Electrochemical oxygen sensors are widely used to measure oxygen concentrations ranging from ambient air (20.9% O₂) down to trace parts-per-million (ppm) levels. While these sensors provide excellent sensitivity and reliability, proper calibration procedures are essential for achieving accurate measurements at low oxygen concentrations.

One common source of calibration error occurs when an operator transitions from ambient air to a low-ppm calibration gas without accounting for residual oxygen dissolved in the sensor electrolyte or remaining within the sample tubing and gas flow path. These residual oxygen sources can temporarily elevate the analyzer reading, creating an apparent calibration offset that does not necessarily reflect the actual oxygen concentration of the calibration gas.

Understanding these effects, allowing sufficient time for sensor stabilization, and ensuring the sampling system is adequately purged can help prevent unnecessary calibration adjustments and improve measurement accuracy.

Understanding the Calibration Offset

Using the example above:

• Certified "zero" nitrogen gas concentration: 1 ppm O₂
• Analyzer reading on zero gas: 45 ppm O₂
• Calculated residual oxygen offset: 44 ppm O₂

If the operator immediately introduces a certified 1,000 ppm oxygen calibration gas, this residual offset must be considered when performing the calibration.

Assuming the residual oxygen offset remains stable, the appropriate offset-adjusted calibration value would be:

1,000 ppm + 44 ppm = 1,044 ppm O₂

In this example, the analyzer should be calibrated using an offset-adjusted value of 1,044 ppm rather than simply adjusting the reading to 1,000 ppm.

Failing to account for the residual oxygen contribution can introduce an error into the analyzer's calibration, particularly when measuring oxygen concentrations significantly below the calibration point.

It is important to calculate the offset using the actual certified oxygen concentration of the zero gas rather than assuming the gas contains exactly zero oxygen.

Important: Verify That the Offset Is Stable

The offset-adjusted calibration example assumes that the residual oxygen contribution remains relatively stable during the calibration process.

If the analyzer indicates 45 ppm while exposed to a certified 1 ppm zero gas, the calculated offset is 44 ppm. However, if the reading continues to decrease as residual dissolved oxygen is consumed, the offset is changing and may no longer be 44 ppm when the 1,000 ppm calibration gas is introduced.

Applying an offset correction while the sensor is still stabilizing can introduce additional calibration errors.

For this reason, Southland Sensing recommends allowing the sensor to operate continuously on the process gas overnight following ambient-air calibration. Once the residual oxygen has largely dissipated, the operator can verify the analyzer's response using a certified ppm calibration gas and make adjustments only if necessary.

This approach is generally preferable to compensating for a changing residual oxygen offset.

Southland Sensing's Recommended Calibration Procedure

For new electrochemical oxygen sensors, or sensors that have recently been exposed to ambient air, Southland Sensing recommends the following procedure:

Step 1: Calibrate in Ambient Air

Expose the sensor to clean ambient air and perform the standard 20.9% oxygen calibration according to the analyzer's operating instructions.

Step 2: Introduce the Process Gas

Install the analyzer into the intended process and establish the recommended sample flow rate. Ensure that the sampling system is properly connected and free from leaks.

Step 3: Allow the Sensor to Stabilize Overnight

Maintain continuous sample flow overnight, allowing residual dissolved oxygen within the sensor electrolyte to be consumed.

During this period, the indicated oxygen concentration should gradually decrease as the sensor approaches equilibrium with the process gas.

Step 4: Verify Calibration the Following Morning

Introduce a certified ppm oxygen calibration gas, such as 1,000 ppm, and allow the analyzer reading to stabilize. (This is only needed if the user prefers to use a certified span gas. Note: ambient air is an excellent source to calibrate against as it is a known value.)

Compare the indicated concentration against the certified calibration gas concentration.

Step 5: Adjust Only if Necessary

If the analyzer indicates the correct oxygen concentration within the required measurement tolerance, no additional calibration is necessary.

If an adjustment is required, perform the appropriate calibration procedure after confirming that the sensor has stabilized and any remaining offset has been properly evaluated. Note: If the oxygen analyzer has been properly calibrated using ambient air (20.9% O₂), the reading obtained when introducing a certified SPAN calibration gas should be reasonably close to the expected concentration and within the analyzer’s published accuracy specifications, once the sensor has fully stabilized.

Why Overnight Stabilization Is Important

Electrochemical oxygen sensors do not necessarily transition immediately from 20.9% oxygen to single-digit ppm concentrations. Note: exposure to ambient air for 2 - 3 minutes, the sensor recovery might take 45 minutes to get back to 10 ppm and up to 4 hours to get below 1 ppm.

The time required for stabilization depends on several factors, including:

• Sensor design and electrolyte composition
• Duration of previous exposure to ambient air
• Sample gas flow rate
• Operating temperature
• Actual oxygen concentration of the sample gas

A sensor that initially indicates 45 ppm while exposed to a 1 ppm gas may continue decreasing over several hours as residual oxygen is consumed.

Attempting to force the analyzer to match the calibration gas concentration before stabilization can introduce unnecessary calibration adjustments.

Allowing the sensor to operate overnight on the process gas generally provides sufficient time for most of the residual oxygen contribution to dissipate, resulting in a more representative calibration under actual operating conditions.

Additional Considerations for Low-PPM Oxygen Measurements

While residual oxygen within the sensor electrolyte is a common cause of elevated readings following exposure to ambient air, it is not the only possible source.

Small leaks in the sampling system, oxygen permeation through unsuitable tubing, contaminated calibration gases, and improper sample flow or pressure can also affect measurement accuracy.

If the analyzer continues to indicate an elevated oxygen concentration after an extended stabilization period, the operator should inspect the sampling system and verify the calibration gas supply before making further adjustments.

For low-ppm oxygen applications, Southland Sensing recommends using stainless steel tubing and appropriate compression fittings wherever practical to minimize the possibility of atmospheric oxygen entering the sample stream.