Polarographic Oxygen Sensor Maintenance

Polarographic Maintenance

The following sections discuss maintenance activities for Hamilton’s polarographic oxygen sensor products.

OxySens™

The OxySens dissolved oxygen sensor is maintenance-free. It is provided with a water-filled storage cap over the sensor tip. This cap must be removed before installation or calibration. If the sensor can no longer be calibrated, it must be replaced.

OxyFerm®

With the exception of the FDA type, the OxyFerm sensors are shipped dry (no electrolyte in the membrane cartridge). The membrane cartridge must be carefully unscrewed from the sensor body, pre-rinsed with the corresponding electrolyte, and then filled with 1.0–1.5 mL of that electrolyte. Care must be taken not to touch or break the cathode/ anode glass assembly. The filled membrane cartridge must be screwed tightly to the sensor body.

After connection to a measuring instrument (process converter, transmitter) the polarization voltage must be set to −670 mV ± 50 mV. After a maximum period of 2 hours, the sensor is ready for calibration.

During external sterilization, the OxyFerm sensor’s bayonet plug T82/D4 should be covered by a protective cap (available from Hamilton). If the plug is partially filled with water despite the protective cap, the plug should be rinsed with ethanol and dried afterward with a forced-air dryer (such as a hair dryer). No protection is necessary when using a VP 6 connection. An operational sensor check can be performed as follows:

Calibrate the polarized sensor in air and adjust the measuring instrument to 100%. Immerse the sensor in a nitrogen gas-filled bag. After the sensor is immersed for 1 minute in nitrogen, the instrument must show a value below 2% of the previous value in air. If the value in nitrogen is too high, the electrolyte might be exhausted or the membrane might be deformed or otherwise defective. In such a case, the electrolyte or the membrane cartridge and the electrolyte must be replaced.

If the sensor still does not calibrate well, i.e., the response is too sluggish or the zero current is too high, the cathode and the front end of the glass body must be cleaned with the polishing strip included in the membrane kit. After cleaning, rinse the glass body under a running water tap and dry carefully.

If the sensor problem persists, the cathode must be replaced. With the cathode, the membrane also must be exchanged to regain the full performance of the sensor. The cathode assembly is replaced as follows:

  1. Place the sensor in an upright position and carefully unscrew the membrane cartridge.
  2. Flush the replaceable cathode assembly with deionized water and then dry the metal parts. Do not touch anode and cathode!
  3. Hold the cathode assembly on the metal part (B) in front of the thread (A) and the sensor on the shaft (C) and pull apart. Do not turn!
  4. Check that all contacts are dry and clean.
  5. Replace the cathode assembly with a new one (blue for OxyFerm®, yellow for OxyGold® B, and black for OxyGold® G). Rotate the cathode assembly slowly and carefully until the correct position is found and then push the cathode assembly into the sensor shaft.
  6. Check the small O-ring (D) above the thread. Replace if damaged.
  7. Use the 1 mL plastic pipette of the membrane kit to pipette 1.0-1.5 mL of electrolyte into the new membrane cartridge. The pipette must not touch the membrane itself!
  8. Screw the cartridge onto the sensor shaft. Rinse away any spilled electrolyte with water.

OxyGold® G & B

OxyGold sensors are shipped dry (no electrolyte in the membrane cartridge). The membrane cartridge must be carefully unscrewed from the sensor body, pre-rinsed with the corresponding electrolyte (G or B), and then filled with 1.0–1.5 mL of that electrolyte. Care must be taken not to touch or break the cathode/anode glass assembly. The filled membrane capsule must be re-screwed tightly to the sensor body.

For the OxyGold G sensor, after connection to a measuring instrument (process converter, transmitter) the polarization voltage must be set to −670 mV ± 50 mV. After a maximum period of 2 hours, the sensor is ready for calibration.

For the OxyGold B sensor, the polarization voltage must be set to 0 mV. After a maximum period of 24 hours, the sensor is ready for calibration. If this polarization time is considered too long, the purchase of a polarization module is recommended.

Prior Article - Optical Oxygen Sensor Maintenance

Next Article - Storage and Cleaning of the Sensor

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