Hach 8450, Instrument Manual

Page: 56 / 66

Page 56
Resolving Calibration Problems
16116Maintenance.fm
Section 4
4.3.2 Timer Expires During Span Calibration
4.3.2.1 Eliminate Possible Mechanical Causes
4.3.2.2 Eliminate Possible Functional Causes
1. Remain in the >CAL >SPAN screen and observe the concentration
reading for one minute. If the reading appears to be increasing steadily by
0.02 ppm/minute or more (not settled), wait approximately 20 minutes and
recheck. Because the calibration is not started, the screen will time out
and return to either the Main or Check screen. After the 20 minutes have
expired, wave the magnet over the
F
key to bring up the Main screen, then
return to the >CAL >SPAN screen.
2. If the observed concentration is settled, but appears to be varying around
a reading by more than ± 0.02 ppm/minute, enter the RDG menu and
increase the damping to 10 seconds or more. Normally, this value will be
less than 60 seconds.
3. When the reading is settled and stable, verify that it is at least 0.02 ppm. If
the reading is at least 0.02 ppm, wave the magnet over the
E
key twice,
re-enter the span concentration, and wave
E
to start the calibration timer.
After one minute, wave the
E
key and observe the message: “SPAN CAL
COMPLETE.” If the message, “PROBE SIGNAL LOW, CAL ABORTED”
appears, proceed to step 4.
4. If after waiting 20 minutes the concentration has not settled to a stable
value of at least 0.02 ppm, or if you have observed the error message in
step 3:
a. Recheck the known concentration and pH levels using laboratory
methods.
b. Double check the probe wiring.
c. Reset the Gpot value to the factory default value of 478.
Note: In step d below, permit the probe to stabilize in deionized water for 30
minutes prior to starting the zero calibration. When complete, place the probe
Cause Solution
Probe temperature not settled.
See Probe Temperature Considerations
on page 35 .
Probe output signal unstable.
Clean probe and repeat conditioning.
Replace membrane if damaged or dirty.
Probe output is low.
Do procedure under Testing for Electrical
Isolation on page 54 .
Add electrolyte if insufficiently filled.
Replace clogged or dirty membrane.
Sample flow rate is out of range or
unstable.
Make certain flow rate is within the range
of 8 to 25 gph when using a flow cell.
Make certain flow rate is between
0.5 and 2.5 fps in an open channel.
pH of sample is unstable. Wait for pH of sample stream to stabilize.