How to Read a Filter Cartridge Flow Curve.
Read flow and differential pressure together. Check the test conditions, identify the element basis and use the curve to ask better sizing questions.
By FiltruneOn this page
A flow curve connects the flow through a filter with the pressure lost across it under stated conditions. It helps you assess resistance at a working flow. Read the axes, conditions and element basis before using the line to compare products.
Start with Flow and Differential Pressure
The horizontal axis commonly shows liquid flow, while the vertical axis shows differential pressure, written ΔP. Differential pressure is the difference between the upstream and downstream pressure at the measurement points. It is not the same quantity as the pressure rating of the housing.
Check the units before comparing two graphs. Flow may be expressed in litres per minute, cubic metres per hour or gallons per minute; pressure may appear in bar, millibar, kPa or psi. Where gallons are used, confirm which gallon definition applies. Keep every calculation on a consistent unit basis.
Check What the Curve Represents
Read the small print around the graph as part of the graph. A curve for a clean element in water at a reference temperature does not describe every liquid, every length or the full operating life. Manufacturers may supply length and viscosity factors for specific products. Follow the instructions attached to that product rather than applying one universal correction. [1]
| Check | Why it matters |
|---|---|
| Liquid & temperature | They establish the reference fluid condition and viscosity. |
| Element length & quantity | A single element and a complete multi-element assembly have different flow bases. |
| Clean or loaded element | A clean-element curve is a starting condition, not a record of service life. |
| Element or complete housing | Identify which components and pressure losses are included. |
Read an Operating Point
Choose the required flow on the horizontal axis. Move vertically to the selected curve, then horizontally to the pressure axis. The resulting ΔP is the value shown by that curve at that flow and under its reference conditions. Read inside the plotted range; extending a line beyond its data is an additional assumption.
Use the slider or arrow keys in this teaching example. At 20 L/min, Curve A reads 0.132 bar and Curve B reads 0.244 bar. Those are authored example values for learning the reading method, not measured ratings for an available cartridge. The lines do not identify micron grades or compare two actual products.
The graph lets you compare resistance at a common flow. It does not establish retention efficiency, chemical compatibility or a service interval. In a real selection, take those requirements from the applicable specification and operating duty rather than inferring them from the height of the line.
Distinguish Element Flow from System Flow
A graph labelled per element must be read using the flow assigned to an element. As a first bookkeeping step, a total flow of 60 L/min shared equally by three parallel elements corresponds to 20 L/min each. Check that the actual assembly supports the intended distribution; equal sharing is an assumption to verify, not a guarantee from a cartridge graph.
The pressure loss of an installed assembly also depends on the housing and flow path. An element-only curve should not be relabelled as total system pressure loss. Use the housing information and the actual arrangement when checking the available pressure allowance.
Reference conditions
Water, 20 °C1One 10-inch element2Clean-element ΔP3
Confirm total flow, element quantity, liquid conditions and housing arrangement.
- 1Fluid condition
Read the stated liquid and temperature before transferring a result to another duty.
- 2Flow basis
Use per-element flow where the chart is supplied on that basis.
- 3Included resistance
Check housing and other system losses separately when they are excluded from the curve.
Keep Viscosity and Loading in Context
Some product literature provides a viscosity-based pressure-drop correction. Its applicability belongs to that product and method. A factor supplied for one media construction or liquid regime is not automatically valid for another. Ask for assistance where the liquid, temperature or operating range differs from the published basis. [1, 2]
Loading is a different issue. A clean-element flow curve alone does not tell you how quickly contamination will accumulate, how long an element will last or the correct changeout limit. Use actual flow, temperature and differential-pressure records alongside the selected product’s service guidance. Comparing readings at different flow conditions can hide the reason for a change.
Use the Curve in a Product Comparison
Make the comparison on a shared basis: the same duty flow, relevant fluid conditions and the correct element configuration. Compare retention requirements first, then examine the flow resistance of suitable candidates. A lower clean pressure drop is useful information, but it is not a complete quality ranking.
Bring these questions together with the actual housing, dimensions, seals and liquid requirements. The aim is an operating point supported by the right data, rather than a flow figure copied from a graph without its conditions.
