[Elecraft] OT: Bird Wattmeter "meter failures"

[hsherriff]

I'm the supervisor of a metrology lab and that is one of the first things I ensure a new technician understands. And many times it isn't the test equipment that is at fault, nor the unit under test, but the methodology of using that test equipment.  Not using the equipment "guard" correctly,  induced voltages, improper test leads for the job.... and many many more.
Harlan NC3C 


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-------- Original message --------
From: Don Wilhelm <w3fpr at embarqmail.com> 
Date: 11/21/2015  7:28 PM  (GMT-05:00) 
To: elecraft at mailman.qth.net 
Subject: Re: [Elecraft] OT: Bird Wattmeter "meter failures" 

That brings to mind an adage advocated by my lab instructors in college 
and has proven to be very worthwhile over the years - it is no less 
applicable today.
"Technician (or Engineer), know your test equipment and especially know 
its limitations".
That means always be prepared to verify your measurement equipment 
readings by some other means rather than trusting the indication of any 
one instrument.

In other words, if your test gear is telling you something that looks 
"funny", verify the test equipment as a first order of business - use an 
alternate measurement method to do a 'sanity check' and remember that 
test equipment can fail just as well as the equipment under test.
Trust no piece of test equipment that cannot be verified for correct 
operation.

As an example, I have a wattmeter that is supposedly good for HF as well 
as VHF/UHF.  The VHF part starts above 30 MHz.  When I had problems 
getting XV50 transverters to develop 20 watts as indicated on that 
meter, I used an alternate power meter and discovered that wattmeter 
took 36 watts to indicate 20 watts on its meter on 50 MHz.  On 144, 220, 
and 432 MHz it was accurate.  So that condition is now plainly marked 
with a sticker on that meter.  Verify before believing the meter in use.

I know that such verification is not always within easy reach for the 
average ham, but usually there are locals with test gear that can be 
borrowed to give you a validation of whatever test equipment you are 
using (test 3 and let the most common vote be the deciding factor).  You 
do not have to pay the cost of a calibration lab to do that kind of 
verification of your test gear, but do be suspicious if you have not 
validated your test equipment in some manner.

73,
Don W3FPR

On 11/21/2015 6:23 PM, Ken G Kopp wrote:
> An earlier posting brings this to mind ....
>
> There is a seemingly little-known failure that frequently occurs to Bird
> meter movements ... or more correctly ... to the connector at the sensor
> casting end of the length of coax used to connect the (30 ua) meter to the
> sensor.
>
> Often a sharp "whack" on the housing will cause the meter to resume
> working, often for long periods of time.  The connector's internal
> connection is made by inserting the (solid) center conductor of the coax
> between turns of the coils of a tiny spring that's part of the center pin
> assembly of the casting end's cable connector.
>
> There's such a tiny amount of current flowing through the coax center
> conductor and the spring "connection" that it seems to corrode easily, and
> most assume the meter movement has failed.  The resulting tiny disturbance
> to the wire/spring junction is why the "slap" on the housing often brings
> the meter back to life.  Many of the expensive proprietary meters have been
> needlessly replaced because of this.
>
>

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