4" to 3" Tri Clamp Reducer with 1/2" Thermo PortConcentric Reducer for 4" Bubble Plate Column with 1 2" Thermo well Stainless Steel 4" to 3" tri clamp concentric reducer for a 4" bubble plate column with a 1 2" thermo port on the side for installing Thermometer. DETERMINING TRI CLAMP SIZE Tri Clamp sizes are determined by the outer diameter of the pipe or tubing, not the outer diameter of the flange or Tri Clamp itself. The flange of a Tri Clamp fitting typically has about a inch greater diameter