What would cause ORP to drop from 370 to 30 in a day?

[marinetankdiver]

Yesterday morning my ORP was reading 377, it has been in the 370's for as long as I can remember, I have my skimmer draw outside air. Then yesterday afternoon it read 30. All other parameters are fine. Temp: 77, Salinity 33.1, PH 8.01, Nitrates, Nitrites and Ammonia are close to 0 as my test kits show, Dkh is at about 9. All of the corals are extended and looking great as are the fish and inverts. I re-calibrated the probe this morning and it is now reading 70. I'm hoping the only explanation for this type of change without any noticeable effect on livestock is a bad probe but thought I would ask for opinions.

Thanks.

[tufacody]

30?!? Broken probe or unit. Perhaps the last digit went out?

[GlassReef]

As I posted in your other thread, sounds like a bad probe to me - 30mV is way low! If you can get a hold of some 400mV calibration solution, stick your probe in some fresh salt water for a couple of hours. If it doesn't settle at approx. 190 to 220mV its the probe

[marinetankdiver]

Thanks for posting on my other question. I figured, after I posted there, that the question belonged in the chemistry forum.

The probe is on my DA Elite so it probably isn't a bad display. I'm going to order some more calibration fluid and test the probe again.

I do know that the ORP goes down when I dose with Mag but within an hour it is back up to the 370 range and I haven't dosed anything but Alk and Ca in the last few days.

I'm sure it is a bad probe, at least I hope that is what it is and not something else way out of wack that I haven't tested for.

[bertoni]

If the tank is still alive, I'm guessing the measurement is wrong. A bad or dirty probe might be the issue. I'd clean and recalibrate.

[zachtos]

Mine used to drop to negative values when I dosed buffered vitamin C. I don't even know why I use my ORP anymore, I just look at the corals and fish to see what's going on.

[marinetankdiver]

Everything is alive and doing great, well the polyps on my cup coral didn't come out today. But I think that is normal every once in a while.

I've decided that it is just a bad probe and disconnected it so I don't stress over the number.

Thanks for the suggestions and support.