Compression caps are used for tightening down on a Viton O-ring. Tightening the cap causes the o-ring to expand, hold whatever is placed into the center of the cap in place. A compression cap can be useful for holding thermocouples in place, for example. This is particularly helpful when you need the thermocouple to not touch the sides of heated glassware, causing damage or incorrect readings.
Note: The joint must be threaded for the compression cap to work properly.