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Old 14-11-2012, 13:08   #1
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Adjusting a needle voltage meter to read correctly

I have recently wired a needle type voltmeter to the starting batteries from my nav table. It reads 13V when it is really 12.57 give or take. I tested it with my digital meter and a second digital meter that I borrowed, both were 12.57 / 12.58. The meter is a Volvo Penta and has no dial in front to adjust the needle. Does anyone know how I can adjust the meter to read lower?
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Old 14-11-2012, 13:11   #2
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Re: Adjusting a needle voltage meter to read correctly

Put a variable resistor in the line and trim it down.
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Old 14-11-2012, 13:13   #3
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Re: Adjusting a needle voltage meter to read correctly

Landon, in general those meters are simply not accurate, and I reckon that you are wasting your time trying to fine tune one. The tricks that they use to get the expanded scale preclude great accuracy.

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Old 14-11-2012, 13:22   #4
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Re: Adjusting a needle voltage meter to read correctly

If it is a standard 90 degree movement type similar to an oil or pressure gauge then it does not have enough resolution (Scale) to be much more that an basic indicator. Some of the gauges volvo used had removable double plastic lenses Twist off the Besel and the lenses come out with a little pick. Then you can simply move the Pointer to the stop and turn it a bit on its shaft to correct it. If you want real accuracy you will need a digital one.
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Old 14-11-2012, 13:34   #5
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Re: Adjusting a needle voltage meter to read correctly

Thank you for the comments. I will first attampt taking the lens off and then the variable resistor. I just got tired of removing the steps everytime I wanted to test the starting batteries. Had the old meter on board and didn't want to spend much money on the digital type. Probably geting what I paid for. Yes the gauge is similar to an oil or gas guage.
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Old 14-11-2012, 13:37   #6
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Re: Adjusting a needle voltage meter to read correctly

You could also just get a cheap MM from harbor freight and velcro it to the wall there and hard wire a set of leads in.
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Old 14-11-2012, 14:25   #7
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Re: Adjusting a needle voltage meter to read correctly

Quote:
Originally Posted by landonshaw View Post
I have recently wired a needle type voltmeter to the starting batteries from my nav table. It reads 13V when it is really 12.57 give or take. I tested it with my digital meter and a second digital meter that I borrowed, both were 12.57 / 12.58. The meter is a Volvo Penta and has no dial in front to adjust the needle. Does anyone know how I can adjust the meter to read lower?
If you want accuracy...


1- Disconnect neg and positive leads
2- Remove bezel fasteners
3- Remove volt meter
4- Throw in garbage or donate to someone who does not want accuracy
5- Replace with accurate meter



In all seriousness I have yet to see more than a hand full of analog volt meters on boats accurate to better than 0.3V when compared to a calibrated Fluke....
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Old 14-11-2012, 14:34   #8
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Re: Adjusting a needle voltage meter to read correctly

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Originally Posted by Maine Sail View Post
If you want accuracy...


1- Disconnect neg and positive leads
2- Remove bezel fasteners
3- Remove volt meter
4- Throw in garbage or donate to someone who does not want accuracy
5- Replace with accurate meter



In all seriousness I have yet to see more than a hand full of analog volt meters on boats accurate to better than 0.3V when compared to a calibrated Fluke....
+1

I do keep a couple of old analog meters around for one specific application. When testing circuits with an intermittent fault or intermittent on (like flashing lights on a trailer) I find it easier to watch for the needle to jump on an analog meter that watching for a flashing number on a digital.

For accuracy, forget it.
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Old 14-11-2012, 14:50   #9
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Re: Adjusting a needle voltage meter to read correctly

Analog voltmeters suck.

A digital meter that's more than good enough for this purpose is about $7, a piece of velcro, screws or ziptie will be another $1, and it takes a few minutes of hacking to figure out how to power it from the 12V bus instead of from its own battery. Sushirama's solution is the way to go, if you ask me.
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Old 14-11-2012, 15:10   #10
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Re: Adjusting a needle voltage meter to read correctly

Landon, it is not a "needletype". Oh, you kids. That is an analog voltmeter, like an analog wristwatch, and the accuracy of a lab-grade analog meter can easily be outstanding. A lab-grade analog meter will have a mirrored strip behind the needle, so you can read the needle "straight down" with no parallax error, and then the accuracy is sometimes as good as one needle-width in either direction, 1% or better.

OTOH I've seen analog voltmeters in boats that sucked. With a 10-20V range, or something similar, if you adjusted it to be dead on at 11.5V it would be off by 1/2 volt at 14.4 volts, which makes it useless.

Adjusting an analog meter is done with the adjustment screw that is part of the pivot mechanism for the needle. There should be a plain (slotted) screw head showing in the cover, at the pivot point of the needle. You apply a KNOWN voltage in the midscale rating, i.e. alternator voltage of 13.8 or 14.4 whatever yours runs at, and then you gently turn that screw to match up the needle to that voltage.

The big trick is to have a calibrated voltage source or reference voltage to compare with, which is not easy.

Digital meters (DMM's) are not necessarily any better. They may be accurate to 1/2% or a full 2% of scale, and with the '12" volt scale usually being 20 volts, that can mean an error of 0.2 volts plus an additional error of 2-3 digits on the rightmost side. A cheapie from Harbor Freight can be off by 1/2 volt. A good Fluke will often be accurate to the tenth of a volt. You need to check the specs on what you buy, if you decide to replace the analog one.

You can also just put two stripes on the face of the analog one: Mark one at "alternator" and a second at "full" or whatever point you need to see. Ignore the numbers, just know "here" means full, or low, and that's often good enough.
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Old 14-11-2012, 15:32   #11
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Re: Adjusting a needle voltage meter to read correctly

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Originally Posted by hellosailor View Post
Landon, it is not a "needletype". Oh, you kids. That is an analog voltmeter, like an analog wristwatch, and the accuracy of a lab-grade analog meter can easily be outstanding. A lab-grade analog meter will have a mirrored strip behind the needle, so you can read the needle "straight down" with no parallax error, and then the accuracy is sometimes as good as one needle-width in either direction, 1% or better.

OTOH I've seen analog voltmeters in boats that sucked. With a 10-20V range, or something similar, if you adjusted it to be dead on at 11.5V it would be off by 1/2 volt at 14.4 volts, which makes it useless.
Hellosailor,

From the OPs op this is not a lab grade calibrated meter but a cheap, panel meter from Volvo. I've had one before and they are just reasonably accurate but very far from calibrated, and have no adjustment on the front.

The one I had did seem to be consistent so the suggestion to mark the meter face might work but would annoy me. Sort of like when my wife sets the bedroom clock 15 minutes ahead so she won't be late.
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Old 14-11-2012, 15:55   #12
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Re: Adjusting a needle voltage meter to read correctly

This wouldn't be like setting the clock ahead. That's deceiving no one including the person who does it. My friend's father did the same thing, damfino who it is supposed to fool.


If you really want to "correct" it, scan, copy, or redraw the needle scale on your computer. Now apply a couple of reference voltages to the meter, so you can mark the needle position for them. Print out a new scale with the correct numbers on it, and just paste it into the meter.

That's the REAL right way to do it, just make a custom scale. Child's play for anyone with a computer and any drawing program and some free time. Or <G> a good reaosn to just replace the meter if you're real fussy about these things and don't have the free time.
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Old 14-11-2012, 16:21   #13
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Re: Adjusting a needle voltage meter to read correctly

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Originally Posted by hellosailor View Post
This wouldn't be like setting the clock ahead. That's deceiving no one including the person who does it. My friend's father did the same thing, damfino who it is supposed to fool.
No not the same as setting a clock ahead. The part I was referring to is a having a device where the given data is known to be incorrect and having to add a correction factor, either mentally or with a piece of tape stuck on it. My nit picky nature would demand that the data and scale be calibrated correctly.

Don't know who cheating on the clock time fools either. Now my wife hasn't reset it for the change in daylight savings time so the clock is one hour and 15 minutes off and she won't let me reset it.




Quote:
Originally Posted by hellosailor View Post
If you really want to "correct" it, scan, copy, or redraw the needle scale on your computer. Now apply a couple of reference voltages to the meter, so you can mark the needle position for them. Print out a new scale with the correct numbers on it, and just paste it into the meter.

That's the REAL right way to do it, just make a custom scale. Child's play for anyone with a computer and any drawing program and some free time. Or <G> a good reaosn to just replace the meter if you're real fussy about these things and don't have the free time.
I would be tempted to pop the cover off and see if there is an adjustment inside. However, unless the meter is part of a matching set I would just toss it and find a digital panel meter to fit. But as you mentioned, just because something is digital and shows data with several significant figures, doesn't mean those figures are accurate. Would have to calibrate that one as well.
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Old 14-11-2012, 17:41   #14
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Re: Adjusting a needle voltage meter to read correctly

Analog voltmeters can be accurate if the manufacturer gives a hoot. My ancient Marinetics is spot on. I cannot say that for the Blue Sea digital or the Raymarine instruments. The Raymarine is about a volt off, and yes I have checked it at the device terminals, which is unbelievable as getting a very accurate digital voltage is trivial to design and build these days. My reference is a Fluke meter of known accuracy and fudge factors in my head for the two nearly useless meters.
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Old 14-11-2012, 18:33   #15
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Re: Adjusting a needle voltage meter to read correctly

Could just adjust the spring bias screw on the front until it reads correctly in the range you want. Small screw driver slot. Normally for zero adjustment but would adjust the needle to give the reading needed.
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