Several models, including the NI PXI-1031, PXI-1036, PXIe-1065, PXIe-1071, PXIe-1078, PXIe-1085 and PXIe-1095 have never incorporated a foam filter as part of their respective designs.
You can continue to use a chassis with a filter installed. NI recommends following the specified maintenance of cleaning the filter routinely (every 6 months) or simply removing the foam filter completely. You can clean the foam fan filter by washing it in a mild soap solution and then vacuuming or blowing air through it. Rinse the filter with water and allow it to dry before reinstalling it in the chassis. Note:
Some chassis have metal filters - these should not
If you need new filters, they can be purchased from The Filter Factory
(formerly known as Air Filtration Products). The material should be 3/32" thick reticulated polyurethane foam with a porosity of 30 ppi.
The images below show the difference between a chassis with a filter (Figure 1) and one without a filter (Figure 2):Figure 1:
PXI(e) with Fan Foam Filter Figure 2:
PXI(e) without Fan Foam Filter