Troubleshooting 2-Electrode Conductivity Sensors

Cooling tower sensors come in either 2 electrode or 4 electrode versions. The 2-electrode sensors are used on the model 140,1575e, and newer Nexsys controllers. They were also used on the old 100 series (101,111,151,161) and some pre-2000 series controllers. The 4-electrode sensors were used on the old 2000 and 2000E series of controllers. They are also used on the Nexsys when in a high fouling environment.

Sensors must be cleaned on a monthly basis (see Tech Tips 2 for more information).

The 2-electrode sensor operates by passing a small millivolt AC current across the sensors. The controller then computes the conductivity based on the reading. The sensor tips are made from a porous conductive graphite rod. If the sensor tips are broken, worn, or missing, the sensor will cease to operate efficiently. The 4-electrode sensor does much the same thing, but the current flows in a cross pattern to compensate for fouling.

Typical Problems

• Broken tips – when one or both tips are broken off, the sensor can no longer measure conductivity correctly. Detritus in the flow can cause the tip to fracture. The sensor must be replaced at this time.
• Uneven tips- When one tip is normal, and the other is shorter, the sensor cannot measure conductivity correctly. The sensor must be replaced at this time.
• Plating – Electromagnetic plating is suspect when material is removed from one sensor tip and deposited on the other. As one tip shrinks, the other grows. This is typically caused by a flow of DC current in the water. This can be caused by a bad ground, another piece of equipment on the same circuit, or even by a bad coil in the armature of the recirc pump. Other causes are a ground being connected to the feedwater pipe (or any pipe) of the tower. In this case, replacing the sensor is a temporary fix. Unless the current issue is resolved, the new sensor will eventually fail also. Consult an electrician to determine the origin of the stray current.
• Rounded tips – Rounded tips can be caused by too much flow in the sample stream. The minimum flow is 1GPM, and the maximum flow is 5GPM. Any higher flow rate can damage components in the flow switch plumbing and cause premature wear to the sensor causing it to fail.

Alarms:

Two alarms that can be caused by the sensors are OPEN TC and SHORTED TC. Inside the 2-electrode sensor there is a small thermistor embedded in the sensor body. It changes value with a change in temperature allowing the controller to compensate conductivity reading based on water temperature. “OPEN TC” means that the thermistor has failed with an open circuit. “SHORTED TC” means that the thermistor has failed by shorting out. Both alarms require sensor replacement.

As always, if you have any other questions regarding conductivity problems, please call our Tech Support line at 800-228-0839. We’d be happy to help.