Ph Troubleshooting - Hach 900 MAX Instrument Manual

pH Troubleshooting

Symptom Possible Cause
Meter continuously Open circuit in either
reads pH 14 or drifts glass or reference
above 14. electrode.
Interface is wired
wrong.
Temperature is
constant or incorrect.
Thermistor is open.
Electrode won't
Gain or offset error.
calibrate.
Very high impedance
in either glass or
reference electrode.
Slow response and/or
erratic readings.
Ground loop problem.
Temperature is
incorrect.
Cracked glass bulb.
No response to pH Short Circuit.
change.
High impedance
bridge.
8990apdx_tbl.fm
Solution
• Inspect the cable and connector of the faulty electrode for evidence of
a crushed or broken cable jacket or brittleness of the cable due to
exposure to heat. Discard the electrode if damage is present.
• Manipulate meter/electrode connections to check for intermittent
continuity. Replace if faulty.
• Inspect the bulb, making sure it is filled with solution. If not, shake
down (like a clinical thermometer) to displace air in the pH bulb. Retest.
• Inspect the bulb for signs of coating.
Check interface wiring.
Check interface wiring. Check for open at electrode RTD wire. Disconnect
to make measurement. (Should read approximately 100–110 ohms.)
• Ensure that solutions are fresh and labeled properly.
• Confirm that electrode and buffer temperatures have stabilized.
• Confirm that the wetting cap is removed.
• Check bulb for cracks or other damage.
• Confirm that interface wires are connected properly.
• Check interface connections for corrosion.
• Inspect the pH bulb for coating or clogging. If present, clean
thoroughly.
• Keep the electrode wet at all times. If it dries out, the impedance will
increase dramatically. To restore performance, soak in 0.1 N HCI
(Cat. No. 1481253) for 30 minutes and rinse well with distilled water.
• Chemical degradation of pH glass can occur rapidly in a high
temperature or high pH environment, yielding sluggish response. Low
temperature environments can double the impedance for every 8 °C
drop below 25°C.
• A high impedance electrode is extremely sensitive to electrical noise,
e.g., oscillating electrical fields generated by motors, generators or
discharges from electrical thermostats. A free-hanging cable swinging
due to air currents will also generate erratic signals.
• Manipulate electrode cable and connections to check for intermittent
continuity. Replace as necessary.
• Check to see if the ground wire is connected properly at the pre-amp
junction box.
• Check for continuity between the stainless steel lug on the electrode
and the ground wire at the interface.
See "Temperature" symptom in this table.
If the electrode gives readings between 5.8 and 6.2 pH in all solutions,
inspect the glass bulb. If damaged, discard.
If a constant reading of 7.0 pH or 0.0 mV is obtained, inspect the cable. If
no visible damage exists, remove the connector and test for a short circuit.
Replace if faulty.
Inspect the connector for moisture or corrosion. If wet, rinse well with
distilled water and dry thoroughly. Determine the cause of wetness and
correct it.
Appendix C
Troubleshooting and Error Messages
Page 131