EEVBlog 121GW Shows Incorrect Negative DC Voltage Polarity in Auto DC+AC Mode

[postfive]

I just noticed when measuring some negative DC voltages while the 121GW meter was in the Auto DC + AC mode that the polarity does not show a minus sign; it stays at a positive sign regardless of the voltage polarity being measured.  When the meter is switched to DC only mode, it works fine.  My meter had 1.04 version firmware and I figured maybe the latest firmware update would fix this so I just downloaded and installed Version 1.05, but it still shows the same incorrect polarity indicator.  Wondering if this is a feature (since in AC mode, there is no polarity, so maybe in the Auto DC + AC mode the meter is forced to ignore polarity) or whether this is an actual bug and if so whether a new version of the firmware could be released to fix this.  It isn't that much of a big deal, but unless you look closely at the mode display you could be led down the garden path by the false polarity indication.

[bdunham7]

You're right, that is an error.  Typically TRMS AC+DC indications leave out the polarity, but theoretically you could have a polarity sign that reacts to the DC part.  However, it would be weird if you had a pure AC signal which would give you a non-zero reading but no polarity.  In addition, the way TRMS AC+DC is typically done--with a TRMS converter chip--the meter won't even know the polarity unless there is a separate DC circuit.  In any case, if it gave you the correct polarity, it would be unusual.  Giving you the wrong polarity is...wrong.  Report it, perhaps they'll fix it, it's just a matter of disabling the polarity indicator totally in that mode.