72-900-06 Rev. F
Page 34 of 38
7.2.2 Sensor Storage
To maintain response, sensors should always remain wet. The preferred storage solution is
pH 4.0 buffer with saturated KCl added. Tap water will suffice for short term storage.
Do not soak in distilled water. The supplied storage boot or bottle will provide an ideal
chamber for lengthy storage
7.2.3 Sensor Cleaning
Sensors which are mechanically intact can often be restored to full response by the
following procedures:
1. Inorganic Scale Deposits. Dissolve the deposit by immersing the sensor first in 0.1M HCl
(hydrochloric acid), then in 0.1M NaOH (sodium hydroxide), and again in 0.1M HCl. Each
immersion should be for a 5-minute period.
2. Organic Oil or Grease Films. Wash sensor tip in a liquid detergent and water. If film is
known to be soluble in a particular organic solvent, wash with this solvent. Rinse sensor tip
in tap water.
If these procedures fail to rejuvenate the sensor, the problem is most likely a clogged liquid
junction. Cleaning the liquid junction involves heating a diluted KCl (Potassium Chloride)
solution to 60-80°C (139-176°F). Place sensor tip in the heated KCl solution for
approximately ten minutes. Allow the sensor to cool while immersed in the solution before
re-testing. If these steps fail to improve the sensor response, replace the sensor.
7.3 ORP Sensor Information
The ORP option provides monitoring and control with a control setpoint in millivolts (mV).
7.3.1 ORP Maintenance and Troubleshooting
ORP standard buffers of 100mV and 465mV are readily available, making it easy to
standardize ORP systems against buffers. Like pH sensors, ORP sensors are subjected to
coating and abrasion by the measured liquid and, in certain instances, are “poisoned” by
chemicals which may be present if the system goes out of control. To improve the reliability
of ORP measurement and control, the following is a means of testing sensors in solutions of
standard potential, which will determine if sensors are responding correctly or need
maintenance attention.
7.3.2 Testing ORP Sensor
Solution A: Use sufficient 100mV buffer to immerse sensors. Potential should be within +/-
10.
Solution B: Remove sensors and rinse thoroughly with water. Immerse sensors in 465mV
solution. There should be a rapid response.
The millivolt difference between the two solutions is theoretically 365mV. The absolute
values may shift upward or downward a few millivolts due to slight variations from
theoretical potential by the reference sensor.
If system potentials are correct, flush sensors with deionized water and measure the liquid
in question. If incorrect by more than 10 mV, sensors should be cleaned with aqua regia
(three volumes hydrochloric acid, one volume concentrated nitric acid.) Repeat above tests.
Once satisfactory readings are obtained, install sensors and make measurements of liquid
in question.
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