How do you calibrate a rectifier?

Rectifier Calibration

Q. Can you suggest an inexpensive method for calibrating the amp and volt meters on plating rectifiers? Our rectifiers range from 0-9 volts and 0-4000 amps.

A. You didn't mention whether the meters were digital or analog, but I'll offer some suggestions that should work for both. You'll only need a good digital multimeter (DMM) for the following procedures.

Note that plating rectifiers typically have high voltage, three-phase power inside, thus present the risk of electrocution. The measurements described below do not require that the tester work inside the rectifier main cabinet, only that he opens the hinged door to the remote control cabinet that houses the voltmeter and ammeter. However, 120 VAC power is typically present in the remote cabinet, as a part of the start/stop circuit, power for digital meters, etc. Only experienced persons that know how to work safely in the presence of live power should attempt the calibration procedures. When in doubt, get a competent and experienced person or firm to do the work.

I am assuming that you have access to the back of the voltmeter and ammeter (where the wires attach). It's best to take multiple readings at different power levels from the rectifier (low output, medium output, high output). If a meter is off and cannot be adjusted to read accurately throughout the entire range, you may elect to just calibrate it to read correctly in the range in which your parts are plated. For example, if a 0-9 VDC voltmeter reads true from 3 to 7 volts, but not at 1 or 9 volts, it might not matter if all of your plating is performed within the 4-6 volt range.

For the VOLTMETER:
Place the DMM in the DC voltage mode, then measure the voltage between the positive (+) and negative (-) terminals on the back of the meter. Record both the DMM reading and the rectifier meter reading. Adjust the rectifier meter to read the same as the DMM, especially in the voltage range that you use for plating. Analog meters are typically adjusted by rotating a small screw on the meter face. Digital meters may have a potentiometer on the rear, or even a calibration routine using a menu system and front panel buttons.

For the AMMETER:
One initial point has to be made. The rectifier's ammeter isn't really an ammeter at all. Although it has a scale that reads in "Amps", it's a DC millivolt meter in disguise! The high current used in chromium electroplating would be much to high to pass through such a small meter movement or circuitry. Normally, plating ammeters are calibrated to read "0" amps with zero millivolts (mV) applied to its input and full amperage (ex. 4000 amps in your case) when 50 mV is applied. Therefore, the ammeter needs a 0 - 50 mV source from somewhere. Typically, the 0 - 50 mV signal originates at a DC "current shunt" located somewhere on the positive or negative copper bus inside the rectifier cabinet. A pair of small, shielded wires connect the shunt to the ammeter. In order to test or calibrate the ammeter, you need only gain access to the back of it. Place the DMM in the DC Millivolt range, then measure the signal across the two meter terminals with the wires from the shunt. Don't worry if a minus sign appears in the DMM display, which indicates that your DMM test lead wires are backwards. Record the DC mV and the ammeter reading in amps.

Now we have to do some math. The DC current shunt generates a DC mV signal that is proportional to the output current from the rectifier. Rectifier manufacturers almost always use 50 mV shunts (but I have on occasion seen 100 mV shunts). Using your 4,000 ampere rectifiers as an example, here's the math. Assume the rectifier ammeter read 1850 amps, and 25 mV DC was measured with the DMM.

25 mV x (4000 amp / 50 mV) = 2,000 amperes by calculation.

The ammeter needs to be adjusted or calibrated to read 2,000 amps. Since 4,000 / 50 = 80, as a shortcut you can just multiply the mV by 80 to get actual amps (for a 4,000 amp rectifier only). I hope this information is helpful.