KasperAero: Magnetic Capture Analysis Tools

Why Model Capture Efficiency?

Every capture-style debris sensor, from a two-prong chip detector to an NZMS, depends on capture efficiency: the share of passing ferrous debris that the magnet actually pulls out of the flow and holds. A sensor can only report the debris it catches.

Capture efficiency is usually measured on a test rig, one detector, one port and one oil temperature at a time. That is slow and expensive, and it says little about a different bore, flow rate or housing. KasperAero's tools predict capture from first principles, with no tuning, compare the predictions with measured data, and then answer the questions a rig test cannot answer quickly.

Two sizes of KasperAero NZMS debris sensor on the left, with a disc-style chip detector top right and a two-prong chip detector bottom right

NZMS sensors (left), disc-style chip detector (top right) and two-prong chip detector (bottom right).

The Shared Physics Core

All of the tools are built on the same magnet and particle physics, so a result from one carries straight into the next.

Exact Magnet Fields

The field of axially magnetized cylinders and rings is computed from a closed-form solution (Derby & Olbert, 2010), so there is no mesh to build and a full field map runs about 100× faster than a surface-charge model. Every magnet is modeled at the temperature of the fluid around it, using its grade's published temperature coefficients. For Alnico the tool finds the magnet's real open-circuit operating point instead of assuming the catalog remanence.

Force on a Steel Particle

Each chip is treated as a soft-steel sphere. The pull is F = V·M(|B|)·∇|B|, with magnetization capped at saturation. Below saturation the force follows the gradient of B², not B itself.

In plain terms, a steel particle in a perfectly uniform field feels no pull at all, however strong the field is. It only lines up with it. The pull comes from how quickly the field changes across the particle. A datasheet or a gaussmeter reports field strength at one point, which says nothing about how steeply that field falls away. A magnet with a high surface reading can still have a weak, short-reaching pull, and that pull fades very quickly with distance. That is why the field strength printed on a datasheet, or read with a gaussmeter, does not predict capture.

Magnet Grades Modeled

The materials library holds remanence, coercivity, recoil permeability and temperature coefficients for each grade, taken from manufacturer datasheets:

Neodymium (NdFeB): N35, N42, N52, N38SH, N42SH and N42UH
Samarium cobalt (SmCo): SmCo5 Grade 18, and Sm₂Co₁₇ Grades 26 and 32
Cast Alnico: Alnico 2 (isotropic), Alnico 5, Alnico 5cc (columnar), Alnico 6, Alnico 8B, Alnico 8H, Alnico 8HE and Alnico 9

Finite Elements Where Needed

Parts with no closed-form field, such as the steel pole pieces of a disc-style chip detector or a steel sensor housing, are solved with axisymmetric finite-element and finite-volume models using saturating B–H curves.

Real Fluid Properties

Oil and fuel viscosity and density are modeled against temperature, for example MIL-PRF-23699 turbine oil through ASTM D341. Viscosity sets how fast a particle can be pulled through the fluid, so it drives capture as strongly as the magnet does.

Tool 1: Magnetic Drift Speed and Capture Volume

The first tool maps the magnetic drift speed around a device: the speed at which the magnet pulls a reference steel particle through still oil. Where that speed is higher than the speed of the oil is a first-cut capture zone. From that map the tool reports reach, capture volume (how much fluid each device "owns"), and capture volume per unit of magnet volume.

The tool also checks each magnet for demagnetization. Both the risk and the safe operating temperature depend on the magnet's shape as well as its grade, so a catalog rating alone is not enough. The tool computes the permeance coefficient point by point inside the magnet, finds where the operating field would reach the knee of the demagnetization curve, and reports the maximum operating temperature before irreversible loss begins. It also reports how much permanent loss a stray reverse field would cause. The Alnico 5 rods of a two-prong chip detector, for example, lose about 1% of their strength permanently after a stray field of only 1 mT, about what a small refrigerator magnet produces a short distance away.

Side-section contour maps of magnetic drift speed for a 100 micron steel particle around an NZMS ring magnet, a two-prong Alnico 5 chip detector and a disc-style SmCo chip detector, all at the same scale

Magnetic drift speed of a 100 µm steel particle around an NZMS, a two-prong chip detector and a disc-style chip detector, all mounted in the same wall and drawn at the same scale. Inside the orange contour the particle is pulled in faster than 100 mm/s.

Bar chart of capture volume where the magnetic pull beats 100 millimeters per second: NZMS 0.0946 cubic inches, two-prong 0.0250 cubic inches, disc-style 0.0936 cubic inches

Measured this way, the NZMS has about 3.8× the capture volume of a representative Alnico 5 two-prong chip detector, about the same as a disc-style detector, and its pull reaches furthest beyond its own surface. The two-prong detector reads the highest on a gaussmeter, yet owns the least fluid.

Tool 2: The Pipe Capture Horizon

Drift speed tells you how hard the magnet pulls, but not where the fluid is actually going. In the second tool, we add the flow. The fluid moves through a pipe in fully developed laminar flow, and anything that stands out from the wall pushes it aside. Each particle then follows the full equation of motion: magnetic force, drag, added mass, and gravity minus buoyancy, worked out step by step along its path.

Particles are released from a grid of start points across the inlet, and each one is either caught on the detector or carried past. The boundary between the two is the capture horizon. Weighting the caught area by the local flow gives capture efficiency in the same terms a rig test measures.

Three-dimensional particle paths in a pipe showing the capture horizon for an NZMS, a two-prong chip detector with rods across the flow, and a disc-style chip detector

Paths of the outermost captured particles. Together they form the capture horizon, and their start points outline the capture footprint on the inlet plane.

Capture footprints looking downstream in a 1.25 inch pipe: NZMS 37.7 percent, two-prong in line 18.9 percent, two-prong across flow 21.2 percent, disc-style 40.8 percent flow-weighted capture efficiency

Capture footprints at the reference flow condition. Every 250 µm particle released inside the blue area is caught.

Tied to a Measured Test Condition

Nothing in the physics is tuned. There are no correction factors on the magnet, the drag or the capture rule. What a lab report does not tell you is the exact flow the detector saw in the test cavity, so the tool picks a reference flow condition instead. In a 1.25 in bore it finds the oil speed (0.137 m/s, laminar) at which the two-prong detector captures 20%, the middle of the range an independent laboratory measured on a production detector in hot oil.

The NZMS and the disc-style detector are then run in exactly the same pipe, oil and flow, with no adjustment of any kind. Their results, 37.7% for the NZMS (about 1.9× the two-prong detector) and 40.8% for the disc, are predictions, not fits. The tool also reports the bore, flow speed and temperature at which each device would match the chip detector.

Sweeps and Self-Checks

Sweeps cover flow speed, bore, temperature, chip size up to 1 mm, mounting position (bottom, side or top of the pipe), capture rule and obstruction model.

Chip metallurgy is an input too. The steel's permeability, saturation magnetization and density are set per alloy, so a bearing steel and a gear steel can each be run.

Built-in checks compare the fast field against the exact solution and confirm that the start-point grid has converged.

Line chart of capture efficiency against oil temperature from minus 40 to 150 degrees Celsius for the NZMS, the two-prong chip detector and the disc-style chip detector

Temperature changes both the oil and the magnet. Drift speed scales as one over viscosity, so in cold, thick oil every particle is pulled in more slowly and more are carried past. The magnets change too: each one is modeled at the oil temperature with its grade's temperature coefficient. Neodymium weakens as it heats, so hot oil costs the NZMS magnet about a fifth of its pull at 100–120°C, while Alnico and samarium cobalt lose only a few percent. Viscosity is still the bigger effect. The NZMS matches the hot-oil two-prong result in oil as cold as about 35°C, and below freezing every magnetic device loses most of its capture.

Tool 3: Capture in a Customer's Housing

Real housings are not straight pipes. They have jets, recirculation zones and dead legs that a pipe model cannot show. For these, KasperAero runs a compiled three-dimensional flow solver directly on the customer's CAD geometry: a D3Q19 lattice Boltzmann method with a WALE large-eddy turbulence model, using interpolated bounce-back walls so curved bores keep their true diameter.

Isometric 3D view of the fuel passages in a customer fuel housing, drawn see-through, with time-mean streamlines and flow arrows running from the inlet across the NZMS sensing face and down to the outlet

The fuel path through a customer fuel housing with an NZMS installed, seen from above. Streamlines of the time-mean flow, colored by speed, show the fuel crossing the sensing face (green) and swirling as it turns down to the outlet.

Debris particles are tracked through the time-resolved flow with the same equation of motion as the pipe tool. A wall-contact rule lets a particle slide or stick under magnetic pull, gravity, drag and friction, so the model reports not only how much debris is captured but how much of it lands on the sensing face, where it is measured.

Particle paths for 100, 250 and 1000 micron steel debris through a customer fuel housing, showing which particles land on the NZMS sensing face, which land on the side of the sensor and which are carried on

Paths of 100 µm, 250 µm and 1 mm steel particles through the same housing. Dark blue paths end on the sensing face, light blue paths on the side of the sensor. KasperAero analysis, not test data.

Cases Run

Fluid temperature, sensing face position, bore and fitting-size sensitivity, turbulence-model choice, grid refinement, and laminar versus turbulent inlet conditions.

Validation

The flow solver is checked against laminar tube flow, the particle tracker against the pipe tool, and the voxelized geometry against the original CAD model.

The result is a direct answer to a practical question: where should the sensing face sit in this specific part to put the most debris on it?

Tool 4: NZMS in Ferrous Housings

The last tool models the sensor rather than capture. A steel port or housing bends the magnet's field, and a careless installation could read as debris that is not there. The tool keeps the exact magnet field, solves the steel's response, and checks how the bore size, the sensor's position in the port and the temperature would affect the reading. The result shows, before a part is machined, whether a steel installation would produce a false debris reading.

Side-section field maps of the NZMS ring magnet in a non-magnetic housing and in a low-carbon steel housing whose bore ends halfway along the magnet, showing field lines from the rear of the magnet bending into the steel wall

The same NZMS in a non-magnetic housing (left) and a low-carbon steel housing (right), with the steel bore ending halfway along the magnet as in a typical installation. The steel pulls part of the magnet's flux into the housing wall.

Why KasperAero Built Its Own Tools

Every NZMS installation raises the same questions. How much debris will the sensor catch in this port? Where should the face sit? Will the steel around it change the reading? How does it compare with the chip detector it replaces? We needed answers in hours, not weeks, and in a form we could check against test data.

The general-purpose route exists, but it is slow. A magnet model in a tool such as CST Studio or ANSYS Maxwell has to be exported and mapped onto the flow mesh of a CFD code such as ANSYS Fluent. The magnetic force then has to be written in as user code, and the particle tracking set up and checked on top of that. Every new housing, temperature or sensor position means repeating much of that chain. So we built tools around the one problem we solve every day, and we use them for the customers we design sensors for.

Rig Testing

Pros

Measured capture on real hardware
The industry reference, as in SAE AIR1828

Cons

Needs specialty hardware (a flow loop, heated oil, seeded debris) and built prototypes before you learn anything
Capture is statistical, so each condition takes multiple trials, and results scatter from run to run
Subject to human error in seeding, collecting and weighing debris
One device, port and temperature per test, and the rig port rarely matches the real installation
Every design change means building and testing again

General Multiphysics Software

Pros

Can model almost any geometry and physics
Widely used and well documented

Cons

Separate magnetic and CFD packages to license and link
Custom code for the magnetic force on each particle
Fine meshes around small magnets and gaps, with long run times per case
Specialist time to set up, and nothing tying the result to a measured capture efficiency

KasperAero Capture Analysis

Pros

Built for magnetic debris sensors and chip detectors
Exact magnet fields, so no magnetic mesh to build or converge
Fast enough to sweep temperature, flow, sensor position and steel in one pass
Run on your own CAD geometry and fluid
Checked against analytic solutions and compared with test data

Cons

A model, not a measurement. Critical results should still be confirmed on a rig
Focused on ferrous capture, not general-purpose simulation

For you, that means faster iteration and less risk. Sensor placement and housing questions get answered before hardware is built, the rig time that remains goes to confirming a design rather than searching for one, and you see how an NZMS will perform in your system before you commit to it.

Read the Full White Paper

KasperAero NZMS debris sensors beside a disc-style chip detector and a two-prong chip detector

NZMS vs Traditional Chip Detectors (KR_WP#261023)

The tools on this page were developed for KasperAero's white paper comparing the NZMS with two-prong and disc-style chip detectors. Its 41 pages cover magnetic strength, capture in fluid flow, magnet reliability and integration, with full methods, validation and appendices.

The white paper is available on request. Contact KasperAero for a copy, or to discuss a capture analysis for your own system.

Request the White Paper »