Process Control and Performance Verification

Outcomes

After this chapter, the learner should be able to:

• separate equipment checks, media checks and overall system checks;
• explain type, batch and in-service control roles;
• select an appropriate field or performance indicator;
• use Hall-effect measurement correctly;
• plan an auditable control schedule; and
• respond to a failed check without testing production parts "to see what happens."

11.1 The three layers of process control

MT quality control is strongest when three different questions are answered:

1. Is each item functioning? Equipment checks address the yoke, bench, ammeter, timer, lamp, meters, agitation and safety systems.
2. Is the detection media still fit? Media checks address product identity, concentration or mixing, contamination, sensitivity, colour, fluorescence, carrier condition and temperature.
3. Does the assembled process reveal the expected condition? An overall system performance check challenges equipment, field, media, application and viewing together.

Passing one layer does not replace the others. A calibrated ammeter does not prove that degraded particles will form an indication. A reference block response does not prove every yoke placement on a complex component has the correct direction.

11.2 Type, batch and in-service testing

The ISO 9934 framework represented by the supplied source separates product control into:

• type testing: demonstrates that a product design is suitable for its intended use, usually performed by a competent external or manufacturer laboratory;
• batch testing: demonstrates that a manufactured batch conforms to its qualified product type; and
• in-service testing: demonstrates continued performance while the user stores, mixes and applies the product.

The Level 2 technician normally verifies required certification and performs or reviews in-service checks. The technician should not recreate a manufacturer's full type-test programme in the field.

11.3 Detection-media reference pieces

The supplied source describes two media-performance references.

Type 1 reference block: a permanently magnetised steel disc with coarse and fine natural cracks. The source describes nominal dimensions of 50 mm diameter, 10 mm thickness and a 10.5 mm centre hole. Fresh qualified media establishes a reference pattern or photograph. In-service indications are compared for length, definition, background and contamination.

Type 2 reference block: a self-contained bar with permanent magnets at each end and a central scale. Media produces indications extending from each end toward the lower-field centre. Combined indication length provides a comparative performance measure. The source describes scale end points corresponding to approximately +100 A/m and -100 A/m.

These descriptions preserve source coverage. Current block design, cleaning, storage and acceptance instructions must be verified against the applicable standard and manufacturer information.

11.4 Open-bath control

A recirculating wet bath changes with use. Evaporation, drag-out, contamination, microbial growth, carrier additions and particle settling can alter performance. Control may include:

• agitation and pump operation;
• appearance under the relevant viewing condition;
• reference-block performance;
• concentration or settled volume when required by product/procedure;
• contamination layer and fluorescence;
• water-break behaviour;
• pH, viscosity, corrosion and foam checks where applicable;
• carrier level and mixing additions; and
• discard/replacement records.

Settling-flask methods appear in older practices and some client requirements. The supplied source notes historical settled-volume ranges but also states that more recent product control may rely on manufacturer recommendations and performance checks. This draft deliberately does not convert those historical ranges into current limits.

Sealed aerosols do not need an open-tank settling check, but they still need identity, expiry, temperature, spray function and performance control.

11.5 Field adequacy

The source lists four broad ways to establish suitable surface magnetisation:

• demonstrate response from fine natural or artificial discontinuities at unfavourable locations;
• measure tangential field strength close to the surface;
• calculate field strength for defined current-flow methods; or
• use another technically established method.

The source links approximately 1 T surface flux density in some low-carbon, low-alloy steels with H near 2 kA/m. This is not universal. Lower-permeability material may require a different H, and excessive magnetisation can generate background that masks relevant response. Formal approval shall confirm the applicable target and method.

11.6 Hall-effect meter

A Hall-effect probe measures magnetic flux density through a sensing element. For tangential surface measurement, orient the probe as specified so the sensitive axis is correctly related to the field. A small angular error reduces the indicated component of the field. Measure while the magnetising force is applied when evaluating the applied field, and survey the least favourable locations rather than only the strongest point.

Good practice includes:

• zeroing away from magnetic influence;
• checking probe and instrument calibration;
• selecting AC, DC or peak-hold response appropriate to the field;
• keeping the sensor as close as practicable to the surface;
• controlling probe orientation and lift-off;
• recording location, direction and reading; and
• distinguishing applied field measurement from residual field measurement.

An eddy-current-based instrument may respond to the magnetisation state within material rather than the air field. It can support comparison on similar parts but does not by itself prove discontinuity sensitivity.

11.7 Field indicators and artificial-flaw shims

Field indicators use magnetic segments or thin foils interrupted by non-magnetic gaps. A suitable surface field creates leakage at those artificial features.

The source makes an important distinction: some strips show direction but are not accepted as proof that field strength meets a specified value. Use each device only for the role assigned by the controlling procedure.

11.8 Equipment and overall system pieces

The supplied source describes:

• an AC yoke lift block with a source-derived nominal 4.5 kg requirement;
• a standard bar approximately 375 mm long and 20 mm square, containing a small cross-drilled artificial condition near the surface, for longitudinal-field system checks; and
• a tool-steel ring with multiple drilled holes at increasing depths for current-flow or central-conductor bench performance checks.

These pieces provide comparative evidence only when their identity, condition, cleaning, magnetisation history and reference response are controlled.

11.9 Control schedule and failed checks

The approved procedure should assign each check a trigger: before first use, start of shift, change of batch, change of setup, after repair, periodic interval or suspected malfunction. Record the result, instrument, reference and disposition.

If a required check fails:

1. stop affected examinations;
2. quarantine or identify equipment/media as appropriate;
3. repeat the check only after eliminating obvious setup error;
4. correct or replace the failed element;
5. repeat the full relevant control sequence;
6. determine the last known valid check;
7. assess work completed since that point; and
8. retest or notify as required by the quality system.

11.10 Common traps

• Substituting a calibration sticker for a functional check.
• Using an indicator outside its intended field type.
• Comparing a dirty reference block with a clean reference photograph.
• Recording "pass" without the observed value or pattern.
• Repeating a failed check until one result happens to pass.
• Ignoring production examined since the last acceptable control.

Chapter summary

Process control is an evidence chain. Equipment, media and the overall system are checked for different reasons. Reference pieces and meters are only useful when their role, condition, orientation and acceptance basis are controlled. A failed check triggers containment and impact review, not hopeful continuation.