tip of the soldering iron.
Clamp the small metal bar onto the barrel of the
soldering iron, using a scrap of metal as a spacer
between, so that the guide stands
2" away from
the iron’s tip (half the width of a hula hoop). Use
small hose clamps to secure the guide (Figure A).
After you’ve made the guide, grind the tip of the
soldering iron to a flat surface of about
2. Build a jig for assembly.
Since an average pool may require 50 or more Lily
Pads, it’s handy to build a simple jig for quicker
assembly (Figure B).