On Mon, 17 Sep 2018 21:12:48 +0100, "Jim GM4DHJ ..."
wrote:

are optical smoke alarms better for less false alarms than ionising
ones...in practice I mean ? .....

No. Neither have significant false alarm rates. Both however can
suffer nuisance alarms, these are not false alarms but real alarms
produced by non- dangerous activities such as steam from a bathroom
or overheated toast.

Optical suffer more from steam, ionisation from the combustion
products of things like singed toast. Used and sited properly neither
should have a significant nuisance alarm rate (unless of course you
are prone to overheating toast with the toaster is in the bathroom
sink while you are taking a shower).