Nominal vs. Absolute

If you’re puzzled by what filter makers mean when they call their filters “absolute” or “nominal,” technical wizard Pure Water Annie will clear the matter up for you. 

Water filtration devices are rated by the sizes of particles that will pass through them. The ratings are normally expressed in microns.

Micron ratings are applied to all types of filtration devices, including even granular filters. It is said, for example, that a sand filter is roughly a 10 micron filter, meaning that it filters out particles ten microns and larger in girth. Carbon block filters are also assigned micron sizes, but micron ratings are most meaningful in comparing sediment filters.

The micron is a standard measure of size that is used by filter makers. The diameter of a human hair is about 90 microns. Sediment filters are used to catch particles 1/300 of that.

Most sediment filters are given ratings by their manufacturers that describe their effectiveness at removing particles down to a specified size. The most common of these are”nominal” and “absolute.”

Nominal, according to the Water Quality Association (WQA), means that the filter will filter out at least 85% of the particles of the size it is rated for. In other words, a filter that is rated as a 1 micron nominal can be expected to pick out 85% of the particles that are 1 micron or larger from the water that passes through it.

Absolute, theoretically, means that the filter will reject virtually all of the particles of the given size. The usual expectation is a 3-log rejection–or 99.9%. Absolute ratings are usually used for the tightest filters and for purposes where efficiency really matters. For example, if a filter maker promises removal of E. coli, more or less 85% efficiency isn’t good enough. If you’re going to trust your life to the filter, you expect an absolute 3-log or 4-log rating at the very least.

The problem with the absolute vs. nominal system is that there is really no universal standard that assures uniformity. Some makers of filters for non-critical applications, for example, might consider 70% rejection suitable for a nominal filter. Definitions vary from one manufacturer to another, and there is really no way for the end user to verify the claim. 

For most common filtration issues filters rated as “nominal” are used.  For example, a nominal 5 micron sediment filter is a standard size used in much residential water treatment.

Keep in mind that tighter is not always better.  The lower the micron rating of a filter, the more it restricts flow.  An absolute 1 micron filter is normally not a good choice for a residential sediment filter because there will be too much pressure loss.  If you put in a filter to remove sand, a filter with a low micron rating is not only unnecessary, but it will also become clogged much faster than a looser filter.

Reference:  The Pure Water Occasional.