Hach 8450, Instrument Manual

Page: 55 / 66

Section 4
Page 55
16116Maintenance.fm
Resolving Calibration Problems
4.3.1 Timer Expires During Zero Calibration
4.3.1.1 Eliminate Possible Mechanical Causes
4.3.1.2 Eliminate Possible Functional Causes
1. Remain in the >CAL >ZERO screen and observe the concentration
reading for one minute. If the reading appears to be decreasing 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 Menu
screen, then return to the >CAL >ZERO 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 the displayed concentration
is lower than the last value entered during span calibration (if known). If it
is higher, this may be an indication of the presence of contaminated zero
water, or an improper earlier calibration. If the zero water is known to be
chlorine free, then attempt to re-zero the instrument. Do not bypass the
stability check.
4. If the timer expires again, navigate the cursor to the SYS menu and reset
the GPOT value to the factory default value of 478. Return to the >CAL
>ZERO screen and start the zero calibration. After one minute (timer =
9:00), wave the magnet over the
E
key to bypass the zero calibration and
observe “ZERO CAL COMPLETE.” You must now SPAN CALIBRATE the
instrument.
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 high.
Do procedure under Testing for Electrical
Isolation on page 54 .
Probe is touching sample container.
Move probe so that it does not touch the
container.