Instruction manual

64
MODEL XMT-A-FF/FI SECTION 11.0
CALIBRATION - TOTAL CHLORINE
11.2 PROCEDURE — ZEROING THE SENSOR
1. Complete the startup sequence described in the SCS921A instruction manual.
2. Remove the reagent uptake tube from the reagent bottle and let it dangle in air. The peristaltic pump will sim-
ply pump air into the sample.
3. Let the system run until the sensor current is stable. The sensor current will drop rapidly at first and then grad-
ually reach a stable zero value. To monitor the sensor current, go to the main display and press  until the
input current screen appears. Note the units: nA is nanoamps, µA is microamps. Typical zero current for a free
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 (oc-
casionally 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.
4. Press MENU. The menu screen appears. Choose Calibrate.
5. Choose Chlorine.
6. Choose Zero.
7. The screen at left appears. The top line is the raw sensor current.
8. Once the reading is stable, the screen at left appears. Sensor zero is
complete and the transmitter has stored the zero current. The screen
remains until the operator presses MENU then EXIT to return to the
main display.
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.
9. 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.
10. This screen appears if the zero current is moderately high. To continue,
choose Yes. To repeat the zero step, choose No. See Section 15 for
troubleshooting.
Cal?
Chlorine
Temp
Cal?
InProcess
Zero
Live 200nA
Zeroing
Wait
Sensor Zero Fail
Current too high
Possible ZeroErr
Proceed? Yes
No
Calibrate
Sim. PV
Program Display
Live 0.000ppm
Sensor Zero Done