ECD DO82, Инструкция пользователя

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HYDRA-DS NO
3
-N Page 31
4.0.4 T
WO
P
OINT
C
ALIBRATION
The second point of a Two Point Calibration sets the slope of the senor, the mV per decade. The slope is calculated by
comparing the millivolt values and ppm values in the “
1NO3-N 1.00 ppm
” line to the values in the “
2NO3-N 100 ppm
”
line of the Buffer Menu. The concentration of the solution used for the second point should be at least 10 times higher
than the value used in the first point of the calibration. The recommended calibration standards for both the ammonium
ion and potassium ion calibrations are 1 ppm and 100 ppm. Perform a two point calibration whenever an electrode is
replaced.
After completing the single point calibration, see Section 4.0.3 above, rinse the sensor with distilled water and gently
dab it dry with a paper towel or soft tissue. Carefully cleaning the sensor prevents either in the calibration due to
carryover from the first solution. Place the sensor in the second solution, either the pH 4.01 buffer or one of the 100
ppm solutions and wait for the reading to stabilize before accepting the value.
4.0.5 C
HLORIDE
I
ON
C
OMPENSATION
Chloride ions have a positive interference on the Nitrate Ion Electrode, the nitrate reading is higher than the actual
value. The Chloride Ion Compensation, Cl comp, adjusts the measured nitrate ion concentration using the measured
chloride concentration and the interference/selectivity value, the Cl coef, and subtracts the resulting amount from the
Nitrate Ion Signal. The Cl coef value in the NO
3
-N Set Up screen allows the interference ratio to be adjusted as needed.
The default setting is Cl coef = .004 which is 250 chloride ions produce the same signal as 1 nitrate ion, Ks = 0.04 would
change the correction to 25:1. The 0.004 correction factor is good for most solutions with levels of chloride greater than
50 ppm.
Each Chloride Ion Electrode should have similar characteristics but adjustments are necessary when the chloride
electrode is replaced or when low levels of chloride are present in the solution. If the nitrate ion concentration is less
than expected when compared to a grab sample measurement then adjust the Cl coef value to a smaller value. Adjusting
from 0.004 > 0.002 for example would increase the chloride to nitrate ion ratio from 250:1 to 500:1 thereby slightly
increasing the displayed nitrate ion concentration by subtracting less in the compensation.
4.1 C
ALIBRATION
–
T80 T
RANSMITTER
The Model T80 transmitter provides three methods of calibration:
4.1.1 AUTO C
ALIBRATION
D
ESCRIPTION
AUTO calibration is the primary calibration method for all measurements. AUTO calibration automatically recognizes
the calibration solution the sensor is in, and proposes the actual temperature compensated value for acceptance. AUTO
calibration can be a single point or two point calibration. A single point calibration sets the zero point or offset value of
the sensor. The second calibration sets the slope or span of the sensor.
When the AUTO key is pressed, the transmitter displays the PV (Process Value) and the associated mV signal from the
sensor. When the reading has stabilized a calibration value is automatically proposed (i.e. 7.00 pH, 10 ppm Fluoride ion,
0.00 mg/L Dissolved Oxygen). The user is prompted to accept the proposed calibration value or enter and accept
another value. Once Cal 1 is accepted, the user is asked to continue to Cal 2 (YES/NO). If yes, then a second calibration
is proposed. When the sensor has stabilized in the second calibration solution, accept the value and the calibration is
complete.
At the end of each calibration, the Offset and Slope are displayed in the respective units: pH, mV, ppm, and mg/l.