Dear Tom,
As my first posting in the above topic mentioned, the
Bird wattmeter is good for observing trends.
Tune it up, watch for the most smoke, be happy.
There ain't any precision, Tom. The average Joe's
watching to see his amp make 500 or a thousand watts
when he screams into the mike or holds the key down
while twiddling the knobs and he could care less
that the meter's off by 5, 10 or even 20 percent.
Lookit: Joe has to spend four thousand bucks for
a Hewlett-Packard or whatever, and even those are
sloppy.
It's a question about the application. I'm happy with
a Coaxial Dynamics analog using Bird slugs for the
ham station. It's just toys, and whatever I end up with I end up
with.
However, for the salt mine, we gotta use a calorimeter and go through
all the math to get a Proof-of-Performance report, but that's
a 250 pound machine that dims the lights when the pump starts.
Hal
-----Original Message-----
From: Tom W8JI [mailto:w8ji@w8ji.com]
Sent: Monday, May 07, 2007 1:13 PM
To: Harold Mandel; amps@contesting.com
Subject: Re: [Amps] Bird wattmeter
Harold,
What is missing from all the "information" given below (besides the source) is
what the calibration reference standard is and what its tolerance of that
standard is.
Without that key piece of information we really have no idea what to expect.
Also let's be fair. The standard Bird 43 Bird is +- 5% full scale accuracy. I'm
not sure where 10% below came from.
Salesmanship? :-)
http://www.bird-electronic.com/products/product.aspx?id=81
and of course you can buy +-1% accuracy meters from Bird:
http://www.bird-electronic.com/products/product.aspx?id=548
By the way the model 4381 mentioned below is not a lab grade instrument. It's
really pretty old. I bought my used 4191A's for $200 each. :-)
But this all the point. It certainly sounds like the Powermaster is a good
meter. So is the Bird. Neither one are perfect, neither one are "standards" and
they both are only as good as the standard they are calibrated against when
calibrated.
Buy either one and you can measure well within whatever is important to a Ham
or two-way service, so there is no need to unfairly exaggerate one while
denigrating the other.
73 Tom
.
> Tom's reply to the PowerMaster wattmeter drove me to inquire as to the
> calibration science behind them, and below is the listed response:
>
> " As shown in Table 2, the PowerMaster is quite accurate over a range
> of frequencies, power levels and signal types. The test signals
> include key down CW, CW at a 50% duty cycle (60 WPM dits) and two tone
> (PEP). The HP microwattmeters themselves are rated at Â5% accuracy, so
> any variations are well within measurement tolerances.
> During the PEP testing we discovered that the PowerMaster is more
> accurate for PEP measurements than the Bird 4381 computing wattmeter
> we normally use for comparison.
> The Birdâs spec is Â8% of full scale for PEP measurements.
> The Lab used its HP microwattmeter and the 14 MHz 2-tone test ïxture
> that we use to check ampliïer IMD to verify the measurements. The
> PowerMaster was pretty much right on the money.â
>
> Basically we do calibrate it very well, and we have taken the steps to
> remove the uncertainty in our measurements and or reduce them to the
> +/- 3% level or better. The one thing that we learned early on using
> the "slug" type line section meters is they are all really pretty bad.
> We even went to one company (name withheld) and purchased a
> "calibrated meter and slugs".
> We found out it was way off, and not ever reproducible in it readings
> due to temperature issues. We ended up finding that the 50 couplers
> we calibrated it in the morning at 65 degrees, were all different when
> we retested them in the afternoon when the temperature in the building
> was a higher 75 degrees. That is when we got serious and purchased a
> NIST traceable meter, calibrated attenuator, and "Thermocouple
> probes". Then we calibrated out our test cables (loss), and other
> fixture properties.
> We are for sure much more accurate then a Bird, which is 10% of full
> scale readings. Thatâs 500W on a 5KW slug!
> And on top of that they don't have a temperature spec which can add
> another 10% error on top of that.
>
> Tell your friend to try this, put in a 1KW carrier into his favorite
> slug meter and measure the power, now while watching it turn on a hair
> dryer on low heat, start heating up the slug and body. It won't take
> long, you can literally see the meter move.
>
> Now try it on a PowerMaster!
>
> Also I might add that the PowerMaster was not designed to just be a
> peak power meter, it is a station monitor with active alarms. Used
> properly it will save some big expensive tubes.
>
> Please feel free to share this with the good folks on the AMPS
> reflector, and if anyone would like to discuss this we will be happy
> to do so.
> _______________________________________________
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> http://lists.contesting.com/mailman/listinfo/amps
>
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