Portion of a humidifier tub

FIG. 1 is a front perspective view of a portion of a humidifier tub according to our design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a right side view thereof; and,

FIG. 6 is a left side view thereof.

Where included, broken lines showing environment or structure are for illustrative purposes only and form no part of the claimed design. Where included, dotted lines represent the border between claimed and non-claimed subject matter and form no part of the claimed design.