Is the PH21 the same as the FU20, just a different color body?

This sensor is specifically designed to be used in water- and wastewater treatment applications.
The specifications of the PH21 are lower than that of the FU20. For example, the PH21 can be used up to 60°C (140°F) and the FU20 105°C (221°F). The expected lifetime is 6-24 months, depending on the process conditions (pressure, temperature).