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.
----------------------------------- 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
