G3rzp@aol.com wrote:
Peter, you are agreeing with *all* the comments I have put here. So
there seems to be little (if any) disagreement between us.
> Another problem with calorimetry is the time it takes, which with a
> sweep tube amplifier would be prohibitive - even pulsed!
1) In response to Gary Smith I said " I suspect Bird use Calorimeters
for internal use, but would not use them to calibrate each and every
element they sell, as the method is too time consuming."
2) "Another big problem for amateur power measurement would be line
voltage variations. Since any such measurement by Calorimetric methods
will take 10's of minutes at least, there needs to be a way of keeping
the RF and DC power constant."
3) I quoted from the book I have " On the other hand, they are bulky,
expensive to construct, require highly trained personnel, slow,
difficult to use, have limited dynamic range, and after therefore
unsuitable for field use outside the laboratory"
>
> I haven't seen a 'scope guaranteed better than 5%, even the new super
> dooper all singing digital things that can give aliases everywhere.
I said that too,
In response to someone I said " I'm not sure I follow your suggested
technique, but scopes are not normally much better than a few percent
on the Y-axis, which since the errors are squared for power, I can't
see that being particularly good."
Rich has said "I used to work in a calibration lab and we calibrated RF
wattmeters by measuring peak-V across a 50.0-ohm termination with a
NBS-traceable oscilloscope and doing the math." Whilst I never replied
to Rich comments, I can't see how anyone can measure power accurtely
with a scope.
> And most of the time, does it matter?
1) I said " At the end of the day, you need to ask yourself what
accuracy do you need. I think for RF power measurements, the answer is
not very much for amateur radio use. You tend to tweak for maximum,
keeping an eye not to exceed the current ratings on the tubes.
Exceeding the anode dissipation by 10-15% will not be a problem if the
temperature is kept down, so knowing the exact efficiency is not such a
big deal. I suspect using the temperature rise across the tubes would be
reasonable to work out anode dissipation."
2) In response to you I said "For amateur use, we don't need much
accuracy."
> 73
>
> Peter G3RZP
So wee seem to be agreeing on just about everything. I would maintain
*IF* you want the accuracy, which the original poster did since he was
considering buying new elements, calibrating his elements regularly with
a Calorimeter would be the best way to do it.
I received the following from someone by private email. I have edited it
slightly, and removed his name, but otherwise i t is intact:
" Hi David
I my long gone misspent youth I worked at XXXX for a while. They used
calorimeters for their high power (1kW & 10kW) transmitters. They didn't
really seem all that complicated - flowing water with a meter -
thermometers on input and output sides - standard plumbing fittings. The
actual dummy load was quite small if I recall correctly. I seem to
remember that the temperature rise was quite small, but the flow rate
was quite large! I quite fancied making one at the time."
Berfor making one, I'd certainly read up on what the sources of erorr
are, and what one needs to do to reduce them, but perhaps someone will
make one, then offer to calibrate other hams meters for a small fee. He
might soon recover the cost of making it.
PS I think Calorimeters should be renamed "Jouleometers", since Joule is
the SI unit of power. However, if a person named Calorie invented the
idea, then it should be kept its original name.
I don't suppose the latter would go down too well with the American
hams, as America seems to keep to an odd mix of units. They use seconds
and Watts, and Watts are just Joules per second. Most of the rest of the
world has gone to SI, so why not America? (That said, astronomers use
Parsecs, which is a pretty odd unit. But perhaps there is not a known
multiplier for metres that makes the use of metres sensible. My
knowledge stops at Tetra, but there might be some multipliers higher
than that.)
--
Dr. David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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