Exhaust Alarm

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.

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