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

[dw]

Thanks Don!
I aggree with you.
I used to calibrate RF-electro-surgery devices in a former life :)
Thanks for the tip on the non-reactive power resistor.
I think I will look to get one.

Good stuff!.
Duane


On Mon, 20 Jul 2009 21:07 -0400, "Don Wilhelm" <w3fpr at embarqmail.com>
wrote:
> 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.  :)
> >   
> >   
-- 
 Bw_dw at fastmail.net