In a message dated 98-06-11 13:38:11 EDT, jim.gambony@eds.com writes:
> As an electrical engineer I have to question your assertion that
> voltmeters are "thermal devices". I think what you're referring to was an
> earlier thread that stated that the older Smiths dash guages were thermal
> devices (as a way to average out the readings) as a way to keep them from
> jumping around as the car ran down the road.
>
> Most mechanical guages and meters are driven by electromagnetic
> force, not thermal. I have plenty of (non-Smith's) electrical guages
> (including voltmeters) that are VERRRRY quick to respond to changes!
Jim,
As far as I know, there are three major types of electrical analog guages in
use, two of which are commonly used in an automobile application.
The first is the thermal type, which was the only type used in the British
cars that we are involved with here on this list (With the exception of the
ammeter, which is a very simple moving coil design, and is very fast. The
coil consists of only one turn of heavy gauge wire). The voltmeter that was
installed in the later model TR6 was of the thermal type, as are the current
aftermarket (Smiths) voltmeters. I assumed that anyone putting new gauges
into their Spitfire would want to match the existing guages, so the new
voltmeter would be of the thermal type.
The second type, not commonly used in automobiles because they are rather
fragile, is the moving coil, or De'arsonval, type. These are very fast and
accurate, but their use is pretty much limited to test equipment and panel
meters.
The third type, used quite often in modern cars, is the dual coil meter. As
the name implies, these meters have two coils, and respond to the difference
in voltage between the two coils. By using the difference in voltage, they
are not effected by changes in the system voltage. If the battery voltage
drops by one volt, for example, it drops one volt on each coil, but the
difference remains the same. The Autometer guages I'm using in my TR6/Ford
302 project car are of this type. Although they are faster to respond than
the old thermal meters (much faster!), they are still vey sluggish when
compared to a De'arsonval movement. I suppose this is a deliberate design to
cope with the shocks they will recieve in an automobile, rather than any
inherent characteristic.
There may be others, but these are the only ones I know about.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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