By Rick Andrew
Manufacturers often offer lines of similar products based on common technology. These product lines are structured in various ways. Some offer multiple systems that use the same replacement element. Others may be based around similar systems that vary in size. And still other product lines involve systems with varying numbers of similar treatment or filtration elements designed to operate in parallel or in series.
In all of these cases, it is possible to certify the entire product line by testing representative product family members, as opposed to testing each and every product. This constitutes product bracketing. Product bracketing is beneficial to all stakeholders with an interest in certification. Manufacturers benefit because their testing and certification costs are lower, as is their time to market. Retailers, dealers and consumers benefit because these lower costs can be passed on to them in the form of lower prices.
The key to product bracketing is development of a sound, scientifically defensible demonstration of conformance to the applicable standards for all non-tested products in the family. NSF pioneered the application of product bracketing and uses it today across every NSF/ANSI Standard. This column will examine various scenarios and the application of these principles for family bracketing decisions.
Requirements of the standards
NSF/ANSI Standard 44 contains product bracketing requirements for pressure drop and softening capacity testing of cation exchange water softeners. The standard requires that softeners must use the same control valve, brine system and distributor baskets, they must use equivalent cation exchange resin and vary only in terms of the size of the resin tank and the length of the distributor tube. Product families meeting these requirements can be bracketed together for pressure drop and softening capacity testing. The standard then contains strict requirements for variation in regeneration cycles, salt settings, flow rates and size between softener models bracketed together as a family. It also contains mathematical formulas used to determine pressure drop and capacity values for non-tested product family members.
Based on these requirements and typical product lines, it is not unusual for manufacturers to test only one of every 10 certified softeners. Obviously, this is a huge cost savings over testing every system. However, there are tradeoffs that must be considered. For example, certain non-tested models would likely have much better capacities than the calculated capacities based on the test models. They may have much deeper resin beds and so on. Experienced certifiers can add value to the process by pointing out these situations, allowing the manufacturer to quantify the tradeoffs in cost savings versus the opportunity to market the full potential of various models.
NSF/ANSI Standard 44 is a great example of family bracketing and guidelines based on fundamental scientific principles. It is convenient to detail this process in the standard because so many manufacturers produce families of softeners based on the same control valve.
However, many other family bracketing scenarios are less commonly encountered. Some of these scenarios may even be unique to specific manufacturers. Because of the endless array of potential family bracketing situations, many of them are not written into the standards. In these cases, product bracketing can still be applied as long as fundamental scientific principles are followed in making these decisions. Certification bodies that embark on such a product qualification process must have the necessary expertise and procedures by which this process is administered and document their rationale when establishing certification for non-tested products.
Case study – parallel flow systems
Manufacturers sometimes produce filtration systems that incorporate water treatment elements in parallel 
(See Figure 1). Manufacturers may offer similar systems that consist of one element; or two, three, or even four elements in parallel. These systems can be bracketed into a family group for contaminant reduction testing purposes. There are two basic criteria that must be met to allow this family bracketing:
· Each element in parallel must be identical. Non-identical elements in parallel could have different flow characteristics, which could cause disproportionate flow to occur through each element.
· The system manifold must be of sufficient bore size to have little pressure drop and ensure equivalent flow to each element.
When these criteria are met, it is possible to test a single element system and establish conformance for the additional multi-element, parallel flow systems through calculations to determine the flow rate and capacity for these systems. These calculations can be found in Figure 2.
Bracketing can vary by test type
When examining a product line that has been bracketed as a family to determine conservative test models for certification purposes, the selection of the specific model may vary depending on which requirement of certification is being considered. For example, the model determined to be the conservative test model for material extraction testing may not be the most conservative model to select for contaminant reduction testing, or the use of fundamental scientific principles may lead to selection of yet another model in the product line to be the most conservative model for structural integrity testing. All of these variables need to be taken into consideration when devising a plan that ensures the least amount of testing to demonstrate compliance for the largest number of products.
Case study – material extraction bracketing
The critical factor influencing the concentration of contaminants leaching from a given material during an extraction test conducted to the NSF/ANSI Drinking Water Treatment Unit (DWTU) Standards is the surface area to volume ratio. That is, the surface area of the material in contact with water compared to the water holding volume of the product.
Consider a product line of cation exchange resin tanks being evaluated for conformance to NSF/ANSI 44. The line varies in size only, with the materials of construction being identical for each model. Given the identical materials of construction, there is a scientific basis to group these tanks together as a family for evaluation purposes.
Application of fundamental scientific principles leads to the conclusion that as the tanks get larger, the surface area of the tank liners increases as a squared function, whereas the volume of the tanks increases as a cubic function. So, the larger the tank, the lower the surface area to volume ratio of the liner material. In order to test conservatively, it is necessary to test the product family member with the highest surface area to volume ratio. Following this logic, the smallest tank will have the highest surface area to volume ratio and is, therefore, the most conservative family member for material extraction testing.
Family bracketing is the ultimate win-win
The NSF/ANSI DWTU Standards are very rigorous in their requirements. Residential drinking water treatment systems conforming to these standards must undergo extensive testing to establish this conformance. Because of the number of different attributes and claims of the treatment systems and the highly sophisticated, data intensive tests required to properly evaluate them, the process of certifying residential drinking water treatment systems involves considerable cost to the manufacturer.
In order to minimize these testing costs and help ensure they are far outweighed by the associated value, family bracketing and selection of conservative test models can be of great advantage. While conformance to the standards is of utmost importance, fundamental scientific principles can be used to limit the amount of testing required to establish conformance of an entire family of products based on the testing of one or a few. The savings in testing costs can then be recognized by the manufacturer and passed on to consumers.
In order to take full advantage of the benefits of family bracketing, it is important that manufacturers work with their certifier to examine the entire family of products prior to initiating any testing. This is key to ensuring that the correct conservative test model is chosen. This analysis can be performed prior to any actual manufacturing of a new product line,enabling the manufacturer to factor in certification cost factors in their design considerations. NSF frequently engages in this type of analysis with manufacturers, such that test models and costs are known before prototypes even exit. The same is true where new models are being added to an already tested and certified family. The new models can be examined and analyzed to see if the previously test model(s) are conservative and appropriate for these new models, or if not, what additional testing will be required to add the new models.
Achieving the full value of certification, at the lowest testing costs possible is the ultimate win – win for manufacturers, dealers, retailers and consumers. It is a practice NSF has successfully applied for many years and for a large number of certified products.
About the author
Rick Andrew has been with NSF International for six years, working with certification of residential drinking water products. He has been the Technical Manager of the Drinking Water Treatment Units Program for three years. His previous experience was in the area of analytical and environmental chemistry consulting. Andrew has a bachelor’s degree in chemistry and an MBA from the University of Michigan. He can be reached at 1-800-NSF-MARK or email: [email protected].