Thanks fred, you are right that it does not make sense to fill the chamber if only making tests of small items. If I don't get a definitive answer from Sintratec, I will try with just a small amount.

You won't believe it but my new switch has just failed in exactly the same way as the last one did!!!!! After just three switch ons it is now permanently on. Not sure why this should happen unless the contacts are not rated for 240v