By Rick Andrew

The term performance indication device (PID) is a formal term for a filter-change indicator that meets certain criteria and requirements, as defined in NSF/ANSI 53. It seems like it would be a good idea to have a filter-change indicator as part of a filter system. It encourages end users to maintain their systems properly, assuring that the system will perform as intended. It is a good idea for a reason that goes beyond this immediately obvious end-user benefit, however. NSF/ANSI 53 requires testing for chemical reduction claims to go to 200 percent of manufacturer’s claimed treatment capacity. But with a PID included in the system, the standard requires testing only to 120 percent of capacity. This essentially means that the manufacturer can claim a higher capacity on the system by including a PID. For example:

  • Filter system A is tested to 600 gallons for reduction of lead.
    — Without a PID, the test must be to 200 percent of the manufacturer’s claimed capacity. Therefore, the maximum capacity that can be claimed by the manufacturer is 600 gallons/200 percent = 300 gallons.
    — With a PID, the test must be to 120 percent of the manufacturer’s claimed capacity. Therefore, the maximum capacity that can be claimed by the manufacturer is 600 gallons/120 percent = 500 gallons.

We see from this example that the exact same filter system that can claim a capacity of 300 gallons can add an additional 200 gallons of claimed capacity by adding a PID to the system—an increase of 67 percent in claimed capacity with the addition of the PID.

Criteria and requirements for a PID
As mentioned above, there are certain criteria and requirements for a PID. They include:


  • The PID must be an integral part of the system. This means that it cannot be an optional kit that is installed aftermarket—it must be a part of the product.
  • It must be based on the volume of water that has been treated. It cannot be based only on time. It is, however, acceptable for a PID to indicate a filter change based on either volume or time, whichever comes first.
  • The PID must be fully automatic, with the exception of a user-activated reset when the filter is changed. This means that in order to qualify as a PID, it must not require the user to push a button with each use of the filter to increase the usage count or any other manual activities that would be needed to trigger the PID to advance.
  • The PID cannot have a user-adjustable end point. For example, there cannot be user instructions indicating how to change the PID from one system capacity to another, i.e., 300 gallons to 500 gallons.
  • It must provide an effective means to warn the user that a filter change is necessary. Examples of an effective means include:
    — Stopping the flow of treated water
    — Reducing the flowrate of treated water by 75 percent or more
    — Sounding an alarm
    — A flashing light
    — Other obvious visual alarm indications

The main requirement for a PID is that it must activate between 80 and 110 percent (95 percent ± 15 percent) of the manufacturer’s rated capacity. There is a test described in NSF/ANSI 53 to make sure that the PID is activating within this acceptable range. Essentially, the filter system is operated normally for 16 hours per day and the volume of filtered water is carefully totalized and recorded at the point the filter change indicator activates. This point must then be within 80 to 110 percent of the manufacturer’s rated capacity.

In terms of operating normally, this means that plumbed-in systems have water flowing through them at 60-psi inlet pressure with a 50/50 on/off cycle, with a 20-minute cycle time or a lower cycle time if requested by the manufacturer. This means that the water is flowing half the time, with flow starting and stopping repeatedly throughout the test. For batch, or pour-through type systems that are not connected to plumbing, operating normally means that the system is simply used according to manufacturer’s instructions.

Regarding PIDs that indicate a filter change based on either volume or time, whichever comes first, obviously volume will come first when tested according to this procedure due to 16 hours of operation each day. So essentially, the test requirement is evaluating the accuracy of the volume indication but not the time indication.

Totalizing the volume of water FAQs
Manufacturers sometimes have questions regarding methods for totalizing water volume for a PID and whether these methods are acceptable. One of the most common questions is whether plumbed-in filter systems can use a timer connected to a dispensing valve that times the valve in the open position and uses an algorithm to calculate water volume. The answer to this question is that if the system passes the test described above, then the PID conforms to the requirements of the standard.

Another question can come up regarding batch pour-through type systems. It has been asked whether an automatic batch counter that converts batches to volumes and triggers the filter change indicator at the correct number of batches corresponding to the system capacity is an acceptable PID. The answer in this case is that the PID test is conducted according to manufacturer’s instructions. So the instructions must indicate that only complete batches are to be processed (no filling the unfiltered water hopper part way). And the batch counter must be automatic, because one of the criteria for a PID is that it must be automatic. So requiring the user to manually count batches disqualifies the device as a PID. Finally, the system must pass the test described above when the system is operated according to manufacturer’s instructions.

System maintenance is important
Industry professionals are all too aware of the importance of system maintenance. There are so many horror stories out there regarding filters and other replaceable components that have not been serviced in years. Without filter changes at the frequency recommended by the manufacturer, these systems will not perform properly and their benefit to the end user will be limited. Having a filter change indicator, or PID, is an important consideration when devising tools to help end users remember to maintain their systems correctly. Beyond the benefit to consumers, a PID can allow the manufacturer to make chemical contaminant reduction capacity claims 67-percent higher than the same system without a PID. These two reasons combine to provide a strong incentive for manufacturers to consider the added cost and engineering effort to include PIDs in their filtration systems.

About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: [email protected]


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