Note that with Tri-Clamp fittings the size refers to the outside diameter of the tubing that it makes a connection for, not the flange diameter (the flat disc part).
For 1.5" Tri-Clamp fittings the diameter of the outer flange is 1.988" (50.5mm). The 1.5" Tri-Clamp flange works directly with 1" (25mm), 1.33" (33.7mm), or 1.5" (38mm) Tri-Clamp fittings. 1.5" Tri-Clamp probes can be used in 2" Tri-Clamp fittings (or vice versa) with a suitable adapter/reducer (not included).