Desiccant Dryer Dew Point Calibration and Karl Fischer Titration Verification
Dew point monitoring alone fails to guarantee dry resin; coulometric Karl Fischer titration per ISO 15512 Method B is mandatory to prevent hydrolytic scission.
Drift Baseline Subtraction
Meaning
Calculation methods in coulometric analysis compensate for the continuous background moisture entering a titration cell during a test run. In polymer moisture analysis, drift baseline subtraction involves deducting the ambient water leakage rate from the total measured water to find the true moisture content of the polymer. This ensures that the instrument does not report false high moisture values for low moisture resins.
The calculation is performed automatically by the instrument processor before displaying the final ppm value.
Leakage Calibration
Atmospheric humidity inevitably enters the titration chamber through joints and seals at a slow but steady rate. Applying drift baseline subtraction compensates for this constant ingress by measuring the drift rate immediately before sample introduction. This background value is then projected across the duration of the sample run and subtracted from the total titration charge.
Without this adjustment, trace moisture measurements in engineering plastics would be highly inaccurate.
Run Validation
Low drift values are necessary for the subtraction algorithm to remain mathematically valid. A sudden spike in drift during a run suggests a physical leak or depleted desiccant rather than a change in sample moisture. In such cases, the drift baseline subtraction becomes unreliable, leading to inconsistent moisture readings.
Molders must reject analytical runs that exhibit drift baselines exceeding ten micrograms of water per minute.
Technical Procedure
Technicians must allow the titration cell to reach a stable baseline before starting any test. This stable state is typically defined as a drift below two micrograms per minute. Once achieved, the system locks in the drift rate.