Is it possible to get area from the image or at least how to get distance between two points?

Ive conducted some evaporator visualization experiment. After filming the refrigerant flow inside the evaporator, wanted to proceed image processing to know the quality of the refrigerant. Tried to binarize the image to get boundary line between liquid and gas. Also tried with gimp to get those lines.

that red box is liquid refrigerant which i want to know the area or at least the distance between boundary line and bottom of the pipe.

The right side image was got by these process, Sobel->dilate and eliminating useless pixels.

This one was got by same process using GIMP. As you can see the pink line which is boundary line.

Want to know if there is any good way to get that red square area(liquid refrigerant area to know the refrigerant's quality).

Asking for any good idea for this problem