ABSOLUTE VS. NOMINAL

NOMINAL: Existing or being something in name only. A nominal 2x4 piece of lumber is really 11/2”x 31/2”.

A 0.4 micron absolute filter will not allow any material larger than 0.4 microns to pass. A 0.1 micron nominal filter could allow some material larger than 0.1 microns to pass. These ratings are the ratings of the Seagull IV Water Treatment Device. When they quote only one figure they quote the absolute rating. This gives no false image to the consumer of the porosity of their cartridges. Compare this with other companies which will quote only the nominal rating (say 0.5 micron) and not mention the absolute rating (say 1.0 micron).

The Natural Solution is advertised as having a 1.0 micron cartridge. This is the absolute rating. Its nominal rating is 0.5 micron.

Filtration is the mechanical removal of suspended solids from water. Six interrelated factors must be considered to properly understand filtration: 1) filter area; 2) quality of the filtered water; 3) porosity of the filtering material; 4) flow rates required; 5) capacity (total gallonage before servicing); and 6) filter construction to eliminate bypass and channeling.

Physical filtration depends on two features: the size of the holes or pores (porosity) that the water goes through and the total amount of filtering area available. The pore size is important because it determines the size of the impurities which will be physically prevented from passing through the filter. Thus, porosity effects the quality of the filtered water. Unless the filtering surface is quite extensive in area, a filter cannot produce an adequate flow of water. Therefore, the amount of filter area is important for proper instantaneous flow rates and total capacities.

Filter construction is equally important because if water can wear away a channel or bypass the filtering material, then the resultant water will not be filtered. Granular activated carbon (GAC) is the most likely to channel. So-called solid carbon block filters are less likely to bypass or channel. They are made of GAC which has been compressed into its final shape in a mold. It can be denser near its surfaces which contact the mold and less dense inside. There is nothing which bonds the GAC together. The least likely to channel are the bonded or extruded types of cartridges. These type of cartridges are very uniform in porosity and are bonded by the manufacturing process, by inert adhesives or by a high-tech firing or heat process.

DATA: 
1 micron = 1 micrometer = 1/1000 of a millimeter = 0.00004 inch = 1um.
Smallest item still visible to the naked eye is 25 to 30 microns.
Milled flour ranges from 1.0 to 30 microns.  Bacteria is from about
0.25 to 30 microns.  Viruses are from 0.01 to 0.1 microns.