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pH / ORP Arc intelligent sensors include advanced diagnostics that can be used to troubleshoot and diagnose problems with the measurement. The messages fall into two classifications – Warnings and Errors.


These messages are normally considered less critical. They can be ignored however the error will continue to be present until corrective action occurs. A good example of a warning is a failed calibration.


These messages are critical. The sensor may be reading erratically or have damage that needs to be addressed. Corrective action should be immediate. A good example of an error is glass resistance too low due to cracked membrane glass.

pH / ORP Arc Diagnostics Table

Fault MessageTypeNotes & Recommendations
PMC1 calibration recommended Warning Perform a new calibration to ensure accurate measurement
PMC1 last calibration not successful Warning The prior calibration had too much deviation from the expected value for an accurate calibration. Try recalibration with fresh buffers. Make sure the temperature is stable and the sensor is in the liquid solution during the full time.
Reading failure Error This error occurs when one of the following errors listed below is present.
Glass resistance too high (pH only) Error This error could indicate a non-conductive coating on the glass membrane (example - fats or oils). Try cleaning with IPA followed by Hamilton cleaning solution.
Glass resistance too low (pH only) Error Short circuit. This is normally an indication of a crack in the in the membrane glass or measurement electrode. The sensor will be fixed on a value close to 7pH and not respond to buffers. Sensor replacement required.
Electrical Potential too high (ORP only) Error The ORP sensor output exceeds +1500 mV. This value is beyond the range typically generated by most chemical reactions and may be an indicator of process induces static electrical noise.
Electrical Potential too low (ORP only) Error The ORP sensor output is below -1500 mV. This value is beyond the range typically generated by most chemical reactions and may be an indicator of process induces static electrical noise.
Reference electrode resistance too high Error Diaphragm may be clogged. This may occur due to protein precipitation. Follow the cleaning suggestions at this link.
Reference electrode resistance too low Error Short Circuit is occurring at the reference electrode. This may be due to contamination within the internal electrolyte. Consult Hamilton Technical Support for advice on alternate sensor designs.
Temperature sensor defective Error The internal temperature sensor has failed. Sensor replacement required.
Sensor failure Error Sensor quality has dropped < 15%. Clean sensor and perform new calibration. If sensor does not recover then sensor replacement may be required.
Internal communication error Error Electronics failure. Consult Hamilton Technical Support.

Diagnostic Notifications

Arc sensors have several methods to alert users when faults occur.

NAMUR – If the 4 to 20mA output is used, the current signal can be configured following NAMUR standards. When a fault occurs the mA output will shift to a value below 3.5mA or above 22mA. Since this value is outside of the normal control range of 4 to 20 mA it is recognized by the process control system as notification of a problem. NAMUR signaling is configurable using ArcAir software. Dedicated mA output values can set individually for either warnings or errors.

Digital outputs – A warning or error can be output digitally through the Modbus signal or via Bluetooth. The actual fault message can be viewed in ArcAir software.

Visual indication – If the Wi 1G or Wi 2G Bluetooth Adapter is used with the Arc sensor then a visible LED indicator light will change color depending on the type of fault. The color codes follow a basic traffic light pattern as shown below.

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