Triton II Swimming Pool Sand Filter - Maintenance Questions

Have purchased a home with an inground pool equipped with a Triton sand filter. The filter has functioned well for several years and been backwashed, as required.
My question is whether the sand in these filters needs occasional additional quantities due to backwashing losses and/or whether the sand needs to be changed entirely on some schedule. The user manual for this equipment seems quite rudimentary and mentions nothing about maintaining/changing the sand, other than backwashing. Also, I can find no web site for the manufacturer, just a blizzard of store sites selling the equipment.
Can anyone out there provide some guidance? and a link to a manufacturer web site?
Thanks.
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Pentair is the manufacturer http://www.pentairpool.com/poolowners.htm
Some people say to change sand after 2 or 3 years. I let my go 15 years and it was still probably OK. Using a wet/dry vac to remove a few hundred pounds of sand is not that pleasant. I'd avoid this unless you see signs of poor filtering.
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The original sand granules have sharp edges which tend to catch debris as it passes. Over time (think years and gallons), these edges become rounded off, thus reducing the ability to filter. Most filters have a pressure gauge that indicates "back pressure" (my term) across the filter. This, with experience, can help determine when the sand needs replacing.
1) Thoroughly backwash the filter. Then return it to normal service. 2) Note the pressure. If no gauge is present, install one. 3) Over a few weeks, note if the pressure rises and how much. Usually when it increases 4-5 psi, the filter is getting clogged and it's time to backwash.
As the sand loses it's effectiveness, the back pressure will not occur because the filter is NOT getting clogged. THEN, it's time to change the sand. My experience in Atlanta, is that I changed the sand after 6 - 7 years of service. It is not difficult. Filter is drained a day before, then the top is removed and a wet / dry shop vac sucks it all out. Once empty, inspect the plastic tubes (radials or laterals) inside. If one of these is damaged it will allow sand to enter the pool, most notable on the stairs. Replaced as required and pour in fresh sand from pool supply place.
BTW, I looked at the old & new sand under a microscope and could not see much difference.

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tacker writes:

Yes, you certainly can slowly lose sand over time. Backwashing can also distort the population of sand grain sizes by preferentially flushing out the smaller grains, which degrades the filtering performance even though it looks like you have a full sand bed.
Only way to know is to open it up to inspect the level of sand in the vessel. Typically requires unbolting a bulkhead plate for the valve assembly on top.
One book I have suggests sand can agglomerate to form "mudballs" from lime deposits and then you get channels through the sand that negate the filtering. I have not observed this myself. But if that's so, it is a reason to change out the sand.
You can put pool acid or lye in there to clean just about anything deposited on the sand. Inspect a sample to diagose if and what is deposited, if anything.
Filter sand is quartz sand with sharp edges and screened and graded to have a certain spectrum of granule sizes. Being quartz it is very hard and durable. It isn't going to "wear out".
My sand filter is huge, and I used a garden hoe to reach down into it to scoop out the sand to change it once and inspect the buried plumbing. Took some time but not that difficult.
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