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harvey4(at)earthlink.net
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 Posted: Sat Sep 06, 2008 1:50 pm Post subject: Ammeter reading oddly |
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When I first start the engine, and then engage the alternator, the
Van's ammeter flips over to +20 for a few seconds.
Then it gradually descends to zero and settles a needle's width on
the -ve side of zero, where it sits for the duration.
If I turn the alternator off, the ammeter goes to -10 so I know it's
sensing something, but the slight -ve reading in flight bothers me.
Do you think it's just an adjustment issue on the gauge, or is
something else going on?
Thanks for ideas.
Neil
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nuckolls.bob(at)cox.net
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| Posted: Sat Sep 06, 2008 2:11 pm Post subject: Ammeter reading oddly |
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At 05:49 PM 9/6/2008 -0400, you wrote:
| Quote: |
When I first start the engine, and then engage the alternator, the Van's
ammeter flips over to +20 for a few seconds.
Then it gradually descends to zero and settles a needle's width on the -ve
side of zero, where it sits for the duration.
If I turn the alternator off, the ammeter goes to -10 so I know it's
sensing something, but the slight -ve reading in flight bothers me.
Do you think it's just an adjustment issue on the gauge, or is something
else going on?
|
Van's ammeter is an electronically signal conditioned instrument
with scale and offset adjustment pots inside. It sounds like
it's just out of calibration. This is ONE of the reasons
I don't recommend battery ammeters.
If you also have a voltmeter . . . THIS is the true guardian
of flight confidence. If the voltage is staying above
battery charge levels 13.8 minimum, 14.2 nominal, 14.6 maximum
then all is well in Electronville.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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peter(at)sportingaero.com
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| Posted: Sun Sep 07, 2008 1:01 pm Post subject: Ammeter reading oddly |
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You probably don't really want to hear this, but Van's gauges aren't the
best quality items on the planet. I have found that the temperature and
pressure gauges are not that accurate (+/- 15%), so no reason why the
ammeter should be any better! I've always found a voltmeter very
comforting to tell the alternator is still at work.
Regards, Peter
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nuckolls.bob(at)cox.net
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| Posted: Mon Sep 08, 2008 6:33 am Post subject: Ammeter reading oddly |
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At 09:56 PM 9/7/2008 +0100, you wrote:
| Quote: |
<peter(at)sportingaero.com>
You probably don't really want to hear this, but Van's gauges aren't the
best quality items on the planet. I have found that the temperature and
pressure gauges are not that accurate (+/- 15%), so no reason why the
ammeter should be any better! I've always found a voltmeter very
comforting to tell the alternator is still at work.
Regards, Peter
|
I have a Van's battery ammeter which I've confirmed does
have a sensitivity to strong RF fields. I disassembled
the gage and was pleasantly surprised at the level of
technology used in this particular instrument. I'll
direct the reader's attention to the suite of photos
at:
http://aeroelectric.com/Pictures/Instruments/Vans/
In particular, note the potentiometers (blue)
in this view . . .
http://aeroelectric.com/Pictures/Instruments/Vans/VAM10_05.JPG
I suspect these adjustments are included in all
the Vans instruments in this style.
In general, I found them well assembled and
artfully designed. These use a 4-quadrant,
moving-magnet motor to drive a pointer. This
is the technology of choice in most mechanical
instrumentation used throughout the automotive
industry. They're inexpensive, rugged and capable
of exemplary accuracy.
They DO require some signal conditioning electronics
and the electronics are subject to the usual
snakes and gremlins. Long term drift (calibration
stability) and RF susceptibility are the
biggest issues. Indeed, the instrument I have
exhibits a characteristic I've never seen before
in an instrument.
A hand-held transceiver waved around the
powered up instrument will drive the pointer
either direction depending on position of
the antenna. See:
http://aeroelectric.com/Pictures/Instruments/Vans/VAM10_09.JPG
http://aeroelectric.com/Pictures/Instruments/Vans/VAM10_10.JPG
Mechanically, I was surprised that they use
thru-hole parts . . . this is NOT the most economical
way to assemble a complex device in volumes.
The largest disappointment was components
that stand up on their lead wires. These instruments
might be at risk for loosing a component under
qualification vibration.
Generally speaking, this series of instrument
appear to be of good value . . . particularly
if they ALL have calibration pots. I can live
with long term changes in calibration as long
as I can RECALIBRATE as necessary.
I hadn't planned to write about these devices
until I've solved the RFI problem . . . but I
didn't want this thread to move forward without
making folks aware of useful details about
how they're built.
The two potentiometers probably adjust offset
(zero) and scale (calibration). They may be
interactive meaning that one has to rock back
and forth between zero and full-scale readings
so that the pots can be set dead-on.
Bob . . .
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frank.hinde(at)hp.com
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| Posted: Mon Sep 08, 2008 7:06 am Post subject: Ammeter reading oddly |
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It's the gauge...The fact the ammeter swings high for a few seconds then returns to normal indicates that everything is working normally.
Explanation....
You have a battery that spits out hundreds of amps in 3 to 5 second burst to run the starter...Now think of that energy as a tank of water...You sunddenly opened a big valve at the bottom and let a lot of water out...Now the ballvalve opens and it gradually gets topped up by the small filler pipe until its full then the valve closes and then it stops.
Well the alternator is is the small filler pipe and the ammeter is measureing the flowrate back into the tank..I mean battery.
That few seconds of +20A is the alternator topping the battery off.
Now if the ammeter goes to -10A and stays there in flight you got a problem.
Realistically ammeters are a bit of a waste of time, My Dynon has one and I never look at it..But I do have an alarm on battery volts..if it drops to 12.5V I get an audible alarm...Then I look at the amps.
Frank
RV7a IO360 electric fuel pumps only. Now featuring mogas+ 10% ethanol.
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