[Elecraft] P3TXMON question?

[Ken G Kopp]

Bob,

The scale calibration --is-- compressed to the right of center,
--not-- to the left as you state, making the left portion expanded
and easier to read.

73!

Ken - K0PP

On Sun, Aug 23, 2015 at 2:14 PM, Bob McGraw - K4TAX <rmcgraw at blomand.net>
wrote:

>
> This seems to have things really screwed up.   Yes, the Bird 43 is a 5% of
> F.S. accuracy instrument.   The Bird 43P is somewhat less accurate,
> although 5% F S with carrier, and more like 8% F S or so in peak mode.
> In measuring a 100 watt radio with a 100 watt F S slug would seem to be
> most accurate.    Total accuracy decreases as the indicated value
> approaches the left side of the scale.   Now using a 250 watt slug to
> measure a 100 watt radio,  the earlier states that most accuracy is to the
> left of center, which by the way, is the compressed side of the scale.
> This makes no sense as the scale markings are much more difficult to
> approximate.   In fact the most accurate area is to the right of center
> toward full scale is the expanded scale  which is where the highest
> accuracy actually occurs.   With a 100 watt radio and a 100 watt F S  slug
> the error is +/- 5 watts at 100 watts.  Using a 250 watt element and
> measuring 100 watts near center scale the error is more like +/- 12.5 watts
> or 5% of the  F S value.
>
> If one measures the same power source with identical elements and gets
> different results, they should refrain from buying ham fest elements for
> indeed the elements are not identical.  To wit, I purchased 3 new 100 watt
> elements and 3 new 2500 watt elements. They all give identical results +/-
> the width of the pointer.  I don't buy ham fest Bird elements as I find
> they are junk and not worth the brass they contain.
>
> And remember, Bird 43 instruments are calibrated for only a 50 resistive
> load.  Any other load being used leaves one with a "best guess" situation
> as to actual power.
>
> 73
> Bob, K4TAX
> K3S s/n 10,163
>
> On 8/21/2015 11:48 PM, Wes (N7WS) wrote:
>
So by your "logic" it would be best to measure one Watt using a 100W
>> element.
>>
>> On 8/21/2015 8:03 PM, Ken G Kopp wrote:
>>
>>> Bird accuracy is specified to be +/- 5% of meter full scale ... a 100W
>>> element would be a poor choice to measure a 100W output level. Take a
>>> look
>>> at any Bird's meter scale.  It's not linear and can be read with the most
>>> accuracy to the left of center scale.  A 250W element is much better for
>>> measuring 100W.
>>>
>>> Wanna chase your tail?  Measure the same power source with identical Bird
>>> elements......
>>>
>>> Don's method is better, but all methods are obviously subject to the
>>> accuracy of the final indicating instrument.
>>>
>>> FWIW .....
>>>
>>> 73l
>>>
>>>
>>>
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>
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