[Elecraft] The weekness of a d'arsonval watt meter

[dw]

If I am correct, most manufactures claim a 10% tolerance on most
d'arsonval type watt meters.
I believe there are also some caveats.
That 10% is at a specified level of applied wattage, and of course
applied into a 50 ohm load.
This means that at 100 watts applied power, the meter can read +/- 10
watts.
This however, does not take into consideration the non-linearity
characteristics of a d'arsonval meter.
And so the manufacture will take a specific meter as his prototype and
mark various wattage readings up through its range.
And those physical points will become the template for the numbers
displayed behind the meter when it goes into mass production. 
If he is really fussy, he will take 10 of those meters on the prototype
bench, and record their physical readings and then make his template
based on the average of those ten prototype meters.

We then have to factor in the variance of tolerance for every d'arsonval
meter assembled into future boxes on the assembly line.
What we might end up with then, in the real world, is a meter, when
activated by the user, points to various numbers on its display with an
inaccuracy that may be 20% or more, based on its individual mechanical
response to various wattages up its range.

The d'arsonval meter is driven with a series resistor (usually a pot)
from the rectified power source.
A manufacture may be so inclined to visibly fix calibration pots with
the eye during the assembly process, and then spot check the meter with
100 watts applied.  If it falls within +/- 10 watts during the
spot-check.....ship it......it meets its advertised accuracy.


This is where the micro-chip can give you an edge up.
The micro-chip can hold within its memory a table of values programed by
the user.
Remove the d'arsonval meter and its series-R and connect the
micro-chip's analog input instead.
As a calibrated applied wattage source is used, "teach" the micro-chip
how to interpret the voltages it sees as the wattage is increased up the
scale.
Now you have a watt meter that is calibrated at multiple points up its
scale instead of at a single spot-check point.
A microchip watt meter can then approximate the accuracy of the bird
watt meter.
Since ( I think this is true ) the bird meter is also rated at a certain
wattage point and that accuracy is (due to the use of a d'arsonval
meter) diminished below that wattage point.

Some day a smart ham is going to manufacture and sell a mico-chip based
watt-meter which the user can calibrate himself using a bird meter, or
better yet...an o-scope. 
Of course that is... if there is a marketable demand for higher accuracy
than currently exists.  :)
-- 
 Bw_dw at fastmail.net