Many boat engines are cooled by freshwater which circulates in
a closed loop. Then a sea-water pump brings cool salt water to a heat-exchanger
that cools the freshwater. The engine temperature gauge measures the
temperature of the freshwater coolant in the engine. When this rises
above normal, an alarm sounds.
However, with a "wet exhaust", the sea-water is injected into
the exhaust pipe - to keep it at a reasonable temperature. Most
commercial exhaust alarms are set at 71 - 74°C. If the sea-water
flow is stopped (or reduced), the exhaust temperature will quicky rise -
and may damage the exhaust pipe (big rubber pipe) or silencer
(fibre-glass object) while the freshwater coolant is still below
its alarm temperature.
So to guard against this (caused by either forgetting to turn
the sea-water inlet seacock on, or debris blocking the seawater inlet
pipe, or a sea-water pump impeller failure), it is prudent to monitor the
temperature of the exhaust pipe. Commercial exhaust alarms consist of a
sender (bi-metallic strip which gives a make/break connection) which
screws into the top of the exhaust pipe or which fits in the
silencer/muffler. This is connected by two wires to the display which
shows a red light and buzzes when the temperature is above the set value
and the alarm is triggered.
One drawback of this system is, that, at low engine revs on
a warm day, the seawater runs gently along the bottom of the exhaust
pipe while the hot exhaust gasses flow along the top: so the alarm may
trigger (as I have found myself).
A combination of problems: electrical connection lugs corroding on
sender, bulb failure in display, as well as the interpretation problem
discussed above, all pointed to scope for improvement.
A thermistor is small, robust, cheap and can reliably _measure_
temperature. The downside is that the resistance of the termistor
(which gives the temperature) is related to the temperature by a known
but rather involved formula.
Optimally a computation is needed to derive the temperature from the
resistance. This is easily achieved by a modest computer: an Arduino
(Uno or Nano) can do this and only costs a few pounds.
I had a spare thermistor (MF52 Series bead type, 2.5 mm
diameter, NTC of 10KΩ nominal resistance, costing a pound or so)
and fitted it (with epoxy) into a hole drilled in the brass exhaust
alarm sender. Then I used the Arduino Nano that I had installed under
the console in the wheelhouse (to monitor the fuel tank level) to measure the resistance and
convert it to temperature - which was then displayed on the NEXTION
display (as well as triggering an alarm by a buzzer and flashing part of
the display). Since the system now gives the temperature value at all
times, I can now monitor that it is working correctly.
More details of installation here. Arduino code.