Myron L PS9TK - Manual (Page 40)

36
viii. caLibRaTiOn inTERvaLS
There is no simple answer as to how often one should calibrate
an instrument. The P
OOL
P
RO
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, MIN/SALT or TDS functions. The pH 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 are best recorded in pH units.
Calibration is purposely limited in the P
OOL
P
RO
to ±10% for the
conductivity cell, as any change beyond that indicates damage, not drift.
Likewise, calibration changes are limited to ±1 pH unit, as any change
beyond that indicates the end of the sensor’s lifetime and replacement
is recommended.
C. Conductivity, MIN/SALT & 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. 55), 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.