Myron L 6Pfc and 4P - Manual (Page 24)

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VIII. CALIBRATION INTERVALS
There is no simple answer as to how often one should calibrate an
instrument. The Ultrameter II is designed to not require frequent
recalibration. The most common sources of error were eliminated in
the design, and there are no mechanical adjustments. Still, to ensure
specified accuracy, any instrument must be checked against chemical
standards occasionally.
A. Suggested Intervals
On the average, we expect calibration need only be checked monthly
for the Conductivity, RES or TDS functions. The pH (6Pfc) function
should be checked every 2 weeks to ensure accuracy. Measuring some
solutions will require more frequent intervals.
B. Calibration Tracking Records
To minimize your calibration effort, keep records. If adjustments you
are making are minimal for your application, you can check less often.
Changes in conductivity calibration should be recorded in percent.
Changes in pH calibration (6Pfc) are best recorded in pH units.
Calibration is purposely limited in the Ultrameter II to ±10% for the
conductivity cell, as any change beyond that indicates damage, not
drift. Likewise, calibration changes are limited to ±1 pH unit (6Pfc), as
any change beyond that indicates the end of the sensor’s lifetime and
replacement is recommended.
C. Conductivity, RES, TDS Practices to Maintain Calibration
1. Clean oily films or organic material from the cell electrodes
with foaming cleaner or mild acid. Do not scrub inside the cell.
2. Calibrate with solutions close to the measurements you make.
Readings are compensated for temperature based on the type
of solution. If you choose to measure tap water with a KCl
compensation, which is often done (ref. an Example of 2
different solution selections and the resulting compensation,
pg. 38), and you calibrate with 442 solution because it is
handy, the further away from 25°C you are, the more error you
have. Your records of calibration changes will reflect
temperature changes more than the instrument’s accuracy.
3. Rinse out the cell with pure water after taking measurements.
Allowing slow dissolving crystals to form in the cell
contaminates future samples.
4. For maximum accuracy, keep the pH sensor cap on tight so
that no fluid washes into the conductivity cell.