Instruction manual

66
MODEL XMT-A-FF/FI SECTION 11.0
CALIBRATION - TOTAL CHLORINE
11.4 DUAL SLOPE CALIBRATION
Figure 11-3 shows the principle of dual slope calibration. Between zero and concentration C1, the sensor re-
sponse is linear. When the concentration of chlorine becomes greater than C1, the response is non-linear. In spite
of the non-linearity, the sensor response between C1 and C2 can be approximated by a straight line.
Dual slope calibration is rarely needed. It is probably useful in fewer than 5% of applications.
1. Be sure the transmitter has been configured for dual slope
calibration. See Section 7.3.3.
2. Place the sensor in the zero solution. (see Section 10.1).
Be sure no air bubbles are trapped against the membrane.
The sensor current will drop rapidly at first and then grad-
ually reach a stable zero value. To monitor the sensor cur-
rent, go to the main display and press  until the input cur-
rent screen appears. Note the units: nA is nanoamps, µA is
microamps. Typical zero current for a total chlorine sensor
is between -10 and +30 nA.
A new sensor or a sensor in which the electrolyte solution
has been replaced may require several hours (occasional-
ly as long as overnight) to reach a minimum current. DO
NOT START THE ZERO ROUTINE UNTIL THE SENSOR
HAS BEEN IN ZERO SOLUTION FOR AT LEAST TWO
HOURS.
3. Press MENU. The main menu screen appears. Choose Calibrate.
4. Choose Chlorine.
5. Choose Zero.
6. The screen at left appears. The top line is the raw sensor current.
7. Once the reading is stable, the screen at left appears. Sensor zero is
complete and the transmitter has stored the zero current. The display
returns to the screen in step 5.
NOTE
Pressing ENTER during the zero step will cause the transmitter to
use the present sensor current as the zero current. If the sensor is
zeroed before the current has reach a minimum stable value, sub-
sequent readings will be in error.
8. This screen appears if the zero current is extremely high. See Section
15 for troubleshooting. To repeat the zero step, press EXIT and choose
Zero.
Process continued on following page.
FIGURE 11-3. Dual Slope Calibration
Cal?
Chlorine
Temp
Cal?
Zero
pt1 pt2
Live 200nA
Zeroing
Wait
Sensor Zero Fail
Current too high
Calibrate
Sim. PV
Program Display
Live 0.000ppm
Sensor Zero Done