Roger Long wrote:

It must work
better with engines 50 hp and up than with my 20 hp Yanmar or it would
have been off the market long ago.

Not necessarily. It might even improve sales & profits. Gadgets of
any degree of uselessness are a big money industry for people who
aren't content with a fully-sufficient temp guage, other direct
observation of normal watchstanding, & the feel & sound of the little
plant. Changes in these things will tell you more than any new
instruments, and they cost nothing with no maintenance. But they do
not bring the emotional satisfaction or bragging rights of another cool
device aboard. ;-)