RO Membrane Size Help

[Cal]

Hi, I have purchased a used RO/DI unit. After doing some tds test I've realised I need new filters. I'm having an issue finding RO membranes the correct size. The original ro membrane is a hidrotek 200g which when looking on website it's a 2812-200 but when I run a tape on it its saying 3inch... Anyway I bought 2 replacement vontron 2812-200 filters but they are too small. So I ordered the next size up, 2 of 3012-400 but they are still too small, in fact they are the same size as the 2812-200 I ordered. Am I missing something? I haven't unpacked the new filters as I'm going to need to return them but I'm certain they are too small for the membrane housing