How is an analytical ultracentrifuge (AUC) counterbalance configured?

An AUC counterbalance not only offsets sample cell weight, but also provides a way to calibrate the optics.

Each counterbalance, which is always anodized red, features four reference holes with removable mask windows.

Inner edges of the reference holes should match up with outer edges of the centerpiece cell. The counterbalance weight can be adjusted by screwing weights into the center of the counterbalance cell. Using the provided screw weights, the counter balance must weigh within 0.5 grams of the sample directly opposing it in the rotor.