Hanna Instruments HI 8000 Series - Manual (Page 57)

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6 CALIBRATION PROCEDURES
6.1 SENSOR CALIBRATION PROCEDURE
The purpose of calibrating the Sensor’s used in conjunction with the Fertigation
Controller is to compensate and adjust the output of the sensors to align with the
software of the controller. As time passes the sensors change output slightly due
to wear and age making calibration necessary for optimum performance. Moni-
toring probe performance with known standards and buffers easily identifies the
need for calibration. The calibration procedure is performed using panel S 54
(CALIBRATE SENSOR) and to each analog sensor a channel number is as-
signed that is the index for panel. For each sensor chosen for calibration two
measurements with two different known standards are used. For pH you can use
7.7 pH buffer and 4.0 / 10.0 pH buffer, depending on the type of measurement
(acid or alkaline). For EC you can use 0 mS (sensor removed from the solution)
and 5 mS buffer. For each of the 2 stages of calibration, the calibration panel
displays the actual reading and a reference input. The buffer value is entered in
the reference input. Prior to calibration rinse the pH and EC sensors with pure
water to insure a clean probe and avoid cross contamination. Place the sensor
in the first standard or buffer and wait for stabilization. For pH calibration stir
gently the buffer with the pH sensor. For EC calibration tap gently the sensor and
stir the buffer to insure that no air bubbles are trapped inside the sensor sleeve.
Once the reading is stable press ENTER to advance to stage 2. Rinse again and
place the electrode in the second standard, wait for stabilization and then press
ENTER. The calibration is complete and the panel will display “done”.
For best results the calibration buffers (standards) should be considerably differ-
ent (e.g. pH 7.01 and pH 4.01 buffer) but not excessively close to the limits of the
sensor range. After the calibration is complete (both calibration stages complete)
rinse and reinsert the probe into the 2 standards to insure the accuracy of cali-
bration. If the probe does not read correctly this may indicate that the same
solution (or nearly the same) was used in both stages of calibration or improper
values were supplied as a reference or the probe is malfunctioning.
Regarding solar radiation calibration, the last digit of the entered reference value
is considered 0 due to the 10 W/m
2 resolution of the reading.