One does not have to immerse the ruler that is being used measure the fluid column height. I have never measured that way. I compute the fluid column height by measuring the distance from the top of the fluid column to the top of the kettle and subtracting that value from the depth of the kettle. That way, I can use any ruler that I have on hand because it does not actually touch the wort. Plus, if one is not using an over-sized kettle, one can usually get away with using an easy to acquire low-cost 12" photo-etched stainless steel ruler.