Digital Multimeter Question

[Joe Myerson]

Thanks, Cruisealong:

You've got it right: I have the yellow meter from Radio Shack, and it does take a 12-volt battery (hard to believe, and not that easy to find, BTW).

I'll follow your suggestions.

This AP has never worked correctly for me. I'm beginning to wonder if the fault could be in the amateurish wiring job, which was done by the owner before the PO. The wire doesn't look to me like it's really marine grade, and, perhaps, the resistance was too high, hence the motor was over-drawing (is that a word?)

And Parfait, too: I'm cobbling together a circuit tester today. I suspect that could well be the problem.

Anyway, I'll check all this out over the weekend.

--Joe

PS I'm probably going to upgrade from this 25-year-old Tiller Pilot to a Raymarine ST-2000, but I'll need to have a working connection for that, too.

[ray b]

I us a simple [junk] china VOM from harbor freight on the boat
they are often on sale for 4 bucks
so no sweat if/when it fails or is dammaged

the varying readings is common wait a few seconds
and you should get a steady reading
common flaw on boat wires is corrosen
that will cause voltage drop and extra amp flow so fuses blow
ccr eltric clean or any contact/turner cleaner or ever WD40
and a wire brush or emory paper/cloth on all links
should help

[Joe Myerson]

I made it onto the boat today, and finished checking out the autopilot circuit.

First, I tested the circuit with my newly made test light. Result: nothing.

Second, I checked both ends of the cable and found that the end at the battery looked pretty bad. I stripped more insulation off the ends of the wires, scraped the copper until it shined, and reconnected it tightly.

Third, I connected my ancient TillerPilot 1600 autopilot (built by Navico, now part of Simrad, I think). Nothing happened.

Fourth, I checked the fuse -- and it was blown, as usual.

Fifth, I hooked up my multimeter to the plug. It read 12.67 volts, meaning, presumably, that there isn't excessive resistance to the circuit.

Conclusions: 1) The Navico TilllerPilot is no good, and
2) the digital multimeter does work.

Apparently, if I just turn the meter on and let it sit, I get all sorts of random readings. But when it was connected to a working power source, it seems to work.

Moral: Check the circuit first.

Net result: I really do need a new AP.

Remaining mystery: I still don't know why the multimeter gives random readings when it's not connected to a circuit. But it seems to work. Perhaps some of the EEs can explain this, not that it matters much.

Thanks again to all who helped me use the multimeter.

--Joe

[Parfait's Provider]

Joe,

The multimeter is sniffing the air for electrons and since it has a relatively high input impedance, it can't decide what to report. If you grab the leads, one in each hand, it will tell you how much of a battery you are. An ordinary VOM with no input buffering wouldn't be able to do this because the input impedance is low enough to drain the stray currents to nealy zero.

Larry DeMers would probably differ on this. John V? John R? OJ?

Having fun cleaning house. Thanks for the diversion.

[Oswego John]

Ken and all.

Put the meter in fine ohms, Rx1K mode. Lick your fingers (umm, yummy), grab a probe in each hand and squeeze.

No, this won't tell you what your body fat ratio is, but it will give you an idea of it's resistance. Heavily calloused finger tips have lots of resistance. Maybe press the probes against other areas of the body, wrists, for instance.

With a 12V meter, while holding one probe, pressing the other probe against the tongue (yuck) will produce a tingle in some people.

Have fun,
O J
PS: And here, lots of people thought that multimeters were only good for trouble shooting electrical appliances , bells and whistles.

[Neil Gordon]

I knew that Joe had a magnetic personality but I didn't know he had an electical one, as well.

[Oswego John]

With tongue (in cheek), I ask. Is Joe's personality Magnetic or True?

Hey, give me a break. I didn't have much to work with.


O J

[Neil Gordon]

With tongue (in cheek), I ask. Is Joe's personality Magnetic or True?

Are we allowed to inquire about his variations and deviations, or is that contrary to this board's family friendly atmosphere?

[Oswego John]

Well thrill seekers, it seems to be a traditional Saturday evening on the BB board. The posts come thundering in, one after the other.

Let's see now, it's about 10:45 PM. The last, previous post arrived, all the way from Beantown, at 06:25 PM.

So, to keep you from nodding off at the keyboard, allow me to mesmerize one and all with another exciting chapter in the ongoing saga of the magical volt/ohmmeter.

This little kitchen tabletop experiment could prove to be of value to those of you who have wet cell batteries on board ship. It is meant to illustrate the conductivity, or lack of it in water. All that is necessary to conduct this test is your multimeter, a drinking glass, common table salt and some regular tap water.

Take a juice glass and fill it about half way with regular cold tap water. Set your ohmmeter on Rx1 and dip the two metallic tips of the probes into the water. Under normal circumstances, you should get no reading. Next, set the ohmmeter on Rx10. Again, you should get no reading.

Next, set your meter on the most sensitive of the resistance positions, Rx1K. Chances are that you will get a reading. What is happening is that the meter's battery voltage is passing through the plain water. But the water isn't plain, it is contaminated by all types of minerals and other impurities and the water is, in actuality, a conductor.

No matter how minimal the conductivity, it is for this reason that it is recommended to use distilled water to top off the electrolyte in the batteries. The more impure the water used, the quicker the battery will self discharge in time. But I digress.

So up to now, we get a reading when the meter is set on Rx1K. We don't get a reading on the next higher reading, Rx10. Reset the ohmmeter on Rx10. If it is possible, have someone hold both probe tips immersed in the water while you shake common table salt into the water.

As you shake the salt into the water, you should start getting a reading on the middle scale setting. The more impurities added to pure water, the higher the rate of conductivity. When the contamination is great enough, it is possible to get a reading on the Rx1 scale.

A sidebar to this test is the realization that the salt water that you sail in has more conductivity than that of sweet, or fresh water. This is relevant when considering lightning protection for your boat. Salt water is safer.

Have a good weekend.
O J

[Joe Myerson]

OJ:

That was an interesting discourse on resistance, conductivity and salt vs. sweet water. Does this mean that you Great Lake sailors are at greater risk of incineration from lightning than those of us who set up our aluminum lightning rods on the briny deep?

If so, would carrying a digital multimeter be of any help?

And, Neil, are you implying in your last post that one should carry a multimeter when visiting the Minneapolis airport? (Uh-oh, I don't want to violate the board's taboo on politics.)

Enjoy the rest of the weekend.

--Joe