The difference between measurable and observable are poor words to use or at least easily confused. I would prefer quantitative and qualitative. They're not vastly different in meaning but I think are treated differently.
Is there a defined size which your sieve will and will not pass? Yes, of course there is. Therefore it is not qualitative. It is not merely "observable" but it is quantitative. The sieve size you have chosen will only let through contaminants of certain dimensions. Of course it is theoretically possible that thin wires could pass through but the same is true for a metal detector and I don't see auditors arguing there. Also in practice, wires tend to get caught on sieves. If you really wanted to, you could get test pieces to prove your sieve was capable of rejecting that size. Which of course you wouldn't because that would be stupid as a visual inspection of sieve integrity proves it's capable of working. Just because you're checking the CCP in a different way to a metal detector, doesn't make it not a CCP. Just means your eyes can see a mesh but can't see a magnetic field (I know there's more to it than just checking the field but you get my point.)
What would count as "observable" or qualitative? Things like vision systems for me, colour changes. Where it all starts getting a bit more vague. Where there is a grey area.
So to my mind, the auditor has misinterpreted the language and tried to be a smart ass. I do not suggest you put that in your CAPA response. Much as it's tempting. I also hate auditors telling people that something should or shouldn't be a CCP, especially when they're only suggesting something might be an oPRP instead. I mean seriously, what food safety risk is going to be changed by that observation by the auditor? Is it the square root of absolutely nothing? In the meantime do you fancy actually trying to find something which can be helpful to us not trying to catch us out on a technicality?
