> In this case with the bird 43 meter all of the meter
> movements are 30
> microamp movements and I would guess that all of the slugs
> are designed to
> put out 30 microamps at full scale into the standard
> resistance of the meter
> movement which is said to be 1400 ohms. I measured 1399 on
> one and 1406 ohms
> on another meter.
It was a normal practice at Prime Instruments to place a
fixed resistor in series with a movement to get the movement
resistance into tolerance. While full scale sensitivity can
easily be adjusted by changing the magnet field strength
with strong external magnetic pulses, it's pretty difficult
to control resistance of high resistance windings in
production. We always used a hand selected multiplier
resistor to set the resistance in applications like this,
where both resistance and current (or voltage and
resistance, or voltage and current) were critical..
> All slugs have to be designed to operate on the same
> amount of current
> regardless of their power rating as all of the different
> power scales are on
> the same relative points on the meter scale. That would
> mean that the same
> amount of current must flow in the diode of all slugs or
> the scales would be
> different.
There are two types of slugs I'm aware of. Of course the 43
series all uses the 30uA FS slug which is designed for a
1400 ohm load resistance. You really need to check both
resistance and current (or any other two parameters like
voltage and current or voltage and resistance) to be sure
the meter is good.
They may even tell you calibration can be verified with a
current source, but the assumption is the load resistance is
correct.
> So it would seem that by applying 30 microamps of current
> to the meter
> movements should result in full scale reading on all bird
> 43 meters in order
> for all 43 type slugs to be compatible.
That's true, but my only point was the resistance would also
have to be correct. The slug is a pretty high impedance
current source so it is very load resistance dependent.
> My original question was if bird actually marked full
> scale at exactly the
> 30 microamp point or at some point close by and should I
> expect to see full
> scale reading with 30 microamps applied or something else.
The proper place to call would be Bird, but as I recall they
were 30uA full scale for the model 43. I can't really read
one that close. About half of the digital microampere meters
I have are out of factory advertised tolerance. :-)
The calibration drift problems we had in meters were mostly
related to flux level changes in the magnets.
73 Tom
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