Today's Message Index:
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1. 01:48 AM - Re: LR3C and overvoltage (Linda Walker)
2. 09:55 AM - Re: Re: LR3C and overvoltage (Robert L. Nuckolls, III)
3. 04:41 PM - Re: LR3C and overvoltage (Clarification) (Robert L. Nuckolls, III)
4. 08:38 PM - Recommend a digital multimeter? (Ralph & Maria Finch)
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Subject: | Re: LR3C and overvoltage |
Bob.
I have a B and C LR3C (?) regulator watching over my electrical system batt
ery and alternator.
I also have a Vision Micro Systems VM1000 instrumentation.
In monitoring the elec system voltage the VM uses a lead from my main bus,
and normally shows between 13.6 and 14.2 volts.
I have recently, yesterday, fitted a Dynon D-10A and on its screen it shows
a voltage of 15 -15.3 volts. The Dynon is powered from the main bus.
I also have two auxilliary outputs from the main bus to power a standby han
dheld gps and handheld radio.
The gps also monitors its input voltage and also shows 15 volts.
My questions are: Could the LR3C be faulty and should it be replaced? How c
an the VM1000 see a different voltage to the Dynon and standby gps? Is the
VM1000 at fault?
Any help in identifying which is correct and which isn't would be much appr
eciated.
Patrick C Elliott
G-LGEZ.
PS: Presently in Indonesia on our rtw trip from the UK, so getting things r
epaired isn't so easy!
Message 2
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Subject: | Re: LR3C and overvoltage |
At 03:44 AM 11/14/2010, you wrote:
Bob.
I have a B and C LR3C (?) regulator watching over my electrical
system battery and alternator.
I also have a Vision Micro Systems VM1000 instrumentation.
In monitoring the elec system voltage the VM uses a lead from my main
bus, and normally shows between 13.6 and 14.2 volts.
I have recently, yesterday, fitted a Dynon D-10A and on its screen it
shows a voltage of 15 -15.3 volts. The Dynon is powered from the main bus.
I also have two auxilliary outputs from the main bus to power a
standby handheld gps and handheld radio.
The gps also monitors its input voltage and also shows 15 volts.
My questions are: Could the LR3C be faulty and should it be replaced?
How can the VM1000 see a different voltage to the Dynon and standby
gps? Is the VM1000 at fault?
Any help in identifying which is correct and which isn't would be
much appreciated.
You are suffering a common malady known as "too much
data". We're all operating vehicles from airplanes
to cars, trucks, boats and lawn tractors that feature
DC power systems. The vast majority do not have voltmeters
and when they do have voltmeters, their veracity is seldom
questioned.
Obviously, only one of the readings you've cited can
be "most nearly correct". I say "most nearly" because
all measurement devices have accuracy specifications
(and perhaps drift with age) that contribute to errors
in displayed readings.
Just for grins, I just gathered up a flock of 5 multimeters
from various hiding places around the shops and vehicles
and hooked them to a power supply on the bench. I set
the power supply to 15.0 volts output based on it's
3-digit display. The various meters read as follows:
Micronta (radio shack brand) 15.00
Digital Instrumets Co 15.05
Radio Shack 15.07
Fluke Model 75 14.96
Fluke Model 8000 14.94
If I average the total of 6 displays I get
15.003 which is a very strong suggestion that
the power supply is indeed putting out 15.00
volts. This isn't a very good illustration
because the devices I choose to keep in my
toolboxes are of known reliability and are
periodically checked. Nonetheless, if I
were to split hairs, I'd have to say that
the REAL output voltage was probably closer
to 14.95 volts. Why? Only because of my experience
with instruments crafted with John Fluke's
name on them.
Now, what's the significance of these tiny variations?
In an airplane or any other vehicle, obviously
none. What you're experiencing is the uncertainty
generated by non-agreement of multiple instruments.
This is compounded by the fact that you don't
know which if any of the instruments were crafted
with the skills and quality exemplified by
John Fluke's test equipment.
The preponderance of evidence suggests that your
bus voltage is indeed on the order of 15.0
volts. Assuming that the VM1000 is not being
"tricked" into some lower reading as a result
of configuration (conductor voltage drops
throughout the system) then the major suspicion
of error is on the VM1000.
The best thing to do is get a multimeter of
some known calibration and use it to referee
the contest. A fresh Duracell AA battery is
very close to 1.60 volts at room temperature.
Get a Radio Shack AA battery holder for 8
cells (Catalog #: 270-387)
Emacs!
An populate with fresh AA cells. Measure
the terminal voltage on the holder. If your
meter reads 12.8 volts, it's close enough
for government work. If it's something else,
make note of that difference. Let's say it's
12.5 volts. Okay, any reading in that range
is 0.3 volts high.
Now, go measure the bus on your airplane.
Apply any observed calibration offset to the
reading and THAT becomes your benchmark for
evaluating the veracity of the other displays
in the airplane.
Your LR-3 senses the main bus so if your
meter suggests some adjustment is called for,
then there's a plug button on the side of
the LR-3 that covers an adjusting screw.
This reading is best taken while in flight
and with normal loads on the system. So it
might take several observations and subsequent
adjustments to achieve what you want.
On the other hand if you find that the bus
is 14.2 to 14.6 volts under all operating
conditions in flight, then I'd leave it alone.
Just make note of the correction factor you
now need to apply to any ONE instrument that
displays voltage and ignore the rest of them.
Many moons ago, the RM series mercury cells
could be depended upon to offer a 1.345 volt
calibration source over a wide range of
temperature. Of course, we can't buy those any
more.
Bob . . .
Message 3
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Subject: | Re: LR3C and overvoltage (Clarification) |
<snip>
. . . populated with fresh AA cells. Measure
the terminal voltage on the holder. If your
meter reads 12.8 volts, it's close enough
for government work. If it's something else,
make note of that difference. Let's say it's
12.5 volts. Okay, any reading in that range
is 0.3 volts HIGHER THAN WHAT IS DISPLAYED
ON THE METER so your correction factor is to
add 0.3 volts to the reading.
One reader asked why you couldn't check the
meter calibration with one cell. Two reasons:
(1) Many meters measure 15.0 and 1.5 on different
scale settings which may not exactly track each
other for %of error. In the example I gave, a
single cell would have been measured on the 2
to 4 volt full scale settings while 15.0 volts
was measured on 20 to 40 volt full scale settings;
same settings as those used to investigate the
situation in the airplane. If Radio Shack had
an 9-cell holder, it would be better yet for
qualifying your investigative instrument.
(2) Instruments have several kinds of errors.
Linearity, scale factor and offset. See instrumentation
chapter in the 'Connection. By doing your calibration
in the same neighborhood as the voltage of interest,
the effects of linearity error are washed out.
Effects of offset and scale factor are largely
combined together in the hypothesized 0.3 volt
error factor. This is a good example of how an
instrument with terrible accuracy characteristics
is still useful as a reasonably precise
'transfer standard' once its shortcomings are known.
Now, go measure the bus on your airplane.
Apply any observed calibration offset to the
reading and THAT becomes your benchmark for
evaluating the veracity of the other displays
in the airplane.
<snip>
Bob . . .
Message 4
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Subject: | Recommend a digital multimeter? |
I just discovered my faithful old Radio Shack DMM is wonky, the little scale
at the bottom of the LCD displays reads correctly between 0-30, but the
digits don't correspond. Time for a new DMM.
I could just go back to the Shack and get whatever they have, but would like
to check here for a recommendation. Mr. Nuckolls, any advice? A brief
search using Google reveals many nifty new features in DMMs these days.
Anything an airplane builder should look for?
Thanks,
Ralph Finch
Davis, California, USA
RV-9A QB-SA
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