Before I contact the supplier I got my replacement cell from I thought it might be better to get an answer from experts. My original cell has 5 plates 200mm (8") long and including the clips is about 48mm wide, 30 amp and self cleaning (reverse polarity).
I decided to get a backup cell just in case it failed and I ordered a cell from a cell supplier with a good reputation from eBay. When I received the cell, first thing I noticed was the length of the plates which are 150mm long, not 200mm. I rang the supplier and he said it would be fine. So, assuming he knew what he was talking about it, I thought no more about it.
A couple of months ago I removed the original cell, cleaned it and stored it away and replaced it with the new cell and right away I noticed the output was about 20% or so lower than the original, so I turned up the chlorine production knob. Even with that full on high it only just gets into the green (Summer) setting by about 15%.
Yesterday I had my water tested at my pool shop and everything was fine except the chlorine level was low. They asked me how long I'm running the pump etc and told them it's been running for 8 hours a day for the past 8 weeks.
After I got home I removed the cell and installed the original longer cell and the meter went almost into 95% green zone so I turned it down so it sites between the two m's of summer, which is about 50%.
I paid top dollar for the new cell and want to know, if my theory of cell plate mass/length missing (50mm x 5 plates) is causing the lower production as per my meter?
EDIT: Images of the original cell are in a post here: salt-cell-clips-t10528.html