I don't usually watch Acorn to Arabella on Youtube - I cringe at the amount of stuff the owners are cramming into and onto a classic boat, but they had an electrical issue and called in Nigel Calder. What is very educational to watch is how he uses a simple multi meter to run various tests and narrow into a solution. Great example of what I should be doing to chase down gremlins (and although I could try to be smug because I have outboards and no metal skin fittings, I know I have magical electrics sometimes).

I learnt a lot just watching a guru at work



Nigel's website is

boathowto.com/about/

Many years ago I was working in my engine room on my fishing vessel sorting out an electrical issue with a digital multi meter. I inadvertently touched the wood framework with the probe. I was surprised to get a voltage reading, I can't remember what it was. I tried other wooden parts and got the same reading. I called a friend down who lives close by. He was still working as an aircraft electrical engineer. He brought an analogue multi meter with him and demonstrated how it was possible to get false positives with digital multi meters. I went out and got one and now use both to sort out stuff.

Yeah an analgue one would be good to have. I had a problem years ago with my fridge. Although the digital one could work out which joints were hot and were an issue it was not good for working out why the fridge was cutting out when it was getting 12 volts. After multiple attempts I was able to barely discern at the fridge terminals that when the fridge started the start up current caused the voltage to drop below 10 volts and intiate fridge cutoff - which cycled over and over again. An analogue ammeter would have been easy to observe the drop to 10 volts.

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Kankama said..
Yeah an analgue one would be good to have. I had a problem years ago with my fridge. Although the digital one could work out which joints were hot and were an issue it was not good for working out why the fridge was cutting out when it was getting 12 volts. After multiple attempts I was able to barely discern at the fridge terminals that when the fridge started the start up current caused the voltage to drop below 10 volts and intiate fridge cutoff - which cycled over and over again. An analogue ammeter would have been easy to observe the drop to 10 volts.

Yeah nothing beats a old Simpson 260 or AVO for finding leaky voltages, motor run capacitors and adjusting things for a peak in voltage or current or anything. Its just so sensitive that you can put your two hands on the probes and its showing the leak or resistance of the skin.

And few people master the basics of measuring things like voltage drop across a things like wiring looms, resistors, capacitors, coils etc etc which quickly identifies faults like wiring or parts. Its actually worthwhile buying a good book on how to use a multimeter even if you an expert.

The old type analog meters have a very low input impedance compared to a digital one. This has the effect of "hiding" voltages that have a high impedance source. Like leakage across damp timber. Actually the digital meter is truly showing what's there but the user needs to interpret it.

A common old type analog multimeter might have an input impedance of 10000 ohms (10K). A typical Digital one 10,000,000 (10M) ohms.

It's possible to make a digital meter read like an analog type by damping it's input with a 10,000 (10K) resistor from Jaycar for 5c.

On boats where we don't want to know about high impedance voltages that's probably helpful. But in modern electronics high impedance is common and the digital meter is vital.

Trek, how can you read a digital voltmeter hitting a spike? It is just a blur of numbers. If you were to damp it with a resistor, would it not take the spike out? The very low duration spike (low voltage) was the thing that was troubling my fridge. I could use a recording oscilloscope, but that is going a bit far (I use them Science teaching)

You actually need a good test light and both digital and analogue multi-meters.

Lots of Digital Multi meters have "Peak Hold" function. If you press it the DMM will continuously read a keep the highest reading.

But for rise and fall monitoring I think analog is much better. In the past when I designed these things I used aircraft instrument panels as a guide since they have been studied and improved a million times. And although the systems are all digital the important displays are translated into analog since it's so much easier to see trends.

And what Ramona said. A test light is good for testing on a boat because its like a 100 ohm on or off meter.

www.fluke.com/en-au/learn/blog/digital-multimeters/under-utilized-functions-how-to-use-fast-peak-min-max-on-your-dmm

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Kankama said..
I cringe at the amount of stuff the owners are cramming into and onto a classic boat, but they had an electrical issue

For me its not the stuff that's crammed in, its that there seems to be valves, displays and switches all around the boat in the living space. It looks like they are sleeping with their fuel filter water separator.