The Chlorinator should use a Viton rubber O-ring. This is much more expensive than the standard NBR rubber O-ring. The Viton O-ring should last much longer than the NBR version.
We offer both versions
Part number 2 on all illustrations
£14.75
Save £1.50 and get two NBR Lid O-rings for £8.00
Whichever rubber quality you choose, the O-ring should be smeared with silicone grease to increase its life.