5.18 Calibration and Adjustment

Calibration and adjustment are mandatory final assembly steps that ensure the product's functional parameters meet the specified tolerance limits. This process corrects accumulated manufacturing variation (tolerance stack-up) in components and assemblies. Failure to perform traceable calibration results in reduced accuracy, performance degradation, and non-compliance with system specifications.

5.18.1 Calibration Flow and Process Mandate

Calibration must be integrated into the final assembly flow, linking directly to programming and safety testing.

Flow Integration

Calibration must occur after the device is programmed and functional, but before the final safety gate:

Program – Functional Smoke – Calibration/Adjustment – Re-verify – Safety

Warm-up: Both the Unit Under Test (DUT) and the instruments must reach thermal steady state (typically 10 – 30 minutes) before measurement begins to prevent thermal drift from contaminating readings. Ambient temperature/RH must be captured and logged.

Instrumentation and Standards

The accuracy of the calibration process is entirely dependent on the test equipment used.

Traceability Mandate: All primary instruments (DMMs, sources, chambers) must be under a strict, scheduled calibration cycle and traceable (NIST/ISO 17025).
Test Accuracy Ratio (TAR): The measurement uncertainty of the reference instrument must be significantly smaller than the tolerance of the DUT (4:1 ratio or better).
Hookups: 4-wire (Kelvin) method is mandatory for low-ohm/low-voltage critical measurements to eliminate lead resistance error. Shielded leads are required for sensitive mV/RF measurements.

5.18.2 The Closed-Loop Adjustment Protocol

Calibration is a controlled, closed-loop sequence executed via automated test software, compensating for variance in specific domains.

Calibration Patterns (The Math)

The mathematical model for compensation must be kept simple and stable.

Offset-Only (Zero): Measure at 0 V or known null, store the offset.
Two-Point: Measure at low and high points; solve the linear equation (y = ~~m \cdot x~~mx + b) to find the gain (m) and offset (b).
Multi-Point: For highly non-linear sensors (e.g., thermistors), use three to nine points to fit a piecewise linear function or look-up table (LUT).

Adjustment Discipline

Measure (As-Found): Apply a known stimulus. The system must average N readings (e.g., 10–32) to reduce noise. Log the As-Found value.
Calculate & Adjust: Compute the constant; electronically write it to the dedicated NVM partition.
Verify (As-Left): Re-measure to confirm the final As-Left value is within spec and meets the guard band (center of spec, not the edge).
Limit the Wrench: For physical adjustments (trimpots), the adjustment must be minimal. After completion, the trimpot must be sealed with an approved compound (e.g., varnish) and marked with a paint dot to prevent vibrational drift.

5.18.3 Data Protection and Traceability

The calibration constants must be permanently protected against field erasure and linked to the unit's identity.

Data Storage and Security

NVM Partitioning: Calibration data must reside in a separate NVM partition from the application firmware. This prevents field updates from accidentally wiping the constants.
Protection: The calibration region must be write-protected and include a CRC/Hash calculation. The system must refuse to run if the calibration region's CRC fails.
Region Flags: Calibration data (constants, region flags) must match the physical label kit and safety test selection.

Logging and Audit Mandates

The complete set of data must be logged and linked to the unit's identity for the audit trail.

Mandatory Record: The MES log must be bound to the unit SN and include: Recipe ID, instrument cert numbers, ambient Temp/RH, As-Found error, As-Left value, and the calculated coefficients/LUT snapshot.
Retries: Any retries or anomalies (e.g., slow chamber ramp) must be captured in the final record.

Final Checklist

Mandate	Criteria	Verification Action
Traceability	Instruments in cal; 4:1 TAR (Test Accuracy Ratio) verified.	Log records the instrument Cert/ID and expiry date.
Process Flow	Calibration occurs after programming, before safety testing.	Warm-up timer enforced; ambient Temp/RH logged.
Data Protection	Final constants written to a separate, write-protected NVM partition.	Verification confirms the CRC/Hash of the cal region is correct.
Adjustment Discipline	As-Found and As-Left values are logged; adjustment targets the guard band center.	For physical trims, the trimpot is sealed after final adjustment.
Measurement Integrity	4-wire (Kelvin) method used for low-ohm/low-voltage critical measurements.	Measurement involves averaging N readings to eliminate noise.
SN Logging	All calibration coefficients and status are linked to the unit's Serial Number (SN).	Audit confirms the data record exists for compliance defense.