Re: Metering

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "davep by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<davep-at-quik-dot-com>
> 
> > Sorry to pick nits, but a recent reference to RMS power set me off, rather
> > like fingernails on a slate blackboard.  Our meters measure measure average
> > voltage and current on the dc ranges, rms voltage and current on the ac
> > ranges,
> 
>         Indeed, care is in order here.
A few nits to be picked still..
> 
>         Some of the 'true AC' meters (Moving vane, moving iron, dynamometer,
>         hot wire, thermocouple) are true RMS (Same caveat on frequency).


Moving vane and moving iron meters rely on magnetic attraction, and are no
different than moving coil meters, just the coil is fixed.  They are
average reading (inherently, some have weird shaped armatures to "linearize").

All of these are basically a low pass filter (mechanical) and, as such, are
linear devices, and can't read RMS directly (which requires squaring and
square rooting)

The hotwire and thermocouple meters ARE true POWER meters (that is, they
directly measure the power dissipated in the meter). Since they have only
two terminals, they can't measure the power in an external device (which
would require at least three terminals), however they CAN measure the power
dissipated internally. 

RMS current is merely the square root of the AVERAGE power dissipated in a
resistive measurement device (where the resistor sets the proportionality
constant).  The averaging in the Thermal RMS devices is done by the thermal
mass. Note that the thermal meters do NOT read correctly on pulsed signals,
because the thermal mass is a low pass filter with an exponential
characteristic which weights more recent values heavier than ones in the
past, not a true "mean" or "moving average" filter, which would be
non-trivial to implement in hardware.)

The thermal RMS meters use the P=I^2*R to do the squaring, mechancial means
(i.e. mass) to do the averaging, and scale calibration to do the square
rooting.

One can build thermal RMS meters with very fast response times (we use them
all the time for pulsed microwave measurements), but they wouldn't work
very well for measuring 60 Hz (because the averaging time is so short,
you'd get a varying reading of the instantaneous power).