Myron L ARH1 User Manual
Page 8
8.
Replace bottom cap plug securely to maintain water resistance.
The pH ZERO Calibration procedure is now complete. You may
continue
with pH Gain Calibration or stop and replace
storage solution &
pH cap.
b. pH Gain Calibration
IMPORTANT: Always calibrate or verify your ARH1 with a pH 7 buffer
solution before adjusting the gain with acid or base buffers, i.e., 4 and/or
10, etc. The pH gain calibration is performed in the same manner as the
ZERO. For maximum accuracy use a buffer value closest to instrument’s
normal area of use, i.e., if you normally measure acidic solutions, use “4”
buffer.
1.
Rinse the sensor well 3 times with acid or base buffer solution.
2.
Refill sensor well again with same buffer solution.
3.
Press
key. If reading is acceptable, end procedure. If
not, continue.
4.
Remove cap plug labeled GAIN CAL on bottom of Instrument.
5.
Refill sensor well again with same buffer solution.
6.
While pressing the
, adjust GAIN CAL Control with
finger until reading agrees with buffer solution.
7.
If the instrument will be used to read both acids and bases,
repeat steps 1 and 6 using opposite buffer solution.
8.
If reading is different by more than is acceptable, split the
difference with the previous setting. (If it is not possible to
adjust Gain, it is an indication of bad buffers or a deteriorating
or damaged pH sensor.)
9.
Replace bottom cap plug securely to maintain water resistance.
The pH GAIN Calibration procedure is now complete.
VII.
CALIBRATION INTERVALS
There is no simple answer as to how often one should calibrate an
instrument. The ARH1 is designed to not require frequent recalibration.
The most common sources of error were eliminated in the design, and
there are simple electromechanical adjustments. Still, to ensure specified
accuracy, any instrument has to be checked against chemical standards
occasionally.
A. Suggested Intervals
On the average, we expect calibration need only be checked monthly for
the Conductivity 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 ARH1 to ±8% for the conductivity
cell because more than that indicates damage, not drift. Likewise, pH
calibration changes are limited to ±1 pH unit because more than that
indicates the end of the sensor lifetime, and it should be replaced.
C. Conductivity or 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. Temperature
Compensation, pg. 19), 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 making measurements.
Allowing slow dissolving crystals to form in the cell contaminates
future samples.
4.
For maximum accuracy, keep the pH cell cover on tight so no fluid
washes into the conductivity cell.
12
13
pH
pH