On the LEFT is a "wet well" sensor and
housing. You can see the tip end of the sensor and usually can touch
it. Water actually touches the sensor so its always stainless steel.
On the RIGHT is a "dry well" sensor
and housing. You might be able to see the sensor but its in the
housing. So the water flows around the sensor but never touches. The
sensor can be black plastic or stainless steel.
Although they do the same thing, they
are different diameters so they are not interchangeable.