Being a med-student a just coming off an ITU attachment i came across a product i thought would interest rebreather world. It is a small plastic box that gets stuck on the end of a ET tube to test if it is located properly by changing colour quite dramatically in response to c02 (it goes from purple to bright yellow).
However although they claim to last for 2 hours this is unlikely to be the case under pressure and it is far too sensitive to track changes in end tidal c02. The one thing i though it could be usefull for is to be brought into the loop toward the end of long dives (where £10 for extra security is more likely to be spent anyway) to check for scrubber breakthrough. So no where near ready to slap into you loop yet but I thought it was interesting nontheless.