Cygnus single crystal probes are for use with the gauge in Multiple-Echo mode to read through and ignore coatings.
These have one piezo-ceramic crystal. This crystal both sends and receives the ultrasound energy.
The S2C single crystal 2.25 MHz 13 mm probe is recommended for the majority of applications and can be used to read through coatings up to 20 mm thick with the gauge in Deep Coat mode.