Have you ever noticed that some filter cartridges are actually different lengths? It may not be something you notice at first glance as the difference is only 1/8". But, the important thing to take away is, yes there are two different lengths a filter cartridge can be, but why?
Filter cartridges come in lengths of 9-3/4" and 9-7/8", depending on the filter cartridge. The nominal 10" standard filter housings are designed for filter cartridges that are 9-3/4" long, which is the distance between the top and bottom knife seat in the filter head and filter bowl.
So, if the 9-3/4" long filter cartridge fits perfectly in a standard filter housing, why is there another length?
Let's take a look.
Standard 9-3/4" Long Filter Cartridges
9-3/4" long filter cartridges are designed to effortlessly fit between the bottom and top knife seat in the filter head and bowl, when the filter bowl is tightened. They are designed with smooth plastic end caps or gaskets on both sides of the cartridge that seamlessly press against the top and bottom of the knife seats. This ensures a seal that prevents bypass.
9-3/4" long filter cartridges are available in pleated polyester, carbon block, briquette 'cyst removal', granular activated carbon and powder activated carbon wrap cartridges.
Why are there are 9-7/8" Long Filter Cartridges?
There are only two types of cartridges that are 9-7/8" in length, string wound and spun poly, which are also referred to as depth cartridges or melt blown cartridges. These types of filter cartridges are purposely made 1/8" longer than the standard 9-3/4" length.
The string wound and spun poly cartridges are softer and the "V" shape of the knife seat will easily press into the string and poly filter media at least 1/16" on both ends. This ensures that there will be no bypass between the end of the cartridge and the knife seat. Bypass can become a problem when these types of cartridge are only 9-3/4" in length. This is especially true with string wound filter cartridges as their end surface is not an even, flat surface.
Bypass: If the filter cartridge is too small, the water will find a faster way around, resulting in unfiltered water getting through. This is something you don't want to happen.
Since this issue has been recognized, there should be no reason to worry about whether your system will allow bypass as the average string and spun poly filter cartridges are only available in the 9-7/8" length. So, no need to keep a ruler handy. Although, being aware of this does allow you to be cautious. If you are noticing that bypass is occurring, this will provide the knowledge on what the issue could be.
Note: These non-compressible types of cartridges, if they were made longer than the 9-7/8" length, would prevent the filter bowl from being tightened enough to properly seal the O-ring between the filter bowl and the filter head. This is why the 9-7/8" length is limited to two types of cartridges.
Replace your Filter Cartridges
Something to keep in mind is that all cartridges have a limited service life. Changes in colour, odor, taste and flow of the water being filtered are signs that you need to replace your filter cartridge. You should always follow the manufacturers recommendations as well.
It is highly recommended that you should change the filter cartridge a minimum of every 6 months to prevent bacteria from building up. A smart idea would be to get in a routine of changing the cartridges at the same time you replace your smoke alarm batteries or change the clock for day light savings!
Although this problem has been revealed and fixed, being aware of an issue that could occur in your filter system allows you to have the confidence that if the issue does occur, you can quickly determine if the length is the issue and fix it.
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