IMB
The original ice mass balance buoy was designed for thick perennial ice. These buoys were not contained in a single buoy hull and were difficult to deploy.
SIMB and SIMB2
These two generations of buoys were very similar and shared the same exterior. These buoys did a better job dealing with thinner ice conditions.
SIMB3
This is the newest generation of buoy which aims provide similar data to older buoy generations, but at a lower price point and with much easier deployment.