How does the Surface Analyst measure over 90 degrees?

Because small drops (less than about 10 microliters) are always spherical, we can use equations of spheres to calculate the contact angle. 

When the contact angle ϴ is <90°:

The camera “sees” the circle where the drop contacts the surface and we use an equation that relates the diameter of the contact circle and the drop volume to the contact angle.

When the contact angle is >90°:

The camera now “sees” the biggest diameter of the drop, not the circle of contact. So for this case we use a different equation that relates the major diameter and volume to the contact angle.

The precision of the measurement depends on how precisely we know the volume and how precisely we measure the diameter. The diameter of a low contact angle drop is easier to measure precisely than a high contact angle drop mainly because it’s easier to measure a larger diameter accurately. So high contact angles are less precise than low contact angles. Here’s roughly how the precision varies with contact angle: