Weston 622 Milliammeter

[retired4now]

A question for the more senior list members...

I have a Weston model 622 precision milliammeter that is consistently reading about 1% low on its four ranges.  I assume this is the result of a gradual loss of magnetism over the 45-years since it was last calibrated.  In removing the shield cover over the hairspring of the movement, I noticed there is what appears to be an adjustable reluctance plate at the front of the magnets.  This is retained by a hex-head screw accessible through a hole in the front of the housing.  There are also two smaller holes on either side, possibly for the purpose of sliding the plate with probes.

Before I attempt to move this plate, I thought it best to ask if this is a linearity adjustment or is it provided for adjusting the sensitivity of the movement.  Can anyone advise me?

Bruce

[Hawaii596]

As I don't know that one in particular, my response is a little generic.  The obvious first piece is the zero adjust.  I will presume it has been mechanically zeroed.  Before looking at other inaccuracies, the zero needs to be right on the money.

Once that is done, looking at for example, 25%, 50%, 75% and 100% of scale, if there is a consistent percent of full scale error, it is a gain issue.  If the accuracy is non-linear, it would be a linearity issue.

Some analogue meters have counterweights or something to adjust gain.  They are really sensitive and easy to break.  I have done it, but you have to tread very carefully with the counterweights. 

If you have GIDEP access, there are some technical manuals with information about adjusting the 622 series.  Unfortunately, I cannot share the document here.

I will say after reading through it, it does say that inaccuracies can be corrected by means of the magnetic shunt.  Not sure how to adjust that, but it sounds like that might help.

[retired4now]

Thanks for Hawaii596 for his help.  The meter is nicely calibrated and checks up and down on all scales to hair-splitting accuracy. I used my Fluke 8842A as a reference.  It turned out only the outer bar was a moveable shunt.  The inner one is fixed and appears to be conducting most of the flux between the two magnets.  Nevertheless, the magnetic field slightly penetrates the moveable outer shunt.  The outer shunt had a remaining movement range of about 1/4-inch to the right.  It took a movement of about 1/16-inch in this direction to increase the gain by about 1 %.