Or how to repair a Raymarine knot-meter with a potato………..
Blue Haven Marina, Providenciales, Turks and Caicos, July 1-3, 2013
Our knot-meter had become increasingly unreliable over the past few weeks. It finally stopped working all together (despite multiple reboots).
A search of the internet revealed that we could check the transducer by disconnecting the wiring and measuring the on / off pulses with a multi-meter while someone (Jane) spun the paddle wheel. We got the pulses! This suggested that the instrument itself had gone bad. The local Raymarine dealer did not have one. It looked like a $350 eBay purchase and logistics planning to get it to wherever we next will be.
Before pulling the trigger on eBay, a quick call to Raymarine UK was made. [Aside: After a slow call to Raymarine USA left me on hold for 40 minutes.] It turns out that the transducer also measures water temperature. If the thermocouple dies, the instrument stops measuring speed as well as temperature. If this is the problem, the solution is to short the thermocouple with a 10-50kΩ resistor.
We drove all around the Providenciales in search of a resistor. Despite scouring a dozen shops, they were nowhere to be found. Luckily Pete had a Eureka moment; he remembered from a high-school science project that potatoes have resistance!
The potato was duly inserted into the circuit:
Prototype number 2:
Unfortunately dimmer switches are no longer rheostats and the resistance was much too high.
But the temperature reads128°; warm even for the Caribbean.