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.