[Amps] Bird wattmeter

[Helge Skram]

I have several Bird 43 meters and the current thru the meter shall be 30
microamp for full scale. If movement is less, the meter will measure lower
than correct. If it is sligthly off you can correct with the needle
adjustment screw. Then it will be sligthly off at zero and low readings.
All slugs will give 30uA at indicated power.
The slugs have a small variable potmeter that can be adjusted for
calibration. Then you need a good reference and gain access to the pot. 
I check Bird meters as you explained with a 12V source connected to the 
Slug connection via a 470k pot and a multimeter reading microamps.     

I have many slugs where most are purchased seconhand. It is not unusual that
readings are off by as much as +-10%, comparing slugs or measuring with a
Narda coupler and HP435 powermeter. With a meter that reads full scale with
30uA and new slugs, power is accurate within +-5%. The Bird 43 is a nice
instrument that last for decades if it's cared for.

73 de LA6MV
Helge


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Gary Schafer
Sent: 6. mai 2007 17:59
To: amps at contesting.com
Subject: [Amps] Bird wattmeter

Has anyone measured the current thru the meter movement on the bird 43
wattmeter? It is a 30 microamp full scale, so it says on the meter face.

I have two very old ones that read low on power. I checked the full scale
reading by putting 30 microamps of current into the meter. They do not read
quite full scale with that amount of current. Both are different too.

 

I am wondering if they should read exactly full scale with 30 microamps or
if bird has the scale slightly off of the normal full scale point of the
meter?

 

I was going to try and collaborate meter current with watts readings but it
does not work out with the square of the current as you might think. It is
probably because of the non linearity of the diodes in the slugs. It looks
like all slugs are set up to provide the same current thru the diode not
matter what power slug is used. That would make the meter scale the same for
all power levels although not linear to current.

 

If anyone wants to check their meter movement it can be done without even
opening up the meter. Just take the element out and attach a small clip to
the element contact in the slug hole side. That gets you directly to the
meter. A 470k resistor and an adjustable 12 volt  power supply and a good
microameter.  I have 3 different bench meters and all read within .01
micromaps of each other. A fluke, HP and keithly. I also tried my fluke 70
series handheld meter and it was off by about 1 microamp at 30 microamps.
Yes 1 microamp makes quite a difference in reading on the bird meter
reading!

 

I also have another old bird meter movement that was replaced many years ago
and it will not go anywhere near full scale with 30 microamps and the
movement is free and does not seem to stick. 

 

I am wondering what happens to these meters? Do the magnets go bad in them
or something else?

There is a 200 ohm resistor inside the meter movement case that is in series
with the meter. I wonder if that is used in final calibration of the meter
movement?

 

There are several articles floating around about adjusting the slugs for the
bird wattmeter to bring them back into spec but I am wondering how many of
those problems are problems with the meter movement rather than the slugs?

 

Thanks for any help

Gary  K4FMX

 

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