What did I need to monitor?
started counting what would be useful to know while under way. The list
grew over time and sorted itself into
two groups. On the one side were the
domestic systems for day-to-day living,
and then there were vessel parameters
while under way.
Domestic wish list:
Hot water tank
Engine room temperatures:
probe on the tank wall quickly tells us
when the and we can shut
down the noise.
In the engine room, important to
know stuff before bad things happen. A
rising engine exhaust temperature provides an excellent warning that
restricted raw water cooling (seaweed
in the intake/a loose hose/a damaged
impellor). Stop, find the fault and fix
it, and you have a major engine
overhaul later. Miss that warning, and
things get very expensive, very quickly.
The same applies to the alternator. As it heats up, the output drops.
Are the amps dropping because a battery
cell has gone short circuit, or are the batteries just hot, or is the alternator at
where its rated output normally drops 50
percent? useful to deduce these things
before hot sulphuric acid starts to spray
around the engine room, or the insulation
on the alternator wires melt!
Creating a System
Any temperature monitoring system comprises two
parts: The sensors at the business ends,
Your list will differ. The point is,
there were quite a few temperatures that would be nice to keep
an eye on. The domestic ones are
pretty self-explanatory. Obviously, nice to know your fridges
The author's homeare functioning properly whether
made display and
selector switch unit.
on a spring morning, or
a sweltering in August in
Desolation Sound. Summer makes the and a display unit. look at the senfridge(s) work a lot harder, which af- sor first.
The traditional way to measure temfects the house batteries.
If like us, we pack our freezer perature is to use thermocouples or
with food for month-long cruises, and thermistors. As the temperature rises,
really need to know OK in there, their resistance changes proportionally.
without opening the lid any more than Both types are cheap and readily available plus, being a two-wire system, you
necessary to check.
Hot water was another issue. Lack- only need two-conductor wire to coning a boiler/hydronic heat system, we nect them back to the display. But they
use the generator to make hot water have an inherent
at night, to wash up and bathe/show- perform well in an electrically noisy
er, long after the run has heated environment, and boats are very noisy.
the water tank via the cooling Alternators, generators, VHF radios,
system. No one wants to run a genera- depth sounders, radars and electrical
tor more than necessary in an anchor- pumps all throw out electrical noise,
age. (OK, we all know a few folk who and unless you are very careful where
hold to this rule.) How hot is hot you lay your two-conductor cables,
enough anyway? Having a temperature or use screened cables, there will be
weird readings. Plus, every connection
between dissimilar metals (between
the thermistor and the wires, between
the wires and the display unit) acts as
a weak thermistor too. Then the
problem of water penetration at the
sensors, which also affects accuracy.
A neat way to side-step those problems is to use a little integrated circuit to
measure temperature. These smart devices, which provide a current for a signal output less affected by electrical noise), are small and are about $1
each. There are numerous types on the
market. One such device is the LM335,
used widely in the industry. (The LM135
is the military grade bit more
expensive and rugged.) They usually
come in a plastic cylinder measuring
millimetres (very small), have three
wires sticking out of them, and work
over the range to
Being a retired electrical engineer,
used them in numerous projects
over the years, but never used a
Dallas Semiconductor DS18S20. This
is similar in size, with
three connections, but
a slightly wider temperature range
to and a fully
digital output, which
makes it effectively immune to noise (similar
to an FM radio compared to an AM radio).
The DS18S20 is a very
smart chip that can do
many things beyond
what I was planning to
do, but it could also be
used in mode
as per this project. By
the way, if you still think in Fahrenheit,
then the DS18S20 is not for you;
strictly a Centigrade device.
For a display unit, I went online to
eBay and found a 12VDC red LED digital car meter thermometer from Skt_
flyer that cost under $13 and came with
a DS18S20 sensor. It had large, easy
to read numbers. I had plans to add a
multi-position switch so I could click
between numerous sensors. I ordered
a bag of 10 DS18S20s through Amazon
for a little under $10.
At my local electrical supplier I
bought an on/off toggle switch, and a
rotary single-pole, 12-position switch,
for a couple of dollars each. If you
like soldering, buy your switches with
screw terminals rather than solder lugs.