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Deems Davis
Joined: 09 Jan 2006
Posts: 925
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 Posted: Mon Jun 18, 2007 11:46 pm Post subject: Loadmeter vs Battery Ammeter? |
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Warning this question is posed by an electron nerd....
I'm planning the installation of probes & sensors for a Z14
architecture. The Z-14 Figures in the 'Connection' seem to depict the
shunts placed for use as 'loadmeters' i.e. they are placed on the
Alternator B lead (which will measure Alt charging only). In reviewing
Bob's notes. He favors a Battery Ammeter, the rationale seems to be that
it measures both charging and discharging of the battery/s. This seems
desirable to me.
However, I'm stumped as to where to locate the Shunts in the Z-14 so as
to be able to connect Battery ammeters.
How about some help?
Deems Davis
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nuckollsr(at)cox.net
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| Posted: Tue Jun 19, 2007 6:17 am Post subject: Loadmeter vs Battery Ammeter? |
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At 12:41 AM 6/19/2007 -0700, you wrote:
| Quote: |
Warning this question is posed by an electron nerd....
I'm planning the installation of probes & sensors for a Z14 architecture.
The Z-14 Figures in the 'Connection' seem to depict the shunts placed for
use as 'loadmeters' i.e. they are placed on the Alternator B lead (which
will measure Alt charging only). In reviewing Bob's notes. He favors a
Battery Ammeter, the rationale seems to be that it measures both charging
and discharging of the battery/s. This seems desirable to me.
|
The Z-figures represent the latest thinking with respect
to the optimal solutions. The chapter on electrical instrumentation
was written a long time ago and will be updated at revision 12. If you have
only one electrical system monitor, it should be a low voltage
warning light. If you add anything on top of that, I'd recommend
an expanded scale voltmeter on the e-bus and/or alternator
loadmeter(s) . . . but both of those are only small peeks at a variety
of test points you'll need to look at for diagnosis. Ammeters
and voltmeters are poor monitoring tools. And as diagnostic
tools, they are only a part of what's necessary for troubleshooting.
| Quote: | However, I'm stumped as to where to locate the Shunts in the Z-14 so as to
be able to connect Battery ammeters.
|
What kind of panel displays are you considering? If they're
steam-gages, then they'll likely use shunts but if they're
electronic displays, they'll most likely come with hall-effect
sensors. Hall sensors can be place ANYWHERE in the system that
you think is most useful to you for knowing how many electrons
per second are passing that point . . . however, adding shunts
in series with the battery as suggested in Chapter 7 is based
on wobbly logic and poor practice. However, a hall-sensor can
certainly be placed on a battery lead if you so choose.
A caveat on battery lead hall-sensors. When the starter hits the
battery with an inrush current of as much as 1000 amps, the magnetic
forces impressed upon the hall sensor are, shall we say, significant.
The hall sensor MUST (by certain pesky laws of physics) be fitted
with a core material that serves to restrain the field flux around
the wire and supply a proportional sample of that field to the
hall device. EVERY magnetic material has a retentivity value . . .
a measure of permanent "set" that the core material will take
on when excited by an extra-ordinary force.
Depending on the materials used in your hall sensor, the act
of starting the engine can cause a small but significant
semi-permanent "set" of the field flux in the core. This
will manifest itself as what is known as "offset" . . .
a fixed error value that shows up most strongly when
the stimulus to be measured is zero . . . the indicator
reads something other than zero due to the core's
retentivity and influence of the starter inrush current.
Based on this expanded thinking, recommendations for ammeter
locations were revised to what you have in your hands today.
Without knowing more about the specifics of any hall-sensor
that may be supplied to you, I cannot recommend that they
be used as battery ammeters EXCEPT where you avoid running
starter current through the conductors. This means backtracking
to the architecture common in 1965 C-172.
The best I know how to do at the moment suggests that active
notification of low volts is #1. A voltmeter on the e-bus
(gas gage for battery during alternator out operations) is
#2. Items 3 and higher have little or no significance in
the OPERATION of your airplane . . . so sprinkle sample
and display items about your electrical system as you see
fit and you budget allows. Just understand that items 3
and higher offer no useful functions in flight and there
won't be enough of them to do a detailed troubleshooting
study on the ground.
Bob . . .
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