Macro Algae Reactor Advice

[oldmonk]

I was just going through the spare parts list on Tunze site for Mar 3181 and 3182. What intrigues me is how come the O ring ( part no 5) dia is same for both models asuuming the 3182 is a bigger model with bigger dia acryclic tube.
Any thoughts? Has anyone ordered a spare O ring or bothered to check the dia of 3182 model?
@Tunze can you shed some light?
Thanks