A working pH system consists of a sensor with mounting hardware, interconnect cable and analyzer/transmitter. Good water-quality measurement is dependent on the health of the sensors used. A properly installed system that is free of ground loops should have a sensor that outputs correct sensor voltages.
A properly operating pH sensor will output close to 0 mV in pH 7.0 buffer (+/- 15 mV). If greater the voltage is greater than 30 mV in 7.0 buffer, replace the sensor.
Sensor Output Voltage (mV) is positive in low pH and negative in high pH.
pH Sensor Diagnostics
Here are the three diagnostics used in pH sensor troubleshooting:
1. Slope – Indicator of pH Sensor Health
- Good Slope: 54-59 mV/pH.
- Borderline Slope: 47-50 mV/pH range. Sensor is nearing end-of-life. Change sensor junction and fill solution. This will extend sensor life. Order a replacement sensor.
- Low Slope: 43-44 mV/pH. It’s time to replace the sensor
2.Glass Impedance – Indicator of pH Glass Health
- Normal Range: 50-200 MΩ on most pH sensors
- High GI = coating or dry sensor, clean sensor or soak
- Low GI = glass bulb wearing out, replace sensor soon
- Zero GI = cracked glass, replace sensor
3.Reference Impedance – Indicator of Reference Electrode Health
- Normal Range: < 20 KΩ
- High Impedance (> 140 KΩ) indicates that the Reference is coated. Clean and soak the sensor. Replace depleted electrolyte and coated junction. Otherwise, replace the sensor.
pH Sensor Poisoning
Aggressive solutions may chemically attack and deteriorate seals and cause reference electrolyte to become diluted, poisoned or washed away entirely. This is indicated when the Reference Offset is maxed-out: 60 mV. Time to replace the sensor. Check chemical compatibility with the senor supplier to make sure you are using the correct sensor. The process may have changed since the sensor was first installed.
pH Sensor Cleaning
- Alkaline or Scale Coating: Soak in 5% HCL solution or vinegar for 30-60 minutes.
- Acidic Coating: Soak in a weak caustic solution < 4% NaOH or baking soda.
- Oil, Grease, Organic Compounds: Clean in mild detergent.
- Use a small soft toothbrush to gently clean the complete sensor head.
- Use DI water to rinse the sensor.
- Soak the sensor in buffer or clean water for 5 minutes.
pH Sensor “Recovery”
Sometimes, pH sensor just need a good cleaning. Soaking a sensor overnight in 4 buffer will often prolong sensor life. Do not store sensors DI water or alkaline buffers.
pH Sensor Storage
When storing an unused sensor, store it upright, with the original shipping cap over the bulb. Inside the bulb, use a cotton ball moistened with 4 pH buffer. If sensor dries out in storage, soak it overnight in 4 pH buffer to “revive” it.