Thank you. I stand corrected on the point. Not physically having it
in front of me, I was recalling another power meter I use more
frequently. It is expanded up scale and compressed down scale.
73
Bob, K4TAX
K3S s/n 10,163
On 8/23/2015 3:36 PM, Ken G Kopp wrote:
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
<rmcg...@blomand.net <mailto:rmcg...@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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