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. :) -- [email protected] ______________________________________________________________ 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
