Plug in voltmeter

[Bob]

I'm looking for some advice on a voltmeter (preferably digital) that
will plug into the "cigarette lighter" of my airplane. I found one on
the J. C. Whitney site, but $50 is too much for what I want to use it for.

I've got a JPI analyzer which reads voltages off the avionics bus, and
also a Davtron clock which does the same. Curiously, they differ, with
the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
avionics bus seems to be less than I'd expect, anywhere from 12.2 to
13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
drops during flight (I guess as the voltage regulator cuts off the
alternator as the battery is recharged after starting) to the point
where I get low-voltage warnings from the JPI.

No other symptoms of electrical problems. Been doing this for many
months (since I had the JPI installed) but still cranks fine.

Anybody know of a reasonably priced gadget that will let me monitor the
MAIN bus feeding the cigarette lighter?

Rich

[Jack Cunniff]

Bob > writes:

>I'm looking for some advice on a voltmeter (preferably digital) that
>will plug into the "cigarette lighter" of my airplane. I found one on
>the J. C. Whitney site, but $50 is too much for what I want to use it for.

>I've got a JPI analyzer which reads voltages off the avionics bus, and
>also a Davtron clock which does the same. Curiously, they differ, with
>the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
>avionics bus seems to be less than I'd expect, anywhere from 12.2 to
>13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
>drops during flight (I guess as the voltage regulator cuts off the
>alternator as the battery is recharged after starting) to the point
>where I get low-voltage warnings from the JPI.

>No other symptoms of electrical problems. Been doing this for many
>months (since I had the JPI installed) but still cranks fine.

>Anybody know of a reasonably priced gadget that will let me monitor the
>MAIN bus feeding the cigarette lighter?

>Rich

www.google.com - "cigarette lighter voltmeter" yielded
http://www.cetsolar.com/voltmeter.htm - $16.99

The "reasonable price" limitation of your query, though, might not help
the situation. I suspect that you want an instrument which will give you a
reference voltage. A "reasonably priced" voltmeter may be less accurate.

Hope the link helps.

-Jack

[[email protected]]

Bob > wrote:
> I'm looking for some advice on a voltmeter (preferably digital) that
> will plug into the "cigarette lighter" of my airplane. I found one on
> the J. C. Whitney site, but $50 is too much for what I want to use it for.

> I've got a JPI analyzer which reads voltages off the avionics bus, and
> also a Davtron clock which does the same. Curiously, they differ, with
> the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
> avionics bus seems to be less than I'd expect, anywhere from 12.2 to
> 13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
> drops during flight (I guess as the voltage regulator cuts off the
> alternator as the battery is recharged after starting) to the point
> where I get low-voltage warnings from the JPI.

> No other symptoms of electrical problems. Been doing this for many
> months (since I had the JPI installed) but still cranks fine.

> Anybody know of a reasonably priced gadget that will let me monitor the
> MAIN bus feeding the cigarette lighter?

> Rich

The problem is you need accuracy and cheap and accuracy don't go together.

I would be surprised if the accuracy of the existing voltmeters are much
better than about 2%, which would easily account for the difference.

Multimeters in the $50 to $100 range have a typical accuracy of 1% for
DC. That translates to +/- .13V at 13V, which gets you close, but is
a bit much to pay for a one-shot measurement (unless you have other use
for the multimeter).

The cheapest option would be to find someone that already has a decent
meter (HAM, repair guy, etc.) that will let you borrow his so you can
make a calibration chart for the meters in the A/C.

As to the voltage, it does sound a little low.

One check you can make is to note the voltage with everything turned
on at runup RPM. Then turn everything you can off and note the voltage
again. There should be no change.

If there is a change, I would check, clean and tighten the connection(s)
from the battery to the avionics bus.

While you're doing that, I would also check to see if the JPI and Davtron
have their power and grounds connected to the same things.

Some installations also have a ground bus where all the avionics grounds
come together with one heavy connection to the A/C ground. If you have
one of those, check, clean and tighten the connection between the bus
and the A/C ground.

Then run the check again. If it still changes you probably need a trip
to the avionics shop to fix it unless you happen to be good at this
sort of thing.

--
Jim Pennino

Remove -spam-sux to reply.

[Bob]

YEAH!
That's the device I'm looking for! Now to see what shipping would be...
or how to find it locally (I'm in SE Mich).

Jack, I agree with your counsel and Jim's, that calibration may be an
issue. However, I've got a pretty good digital multimeter, and a REALLY
GOOD Hewlett Packard laboratory power supply that should allow me to
calibrate it (or create a deviation card) sufficient for my purposes.

Good suggestion to check grounds, too. I'm SURE they are grounded to
different places. I can see where the Davtron mounts... gonna have to
figure out the wiring on the JPI (which is using a previous GEM 602
harness).

Thanks again!

Rich

Jack Cunniff wrote:
> Bob > writes:
>
>
>>I'm looking for some advice on a voltmeter (preferably digital) that
>>will plug into the "cigarette lighter" of my airplane. I found one on
>>the J. C. Whitney site, but $50 is too much for what I want to use it for.
>
>
>>I've got a JPI analyzer which reads voltages off the avionics bus, and
>>also a Davtron clock which does the same. Curiously, they differ, with
>>the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
>>avionics bus seems to be less than I'd expect, anywhere from 12.2 to
>>13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
>>drops during flight (I guess as the voltage regulator cuts off the
>>alternator as the battery is recharged after starting) to the point
>>where I get low-voltage warnings from the JPI.
>
>
>>No other symptoms of electrical problems. Been doing this for many
>>months (since I had the JPI installed) but still cranks fine.
>
>
>>Anybody know of a reasonably priced gadget that will let me monitor the
>>MAIN bus feeding the cigarette lighter?
>
>
>>Rich
>
>
> www.google.com - "cigarette lighter voltmeter" yielded
> http://www.cetsolar.com/voltmeter.htm - $16.99
>
> The "reasonable price" limitation of your query, though, might not help
> the situation. I suspect that you want an instrument which will give you a
> reference voltage. A "reasonably priced" voltmeter may be less accurate.
>
> Hope the link helps.
>
> -Jack

[Jack Cunniff]

>YEAH!
>That's the device I'm looking for! Now to see what shipping would be...
>or how to find it locally (I'm in SE Mich).

Glad to help. Someone on the Piper mailing list has mentioned this device
before, but the groupthink has been that the JPI would be more accurate
than a cigarette-lighter voltmeter like this. Still, if one didn't have
another way to get a quick voltage reading, it's better than nothing.

>Jack, I agree with your counsel and Jim's, that calibration may be an
>issue. However, I've got a pretty good digital multimeter, and a REALLY
>GOOD Hewlett Packard laboratory power supply that should allow me to
>calibrate it (or create a deviation card) sufficient for my purposes.

Again, glad to help.

-Jack

[Stu Gotts]

On Wed, 17 Dec 2003 17:28:37 +0000 (UTC), Jack Cunniff
> wrote:

>Bob > writes:
>
>>I'm looking for some advice on a voltmeter (preferably digital) that
>>will plug into the "cigarette lighter" of my airplane. I found one on
>>the J. C. Whitney site, but $50 is too much for what I want to use it for.
>
>>I've got a JPI analyzer which reads voltages off the avionics bus, and
>>also a Davtron clock which does the same. Curiously, they differ, with
>>the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
>>avionics bus seems to be less than I'd expect, anywhere from 12.2 to
>>13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
>>drops during flight (I guess as the voltage regulator cuts off the
>>alternator as the battery is recharged after starting) to the point
>>where I get low-voltage warnings from the JPI.
>
>>No other symptoms of electrical problems. Been doing this for many
>>months (since I had the JPI installed) but still cranks fine.
>
>>Anybody know of a reasonably priced gadget that will let me monitor the
>>MAIN bus feeding the cigarette lighter?
>
>>Rich
>
>www.google.com - "cigarette lighter voltmeter" yielded
>http://www.cetsolar.com/voltmeter.htm - $16.99
>
>The "reasonable price" limitation of your query, though, might not help
>the situation. I suspect that you want an instrument which will give you a
>reference voltage. A "reasonably priced" voltmeter may be less accurate.
>
>Hope the link helps.
>
>-Jack

I think that wherever he takes a voltage reading, there will be a
difference. What he needs to do is to hook up the Davtron and JPI to
the exact same post, both + and -, use the exact same type of
connector and insure that the same wire size is used as lead.
Incidentally, I've seen about a dozen Davtron/JPI equipped planes, and
none agree on the voltage. One may have built this "flaw" to drive
some of the flying population crazy.

[[email protected]]

Stu Gotts > wrote:

> I think that wherever he takes a voltage reading, there will be a
> difference. What he needs to do is to hook up the Davtron and JPI to
> the exact same post, both + and -, use the exact same type of
> connector and insure that the same wire size is used as lead.
> Incidentally, I've seen about a dozen Davtron/JPI equipped planes, and
> none agree on the voltage. One may have built this "flaw" to drive
> some of the flying population crazy.

Theoretically, if you have a sensitive enough voltmeter, that's true,
but for the currents drawn by modern solid state avionics if you get
..1V drop accross a connection, it's probably a bad connection.

The biggest source of error, assuming both the Davtron and JPI are installed
correctly, is their basic accuracy.

Most of these things read to .1V resolution, but to get .1V accuracy at
13V means the meter accuracy must be better than .1/13 which is about
..8%.

To further complicate the discussion, since these things are digital, you
need to have half last digit accuracy to ensure the last digit changes
halfway between two values. That is, you need to have .05V accuracy to
ensure the last digit switches from .2V to .3V as the voltage passes
through .25V for example.

At 13V, that means .05/13 which is about .4% accuracy. Most bench test
equipment is only good for about 1% accuracy; .4% is starting to get into
the realm of lab equipment.

On top of all that, there are also the problems of temperature coefficient,
how much the reading changes as the temperature of the instrument changes,
and periodic calibration to compensate for compenent aging.

Anyone taken their Davtron or JPI out to check calibration lately?

The practical thing to do about the Davtron and JPI is to get the most
accurate meter you can find and note the difference between the 3 meters.
For the range of interest, i.e. about 11-13V the difference is going to
be linear so all you need is a single correction factor for each meter
as opposed to a correction card covering a range.

The bottom line is these in panel meters are just reference, not lab
equipment, unless someone went to heroic lengths (and a heroic price to
match) to get the accuracy and calibration that close and the periodic
recalibration to ensure it stays there.


--
Jim Pennino

Remove -spam-sux to reply.

[Michael J. Myers]

If you already have a good DMM, do what I did--

Go to Wal-Mart and buy a cigarette lighter plug for $0.99. Strip the leads
and clip to your DMM. Start your motor and go.

With my Fluke 87A, I could even trap voltage spikes on the system.

Mike M.

[Rich]

That was going to be my "fall back" plan... and I may still do it,
depending on the readings I get from the Plug in Voltmeter. Reason for
the plug in unit was to have something I could leave in the plane and
monitor over the course of several flights.

Unit is on its way to Michigan from "Auto Barn" in New York. Couldn't
find one at several Auto Specialty shops locally.

Rich

Michael J. Myers wrote:
> If you already have a good DMM, do what I did--
>
> Go to Wal-Mart and buy a cigarette lighter plug for $0.99. Strip the leads
> and clip to your DMM. Start your motor and go.
>
> With my Fluke 87A, I could even trap voltage spikes on the system.
>
> Mike M.
>
>

[mikem]

Bob wrote:

> Anybody know of a reasonably priced gadget that will let me monitor the
> MAIN bus feeding the cigarette lighter?

If you connect a third voltmeter, you will now have three different
readings. Which are you going to believe? It like wearing three watches,
which one is "closest" to the correct time?

The only thing that really matters is what is the voltage at the battery
terminals. If your batteries are lasting more than three years, and as
long as the starter consistently cranks, then the voltage cant be
too far off.

If you are not consistanly adding water to the battery, then your bus
voltage is not too high.

If your starter is not laboring, and your airplane starts consistantly,
then your bus voltage is not too low.

The "acid test" (pun intended) is to check the Specific Gravity of the
battery with a hydrometer.

MikeM
Skylane '1MM
Pacer '00Z

[David Lesher]

writes:


>The problem is you need accuracy and cheap and accuracy don't go together.

>I would be surprised if the accuracy of the existing voltmeters are much
>better than about 2%, which would easily account for the difference.


I'll disagree. You don't need absolute accuracy. You want repeatability.
If it says 13.4vdc at cruise today, and it did yesterday; and tomorrow
it says 12.9v - Houston, we have a problem.

And unlike analog meters, a DVM can be within 2% and still dirt cheap.

But all you really need is a array of say 5 LEDs:

<12.8v RED
12.9-13.5 YELLOW
13.6-14.4 GREEN
14.5-14.8 YELLOW
>14.9 RED

{We can argue re: the setpoints...}

So it's Y before starting, G when running, maybe Y at high loads/low RPM,
etc.

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[mah]

Joining this in mid-thread as an interested onlooker.

Was working a similar problem on my van recently. I'd see a difference
of 1.5 volts depending on where I tapped into the circuit, the ground
location, and load in the system.

That's the one item, load, not mentioned in the previous posts. When
I'd fire up my gps, laptop, and cell phone charger the voltage on that
circuit would become 12.2 volts while it read 13.5 at the battery.

Be thinking of what load you are drawing as the voltage will drop as it
pushes more amps down the wire.

MAH

[Rich]

Yep... that's the issue. I'm attempting to use the readings on the JPI
and the Davtron to ascertain the condition AT THE BATTERY. Because of
these differences (probably due to different grounds; maybe due to
voltage drop from main bus to avionics bus), I need some way to
calibrate JPI readings to battery voltage; best easy way for me is to
measure the main bus, at the cigar lighter.

I got the plug in voltmeter and calibrated it using the HP power supply
and a real nice digital shop multimeter. To my surprise, the plug in
unit was VERY accurate... right on below 10 volts, and maybe 0.1 low at
12 volts (some lag, though... a second or two to stabilise).
Sufficiently close for my purposes.

Too busy (Christmas) and too cold (Michigan) to check it out right now.
Further reports to follow.

Rich


mah wrote:
> Joining this in mid-thread as an interested onlooker.
>
> Was working a similar problem on my van recently. I'd see a difference
> of 1.5 volts depending on where I tapped into the circuit, the ground
> location, and load in the system.
>
> That's the one item, load, not mentioned in the previous posts. When
> I'd fire up my gps, laptop, and cell phone charger the voltage on that
> circuit would become 12.2 volts while it read 13.5 at the battery.
>
> Be thinking of what load you are drawing as the voltage will drop as it
> pushes more amps down the wire.
>
> MAH