Expert’s answer: Yokogawa AXR electrodes!
Ah, the mystery of the extra electrode! Actually, it’s not that big a mystery. But yes, there are some differences between devices with magmeter and Yokogawa AXR electrodes. Usually, a magmeter with three electrodes has one as a ground, which means you can skip a ground disc. Depending on the installation, the AXR will probably need a ground disc, but it’s really no big deal. The extra electrode just comes as a “nice-to-have” option.
Even so, magmeters with three electrodes sometimes need ground discs in specific installations, like on plastic pipes. Gotta ground somewhere! No zappy! On a plastic pipe, you should always install with a ground disc, no matter how many electrodes you have.
Heck, you can even find options on the market with four electrodes. The fourth registers an empty pipe. The AXR has a different way to check for an empty pipe, so you don’t need an extra electrode with it! Hope that covers everything!
Other resources that might interest you
Can you do a Yokogawa AXR loop test with the display?
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Yokogawa AXR flow units!
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Yokogawa AXR field zero adjust
I want to do a zero adjust on our Yokogawa. How do I do it exactly right?