How to Measure Your Hot Tub Filter
Measuring your filter cartridge may not be as straight forward as it at first seems.
Please follow the steps below to be sure that you get the correct measurements.
Remember that all measurements may vary by about 1/8" (30mm) due to expansion/shrinkage of the cartridge over time.
1. Measure the Diameter (Width) of the Filter
Measure the outside diameter (width) of the filter cartridge (in inches or centimetres), that is the maximum distance from one side to the other, including any 'over-hang' of the plastic end caps which might make the filter wider than the actual filter element itself.
2. Measure the Length of the Filter
Measure the length of the filter cartridge from end cap to end cap. the length should include the thickness of the plastic end caps, but do not include handles or screw threads in the length measurement.
3. Measure the diameter of the Hole
Does the filter have an open hole going all the way through it from top to bottom or does it just have a hole at the bottom with a closed top or handle?
In either case you will need to measure the inner diameter of the hole, excluding any taper.
4. Identify and Measure the Thread
If the filter has a thread, identify which type of thread it is. There are only two types of thread fitted to hot tub filters:
The most usual type of thread is fine and tightly spaced with relatively sharp edges. these are called MPT or male pipe threads. Fine threaded filters are usually found in 3 nominal sizes: either 1 1/2", 2" or 3 1/2" MPT.
The alternative is a very coarse, widely spaced, flat-edge thread and again these are usually found in 2 nominal sizes: 1 1/2" or 2" SAE.
[Please note that it is actually the diameter of the thread shank that is measured, so for example: a 1 1/2" MPT or SAE thread actually measures 1 3/4" outside diameter and a 2" MPT or SAE thread measures 2 1/4" outside diameter!]
5. Does the Filter have a Removable Bezel?
Some Pleatco brand filters have a removable male pipe thread fitting which slip-fits into a 1.9" hole in the filter bottom. You may need to unscrew the male pipe fitting from your spa's filter housing if it did not come out with the old filter (many hot tub owners are not even aware that the fitting is still in there). Most other brand filters are molded as a single piece, so that the threaded fitting comes off with the filter
6. Are there Two Filters availabe that Appear to be the Same Size?
If you are still having trouble identifying the correct filter because there appear to be two availabe of the same size then the difference will be the filtration area available within the filter. If you don't know which filter you need then the only way to be sure is it count the number of pleats! It might take a while but the pleat count determines the filtration area of the filter and so enables you to distinguish between filters with the same dimensions. When you view the detailed information about each filter the pleat count is usually given.