Duane, While you are correct on the meter movement specs, take a hard look at the wattmeters available to hams.
The LP-100 wattmeter does almost what you are referring to. It is quite accurate when calibrated. I have a general mistrust on analog reading wattmeters. Many wattmeters on the ham market are speced for 20% of the full scale reading - and that means an error of up to 20 watts on a 100 watt scale (often the lowest scale). Even the revered Bird wattmeter is speced for 5% of the full scale reading right after calibration, and that is up to a 5 watt error with a 100 watt slug - and how many Bird wattmeters in the hands of hams have a current calibration sticker? The Elecraft W2 wattmeter promises to make the power measurement capability better, but it is not yet available (ship in August was the target). You can certainly do better than that with an oscilloscope to read the RF voltage across a precision 50 ohm non-inductive resistor. Caddock makes 50 ohm Thick Film Power resistors that are good to 100 watts with a heat sink - attach one to a heat sink salvaged from a defunct computer CPU cooler and attach a BNC connector with zero length leads and you will have an inexpensive precision dummy load that is flat to at least 200 MHz. Ridge Equipment (use Google) often has dummy loads at a very inexpensive price that are good to at least 200 MHz as well. So for under $20, you can have a piece of precision equipment that is the basis for power measurement (just measure the RF Voltage across the precision dummy load and compute the power). That device can be used for calibration of wattmeters, and the result should be within 5% even allowing for errors - and the results have nothing to do with the "full scale". 73, Don W3FPR 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. > 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. :) > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
