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

[Ron D'Eau Claire]

DW wrote:

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.

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That has nothing to do with the d'Arsonoval meter itself. Good quality
movements are quite linear and accurate. (There is a different type of
"moving needle" meter that is not as accurate: the so-called "moving vane"
type of meter that was cheaper to produce. But they are very rarely seen any
longer.)

Most wattmeters, including the Birds, specify the accuracy as a percentage
of "full scale". 

Birds are great wattmeters, but one *must* use the slug rated for the
correct wattage and frequency range for any accuracy at all. For example, a
"Bird" is spec'd at +/- 5% of full scale. So, if you use a 100 watt slug,
it'll be as much as 10 watts off across the entire range. So, at 100 watts
it'll be within 10% of what it indicates. And at low powers the error may
reach 100% of the applied power. 

It is true that many meters using a simple detector circuit tend to get
non-linear at low levels because the diode isn't linear. Actually, the
series resistor helps that but it's not a cure-all. Still, proper design
keeps the RF voltage at the detector at a good level for good accuracy. 

I've found that most medium priced wattmeters are quite accurate - often at
least as accurate as any other good meter, including a "Bird" (speaking of
the popular Model 43 "thruline" meter, recently calibrated and using the
proper slug). For example, I have a Daiwa CN801 that sold for about $150
new. My comparisons with other quality wattmeters suggest it's as accurate
as the 801 the ARRL tested in their product reviews. They found it indicated
between 4.9 and 4.8 watts from 2 to 30 MHz with 5 watts applied: a truly
great accuracy. At 144 MHz, the top end of its specified range, it showed
6.3 watts with 5 watts applied.  

The accuracy is as good or better at higher powers. 

I suspect that's true of most of the meters on the market today. 

I suppose that a digital readout can be made more accurate, but there's also
a caveat with digital readouts: many people look at the numbers and assume
they are correct. If a DMM says 10.134 volts they think 10.134 volts is the
voltage applied. That's almost *never* true for many reasons including that
the A to D converters used have a finite accuracy, just like the
moving-needle d'Arsonval meters. 

Personally, I like the d'Arsonval type movements for most things because
they let me see changes quickly and don't "fool me" with an implied accuracy
that simply is not true.

Ron AC7AC