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topic 6569

Calibration of Plating Rectifiers


(2000)

I work for a small aerospace manufacturing company which has a small plating shop. We currently have two rectifiers, one for a small cad plate line and one for chromic acid anodizing. My question is in regards to the calibration of this equipment. Up until now we have been using a shunt and multimeter to determine the output of the rectifiers and using this as calibration data. Basically we are making sure that when the meter says 50 amps there is truly 50 amps at the bar. I heard there is a piece of equipment available which will measure the output at the bar. I am told it is like a wand you simply hold over the bar but I cannot find any information on it. Can anyone point me in the right direction.

Todd Sjerven
- Victoria, BC Canada


(2000)

You are probably talking about a tong tester, but that is not how I would calibrate a rectifier meter, as it would surely be significantly less accurate than the more sophisticated meter already on the rectifier.

The rectifier should have a built-in shunt, usually mounted right at the outside of the rectifier housing, just where the busbar connects to it. This shunt is a block of metal of fixed size and resistivity that should not change much in resistance over decades, although I suppose you could remove it and have it tested for accuracy by a certified lab if you really needed to. I am not sure if this is the same shunt you are referring to.

The shunt has a very specific resistance, usually chosen so that it will introduce a voltage drop of 50 mV when the maximum rated current of the rectifier is flowing. Once you have determined and verified this mV-to-Amps factor, then measuring the millivolt drop across the shunt will give you a dead accurate measurement of the current flow. Now you just make sure that the rectifier's ammeter is accurate. Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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