Filtering cartridges are very common products used in many industrial processes to clarify/decontaminate a broad variety of liquids (beverage, pharmaceutics, chemicals, paints, varnishes, blood, drinking water,…) and on as many types of machines (aircrafts, engines, boats, fluid power systems, road and off-road vehicles, machine tools,…).

If efficient, all filtering cartridges, whatever their type, clog, what means their differential pressure increases and/or flowrate decays. Then they cannot fulfil their function correctly and need being replaced. This creates a huge market of renewable products with plenty of suppliers offering very diverse cartridges.
 
Criteria to choose a cartridge and its supplier rather than another one strongly depend on its application. But in all cases, the key criterion is filtration efficiency. another one possibly being retention capacity.
 
Filtration efficiency guarantees the quality of the filtered liquid. It is specified/expressed by a percentage of efficiency in percent or a filtration ratio (e.g. ß) at a given particle size or by a rating, i.e. a particle size supposed to correspond to a given, often not claimed, efficiency.
 
Since the efficiency measurement method, equipment and products directly impact the values of efficiency, many professional sectors, including filter cartridge manufacturers and their end users agree on standard methods, often a compromise between technical and cost requirements, such as ASTM, NFPA, SAE or ISO standards.
 
All standard procedures allow variations of different details, e.g. accuracy of measuring instruments, variations in test conditions, type of equipment, validation criteria, etc. This means that performances claimed according to any standard by any manufacturer may not be comparable to those claimed by a competitor for the same application.
 
Thus only independent third party testing centre results can really be trusted when evaluating competing products.
 
An independent qualification campaign performed on after market filters sold in specialised stores to fulfil a famous car manufacturer’s specification has shown major differences in filtration efficiency, retention capacity and sales prices.
 
Figure 1 allows comparison of initial filtration efficiencies vs. particle sizes of the six filters, i.e. the one measured at initial stage of running with a clean media, without clogging it.
 
Figure 2. shows how their efficiencies at a particle size of 25 µm(c) have evolved with time, or with the amount of contaminant captured by the filters. Due to this capture, all six filters have plugged but at quite different speeds. 
 
Since the car manufacturer’s specification requires changing the filter at a differential pressure of 1600 hPa (1.6 bar) it appears from figure 3, the curve of differential pressure as a function of the mass of contaminant captured by the filter, that filter E has to be changed nearly twice earlier than filter B, however 30% cheaper.
 
Since the filtration efficiency of a filtering media, thus of a filtering cartridge, can significantly change, becoming either better or worth, with time and degree of clogging (fig 2), it is common practice to only report the overall filtration efficiency curve, that is the average efficiency measured all along the life of the cartridge. This is shown on figure 4 : filter F has an average filtration rating at 50% efficiency of 16 µm(c) whereas, at opposite side, filter B, more than 2 times cheaper, has an average rating of 27 µm(c).
 
By combining prices with all data relative to instantaneous efficiency, overall efficiency and life, the classification from better to worth leeds to E>F>D>C>B>A with a price ration of 1 to 2.2. Such unbiased test results, only available at an independent test centre, are really useful for car manufacturers’ purchasing departments willing to extend their supply sources by reducing.their purchase costs while guaranteeing full system (engine and car) operation on the largest period of time, thus giving the market an image of undisputable reliability.

Figure 1 : Graph initial filtration efficiency 
Figure 1 : Graph initial filtration efficiency

Figure 2 : Graph of efficiency at 25µm(c) function of mass of contaminant retained
Figure 2 : Graph of efficiency at 25µm(c) function of mass of contaminant retained
 

Figure 3 : Graph clogging curve in function of contaminant retained

Figure 3 : Graph clogging curve in function of contaminant retained

Figure 4 : Graph of global filtration efficiency vs particle size
Figure 4 : Graph of global filtration efficiency vs particle size

Such an approach becomes everyday more required with the appearance on US market of low if not very low cost replaceable filters, often copied by unqualified manufacturers which claim performance measured in uncontrolled conditions if not measured at all.

An independent test centre is the most efficient and reliable friend of national manufacturers, manufacturing /assembling workshops and workers since it allows knowing precisely how performances compare and what filters really comply with customer’s, e.g. car manufacturer or any large end user of cartridge filters specifications and requirements.