"COLIN LAMB" wrote in message
hlink.net...
Well, I was trying to be helpful. Instead, I am condescending.

I have been a ham longer than 41 years, and I learn things all the time.
Sometimes, I forget simple ways to do things.

The Bird is not a model of accuracy. Accuracy is specified as a % of full
scale. I do not have my books handy, but I recall it is 5% of full scale.
If my recollection of 5% is correct, that means that it could be off (oh,
oh, here is simple math) 125 watts at full scale or 125 watts when you
are reading 500 watts. Using another Bird is not a good method of setting
the accuracy.

You have to use the dvm to check out the dummy load, anyway. If the dummy
load is off in resistance, the Bird wattmeter will not read correctly -
since it is assuming that the load is 50 ohms. I have checked dvms and
found even the cheap ones are quite accurate - much higher than the
accuracy specified for the Bird. I have also check dummy loads and found
they are often off. A friend asked me to come over because he was
measuring 3200 watts out of his amplifier and thought it was high. It
was - the dummy load was also high.

If you are interested in an accurate reading slug, then calibrating it
against another Bird slug does not make much sense - since that might be
inaccurate. Forget the surplus rf voltmeter and simply measure the heat
rise of the dummy load. That will be an exact measurement of the power
going in and yield much more accurate readings than the Bird. It will
simply take a thermometer and a bit of math. I will not detail how to do
it, because that might be condescending.

I have a number of Bird wattmeters and slugs. I use them for quick and
crude measurements - because they are handy. Before I could afford the
Birds, I designed and built my own - and calibrated them. In fact, I
wrote an article for QST in 1973 on how to build your own using ordinary
plumbing parts. They were hand calibrated and as accurate as any Bird
meter.

If, you want your meter to be calibrated like Bird would do it, send it to
them.

Colin K7FM


I "started" to reply last night and I thought I'd let Colin speak for
himself. He has. "I" personally didn't find him to be "condescending". You
have to understand something. Bird Wattmeters or ANY equipment - as you have
found "can" fail or come under question. OK - so when it does - now you're
faced with the exact predicament that you are. So - do you stop working and
wait and wait and wait until you can figure out what to do about THAT piece
of equipment, OR do you finagle another method to check the results of the
"suspect" equipment and to get a comparison reading? I can't speak for you -
but I choose option number 2. I can't afford to wait. Time is money.

As for "my" interest - though right now I have no doubts of my equipment, it
is always nice to know what to do or where to turn to remedy any problems.
I've never bothered to check into this before - regarding the bird - and
your question gave me the initiative THANKS!. I believe now I can do it
myself - BUT - if not - just do what needs to be done - bite the bullet and
have Bird do it. I've forwarded each response to my mailbox as I always do
with valid info to build my library of "Useful" information for future
reference.

Back to point number one....... When I got into Ham and 2 way radio - I
learned how to do without the fancy equipment I couldn't afford "then". That
included hooking up with those who did have it on an as needed basis. I made
mock up equipment and set ups which got me pretty far along without much
problem. An Eico 232 with an RF probe validated quite a few cheaper
Wattmeter readings. As Colin said, you have to also make sure the Dummy Load
"is" at the reference it is supposed to be OR your readings are shot in the
ass. A bad dummy load can make things look pretty goofy and if you don't
"check" it, then you're there scratching your head wondering what the hell
is going on. Now I have all that fancy stuff and am fortunate to have it -
but should it flake out - then it is back to using the other set ups until I
can get this stuff back on line.

As to Colin's article, I "think" I have it here somewhere - I recall reading
it. Pretty good article - though with the plumbing, I chuckled due to myself
not being much of a plumber! Those articles always give you room to think
and to expand - maybe to create your own way of doing things.

Just my 2 cents - taken with a grain of salt and CERTAINLY not meant
either - to be condescending.

L.