The Class 3C diaphragm has characteristics similar to the Class 3 except the convolution is molded in. The information concerning flange design described for Class 3 also applies to Class 3C diaphragms.
Since this diaphragm is made in the “as-installed” shape with a permanent convolution it does not require inversion at assembly. It has a small centering force which may tend to return the diaphragm to its “as-molded” or neutral plane position. Because no inversion is required, only a flat retainer plate is needed to retain the head of the diaphragm to the piston.
The stroke of the Class 3C design is limited since deep molded convolutions are presently impractical to manufacture. It is recommended this design be used where differential pressures do not exceed 150 psi.