Fluorescent Bench Losing Sensitivity After Forty Parts
A wet horizontal fluorescent bench has processed about 40 production parts since the bath was made up. A settling test (100 mL, 60 minutes) reads 0.2 mL, inside the 0.1-0.4 mL acceptance range. A calibrated black-light meter reads 700 uW/cm2 at the inspection surface. A white-light meter reads 30 lx at the part surface. A master part with known EDM slots that previously produced crisp indications now shows faint, hazy indications over a green background glow covering the whole part. Last week a cleaner labeled 'fluorescent penetrant remover' was used at the bench and some rinse water was dumped into the particle tank. The Level II proposes adding more particles to brighten the indications.
1. List the deficiencies in this setup and identify the true root cause. Is adding particles the right response?
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Three separate deficiencies, and adding particles treats none of them:
- Black light at 700 uW/cm2 is below the 1000 uW/cm2 minimum used by ASTM E709/E1444 and by most purchaser specifications. Either the bulb is aging, the filter is dirty, the distance is wrong, or the meter is out of calibration.
- White light at 30 lx exceeds the 20 lx maximum permitted for fluorescent inspection; ambient light is washing out contrast.
- The settling test reads in range but only measures total settled solids by volume. It cannot distinguish active magnetic particles from inert dirt, rust, or fluorescent dye, so it is a weak indicator of bath health.
Root cause is contamination: a dye-bearing cleaner entered the tank, so the whole bath is now fluorescent. That produces the green background haze, coats the part, and masks the true indications. Adding particles increases background as fast as signal and makes it worse.
Corrective action: drain, clean and rinse the tank and recirculation lines, remake the bath with clean water and the specified particle concentration, replace or re-lamp the black light and verify with a second meter, and install light control (curtains or a darkened booth) to get below 20 lx. Re-qualify with the master part before returning to production, and prohibit cleaners and dye products at the bench.
2. Give a discriminating test for each of the three candidate causes of the loss of sensitivity.
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Design each test so it separates one cause from the others:
- Bath contamination: draw a sample, filter it through clean white filter paper, and view under UV in a darkened area. A uniform fluorescent stain over the paper (not discrete particles) confirms dissolved fluorescent contamination. A clean sample from a freshly made bath run in parallel as a control gives the comparison.
- Black light output: measure with a second, independently calibrated meter at the specified distance and at the filter face as well as at the part surface. A reading that is low at the surface but acceptable at the filter face points to geometry/distance; low at both points to lamp or filter degradation. Also verify the meter certificate date.
- Ambient light: measure with the booth lights on and off, and with the part in the actual inspection position rather than at the bench edge. A drop of 10 lx or more between on and off confirms the white light contribution.
Sequence them cheapest-first: white light and black light meters take minutes; the filter-paper test requires a bath sample. All three should be completed before any part is accepted, because a 700 uW/cm2 reading alone invalidates the inspection regardless of bath condition.
3. Write the daily and weekly process-control checks you would put on the technique sheet for this bench.
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Daily, before production:
- Black light intensity at the inspection surface, minimum 1000 uW/cm2, at the working distance.
- Ambient white light at the part, maximum 20 lx, with the booth closed.
- Master/reference part run to confirm the whole system (bath, light, technique) detects known slots.
- Visual check of the bath for sludge, oil film, or odor, and of the recirculation flow/agitation.
- Yoke or bench magnetizing current and lifting power where applicable.
Weekly or per batch, whichever is shorter:
- Settling test, 100 mL in a pear-shaped tube, 60 minutes; acceptable range 0.1-0.4 mL for the fluorescent bath, recorded with the tube identification.
- Particle concentration by the same settling method plus a visual comparison against a known-standard tube.
- Contamination check with clean filter paper under UV.
- Black-light meter calibration verification against the certificate due date; bulb/filter condition.
Record every check with date, time, initials, and the result, and define the action limit that stops production for each one. A check with no stop-work criterion is not process control.