A man with three watches has no idea what time it is....

jviss · Jul 6, 2012

Rick486 said:
I have a riddle here I'd like to get some thoughts on.
I have an analog voltmeter on my DC panel of which I have been suspicious of for some time. Yesterday I dug out the digital voltmeter and made some measurements. With the boat plugged in at the dock and batteries showing "Float" as per the Freedom 25 panel, the AVM read about 13.4 volts, give or take a hair. The DVM read 13.88 volts on all three 4D Gel house batts. The Freedom 25 spec says at Float for those batts with the settings I have, voltage should be 13.8 volts. So it looks like the DVM and Freedom 25 are in pretty close agreement, but the AVM on the panel is out to lunch.
So the question is:
Who should I believe, and is the apparent inaccuracy of the AVM on the panel just due to the nature of the thing or could there also be voltage drop along the way to the panel. My next step is to open the panel and see where the AVM is reading across.

Thoughts?
Thanks

There would only be a voltage drop in the wiring to from the batteries to the AVM if there is current flowing in those wires. It should be wired directly with wires that aren't shared with any load.

It could be that the meter is out of calibration. Is it a full-scale meter or an expanded scale? Both should have an adjustment screw.

Maine Sail · Jul 6, 2012

Analog volt meters on boats are horribly inaccurate. I just dealt with both DC and AC analogs yesterday that we're horribly out of whack on a Defever 41. The were able to be some what calibrated but still fairly inaccurate. The volt meter sense wire should not sense anything but the battery, meaning it should not sense at the end of the wire that feeds your DC panel or it will also read the voltage drop... Many builders were lazy and did install them to sense at the panel or back side of the battery switch...

Many AVM's are a 2" round hole and can be filled with a true battery monitor such as the Victron BMV-600....

amry69 · Jul 6, 2012

I agree that the analog meter needs to be calibrated periodically. Use the volt meter as a base a adjust the screw. Should be a fairly reliable backup (I like using volt meters) for years to come.

kloudie1 · Jul 6, 2012

The only analog meter I've ever trusted was my Simpson 260 Vacuum Tube Voltmeter.(1967 vintage, Navy issue). It was calibrated before each use.

Ross · Jul 6, 2012

You can get accurate and reliable analog meters but you always get no more than what you pay for in quality.

Add link: http://www.newark.com/jsp/search/br...etricAttributeId=&prevNValues=422+2203+201271

Calif. Ted · Jul 6, 2012

Analog volt meters on boats are horribly inaccurate.

Amen
Most I've had in panels are cheap enough to require a couple finger taps to calibrate "old school".

kenn · Jul 6, 2012

Before putting full trust in a handheld digital multimeter, be warned that the cheaper the DMM the less accurate it may be.

For our boat, I found a good-quality surplus analog edge-scale meter movement, added the circuitry to make it an expanded-scale meter (displays 10 to 15 v on its scale), made a custom scale, then calibrated it with a good DMM while across our battery. See, you can have an accurate analog voltmeter

Ross · Jul 6, 2012

kenn said:
Before putting full trust in a handheld digital multimeter, be warned that the cheaper the DMM the less accurate it may be.

For our boat, I found a good-quality surplus analog edge-scale meter movement, added the circuitry to make it an expanded-scale meter (displays 10 to 15 v on its scale), made a custom scale, then calibrated it with a good DMM while across our battery. See, you can have an accurate analog voltmeter

Counting your labor and material you have a two hundred dollar meter.

FastOlson · Jul 6, 2012

Here's a good solution

When our original NewMar meter stopped working after a lotta years/decades, I found that they still sell an analog meter with the same (perfect!) expanded scale. Exact fit for our original panel opening, too.

I replaced ours to continue serving as a lower-tech adjunct to my fancy new AH meter.

On our boat this meter is direct-wired to each battery bank with a momentary selector switch for checking one bank or the other.

IMHO, it's an essential primary trouble shooting gauge on our boat.
http://www.newmarpower.com/Meters/Meters.html
Look down the page a ways : analog meters, dc volt 8 - 16.

It agrees Very closely with the digital readout, BTW. Admittedly, it does lack the mirror behind the pointer on an old Simpson face!

Stu Jackson · Jul 6, 2012

That makes sense. It's what we do. I just calibrated the analog to the digital and use it for a quick check. It's only there 'cuz when i stalled my Link 2000 I decided it would replace the old AutoMac instead of the voltmeter. It's all relative in analog land anyway.

kenn · Jul 6, 2012

Ross said:
Counting your labor and material you have a two hundred dollar meter.

Fish gotta swim, birds gotta fly, I have to futz with stuff. Makes the winter fly by.

kenn · Jul 7, 2012

Rick486 said:
Thanks guys for all the great ideas. First of all I didn't know I could calibrate the AVM. So today I'll open the panel and take a DVM reading across the AVM and look for the calibration adjustment. Thanks again.

If you're lucky, there will be a trimpot on a small PC board behind the AVM which can be adjusted. But even on a cheap meter lacking a PC board, you can still improve the accuracy by tweaking that little screw on the front of the meter. Normally this screw is used to set the '0' or left-hand needle position, but since you're only interested in a narrow range of the meter scale (approx 12 to 14 v), you can tweak it to match the reading of a good DVM across the same battery bank. You shouldn't have to turn the front screw more than maybe 45 degrees either way, so turn it very carefully.

Don12364 · Jul 7, 2012

In your case you are really only interested in accuracy when measuring voltages between about 12.4 and 14.4.
Every moving coil analogue meter I have seen has a small adjustment screw in the centre of the front panel. This is used normally to simply preset the pointer to zero. These screws are a great temptation to idle fingers. However I expect your meter is of the suppressed zero (or expanded scale) type so has no zero on its scale.
This type of moving coil analogue meter rarely goes out of calibration internally so why not just reset its adjustment so it reads the same as your digital meter? It will then be pretty near perfect at voltages on either side of this recalibration point.