What is the formula for magnetic particle bath concentration?

Concentration = settled volume x 100 / sample volume (mL per 100 mL); Fluorescent particles: 0.1 to 0.4 mL per 100 mL; Non-fluorescent (visible): 1.2 to 2.4 mL per 100 mL; Carrier to add = bath volume x (concentration / target - 1); Settling time: 30 min for water-based suspensions, 60 min for petroleum distillate suspensions. Settling test result in mL per 100 mL against the 0.1 to 0.4 mL fluorescent and 1.2 to 2.4 mL visible acceptance bands, with the carrier volume needed to correct a rich bath.

What should I check before relying on the result?

Draw the sample from the flowing bath after the pump has been circulating long enough to fully agitate the tank. Settling time is 30 min for water-based suspensions and 60 min for petroleum distillate suspensions - reading a distillate bath at 30 min gives a falsely lean result. Demagnetise the sample before settling. Magnetised particles clump and read falsely rich. Read the settled solids only. Bands of different colour, flocculation, or an oil or dirt layer above the solids mean the bath is contaminated and must be replaced, not adjusted. Water-based baths also need their wetting agent, corrosion inhibitor and conductivity checked, and a water break test on a clean panel. Check concentration at the start of each shift and at the interval required by the governing code, and again after any addition. A rich bath is corrected by adding carrier, not by discarding bath. Verify against the…

Magnetic particle bath concentration

A wet bath is a suspension of magnetic particles in water or a light petroleum distillate. Concentration is what decides whether the bath works: too lean and there are not enough particles to build a visible indication over a fine leakage field; too rich and the particles form a general background that furs over the surface and buries the very indications they are meant to reveal. The margin is narrow, and it drifts through a shift as particles are carried off on parts and as carrier evaporates.

Concentration is measured by settling, not by weighing. A 100 mL sample drawn from the flowing, well-agitated bath is left undisturbed in a pear-shaped centrifuge tube and the settled solids are read off the graduated stem. Fluorescent particles are far smaller and are used at a much lower loading, so their band is 0.1 to 0.4 mL per 100 mL; non-fluorescent visible particles are coarser and run at 1.2 to 2.4 mL per 100 mL.

The settled column also tells you about condition, not just quantity. Bands of different colour, a fluffy or flocculated layer, or a clear layer of oil or dirt above the solids all indicate contamination, and the bath must be discarded rather than adjusted. Magnetised particles clump and settle high, giving a falsely rich reading, so the sample tube is passed through a demagnetiser or held away from magnetised parts before the test.

Correcting a rich bath means adding carrier, not throwing bath away. The dilution needed scales directly: to bring a bath of volume V from concentration C to target T, add V x (C/T - 1) of carrier. A lean bath needs concentrate added and re-tested; do not estimate it.

Worked example

Particle type	fluorescent
Carrier	water
Sample volume	100 mL
Settled solids volume	0.3 mL
Bath volume in the tank	20 L
Concentration	0.3 mL/100 mL
Minimum acceptable	0.1 mL/100 mL
Maximum acceptable	0.4 mL/100 mL
Mid-band target	0.25 mL/100 mL
Carrier to add	4 L
Settling time	30 min

A 100 mL sample of fluorescent bath in a water-based carrier settles to 0.3 mL after the 30 min settling period that water-based suspensions require (a petroleum distillate bath is left 60 min before reading), so the concentration is 0.3 x 100/100 = 0.30 mL per 100 mL. That is inside the 0.1 to 0.4 band, on the rich side of the 0.25 mid-band target. To bring a 20 L tank down to mid-band, add 20 x (0.30/0.25 - 1) = 20 x 0.2 = 4 L of carrier.

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