If you have the equipment, the proper environment, know your heat
losses, it is possible to measure power to a greater accuracy than you
can read on an analog meter. BUT to do this takes time and effort.
Proportionately a lot of time.
The ability to measure accurately is determined essentially by equipment
and technique. IOW, you could measure power to a useless accuracy.
That begs the question as to what accuracy is useful? Then is it what
the user would like to see, or what they need to see? With a freshly
calibrated VNA I can see the parameters of a wire antenna to 3 decimal
places. I can see those parameters vary in a breeze. It's impressive, a
great conversation starter, but hardly useful. Many students obsess over
decimal places on their calculators, yet for many decades engineers
seemingly got by with slip sticks. Now days there are instances where
those decimal places are important, but not many that I know of.
Measuring the temperature rise in a known quantity of water with a known
heat loss is limited by the system's heat loss. Water is straight
forward as an instrument as 1 calorie will raise the temperature of 1cc
or gram of water 1 deg C. (I'd have to look up the number of joules
required) You can graph the heat rise vs time, and or reach
equilibrium at a given heat loss (the temp of a given volume of cooling
water at a given rate. Then you are limited by how accurately you can
measure the flow and temp.
I'm pretty sure I'm missing a few parameters. As I've often said, "It's
been a long time"
73
Roger (K8RI)
On 5/3/2015 9:22 AM, Jim Garland wrote:
Joe and Roger are correct about Bird Wattmeter accuracy. It's 5% of full
scale. I've not been following this thread closely, so I apologize if I'm
duplicating what others have said, but I find the only convenient way to
calibrate a wattmeter accurately is to use a good oscilloscope and measure
the p-p RF voltage across a 50 ohm dummy load. A calibrated Tektronix analog
scope has a rated accuracy of about 3%, which translates into a 6% error in
power. I have a scope calibrator and, if I use it as a calibration standard
and I'm careful, I can usually measure an RF voltage to about 2% on my Tek
2465B, which translates into a 4% power error. I doubt if lab calibration
methods using thermocouples, etc., can do better than that. RF power is
really hard to measure accurately.
73,
Jim W8ZR
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Joe Subich,
W4TV
Sent: Sunday, May 03, 2015 6:44 AM
To: amps@contesting.com
Subject: Re: [Amps] Time for New Power Meter
> Rarely does Roger even need correcting, but this is one of those
> times.
No, Roger is correct. Bird's specification is 5% *of full scale*.
That means the Bird's accuracy is +/- 125 Watts *anywhere* using a
2500 Watt element. While in practice the accuracy may be higher
at other places on the scale, the Bird specifications allow that
percentage accuracy may be worse than +/- 5% of the reading below
full scale.
73,
... Joe, W4TV
On 2015-05-03 12:40 AM, Bill Turner wrote:
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Sat, 02 May 2015 21:39:04 -0400, K8RI wrote:
The nearest slug I can find to the legal limit is 2500 Watts. 5% is 125
watts, 125 watts is a bit over 8% (8.333%...)+/- 125 Watts, or
1500-125=1375 to 1500+125= 1625
REPLY:
Rarely does Roger even need correcting, but this is one of those
times.
The 125 watt figure is applicable only at full scale, not part scale.
Assuming the meter is linear across its full scale, the 5% spec
applies to any lower reading. For example, 5% of 1500 watts is 75
watts, not 125.
Most RF meters are not perfectly linear (especially at low scale
readings), but for our purposes we can assume they are close.
73, Bill W6WRT
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