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Sign inMagnetic 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.
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
- 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.
Reference: ASME BPVC Section V (2023 Ed.), Article 7 - wet particle concentration: 0.1 to 0.4 mL in a 100 mL sample for fluorescent particles and 1.2 to 2.4 mL for non-fluorescent particles. ASTM E1444/E1444M-22 and ASTM E709-21 give the same bands and the settling procedure using a 100 mL pear-shaped centrifuge tube (ASTM D96 type). Settling time per ASME V T-764.1.2: approximately 60 min for petroleum distillate suspensions and 30 min for water-based suspensions.
