If your Nitrox blend reads correctly during calibration but begins to fluctuate during production, the problem may not be the blending system at all.
In many cases, what appears to be a percentage instability can be a sign of a worn or aging oxygen sensor inside the analyzer. These electrochemical cells degrade gradually over time. As performance declines, readings can drift, respond slowly, or become inconsistent under higher oxygen concentrations.
Before adjusting blending procedures or questioning mechanical components, it is important to consider the most common variable in the system: the sensor itself.
How Oxygen Sensors Change Over Time
Oxygen sensors inside analyzers work like small batteries. They generate an electrical signal based on how much oxygen they detect. Over time, the internal materials of the sensor are gradually consumed. As this happens, the sensor becomes weaker.
Over time, the sensor’s electrochemical components are gradually consumed, resulting in subtle shifts in output.
1. The Sensor Cannot Read High Oxygen Levels Accurately
As an oxygen sensor ages, it may no longer be able to generate enough electrical output to correctly represent higher oxygen concentrations.
At normal air levels, around 21 percent oxygen, the analyzer may still appear to function normally. It may even respond when briefly exposed to pure oxygen at the surface. This can create a false sense of reliability.
The problem becomes critical when measuring higher oxygen mixtures. An aging sensor may under-report the true oxygen percentage in the cylinder. This means a diver could believe they are breathing a moderate oxygen mix when the actual concentration is significantly higher. If the dive is then planned based on the incorrect reading, the diver may unknowingly exceed safe oxygen exposure limits.
2. It Passes Calibration but Fails in Real Use
When calibrating an analyzer in atmospheric air (21% oxygen) or in pure oxygen at surface pressure, the sensor is being tested at the lower end of its operating range.
An aging electrochemical sensor may still retain sufficient chemical activity to respond linearly within this limited range. It can calibrate correctly in air and may also respond appropriately in pure oxygen at surface pressure. Because calibration succeeds, the sensor might be assumed to be healthy.
The limitation appears at higher oxygen levels. As degradation progresses, the sensor’s electrical output may no longer increase proportionally beyond the calibration point. When exposed to richer oxygen mixtures, the signal can show an incorrect evaluation of the oxygen level.
3. The Reading Becomes Slow, Unstable, or Drifts
Worn sensors often stop responding quickly. Instead of locking onto a stable value, the display may fluctuate or slowly wander even when the gas mixture remains constant.
This happens because the internal chemical reaction has weakened. The electrical signal can no longer become accurate, making it harder for the analyzer to settle on a precise reading.
When Should You Replace the Oxygen Sensor?
If your oxygen analyzer keeps giving inaccurate or unstable readings, it may be time to replace the sensor.
Oxygen sensors are consumable components with a limited lifespan. Over time, the internal materials are gradually used up, which reduces the sensor’s ability to produce a strong and accurate electrical signal. As this happens, the sensor may no longer respond properly across the full range of oxygen levels. Rather than displaying a clear error, it may simply stop reading accurately at higher oxygen concentrations.
You should replace the sensor when:
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The analyzer passes calibration at atmospheric pressure, but does not behave as expected at higher oxygen partial pressures
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Readings are inconsistent when compared with another properly functioning analyzer under the same gas sample
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The sensor takes longer than usual to stabilize on a reading
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The cell no longer performs linearly across its expected range
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The sensor has reached or exceeded its recommended service life
Replacing a questionable sensor is often faster and safer than attempting to troubleshoot the entire blending system. If stability returns after installing a new sensor, the issue was likely component-related. If not, further system evaluation may be required.
Ready for a Reliable Oxygen Sensor?
If your oxygen analyzer begins to show unstable or inaccurate readings, replacing the sensor is often the simplest and safest solution. NRC International supplies high-quality oxygen sensors designed for stable output, consistent linearity, and dependable performance in professional Nitrox operations.
With decades of experience in Nitrox and rebreather technology, NRC delivers reliable, German-engineered components trusted by professional divers in more than 35 countries. Choosing the correct sensor ensures your analyzer performs as intended, without drift, delay, or uncertainty.
Find the perfect oxygen sensor that fits your oxygen analyzer!
Frequently Asked Questions
Is Nitrox Oxygen Percentage Instability always caused by blending errors?
Not really. In many cases, instability is caused by an aging or weakened oxygen sensor rather than the blending process itself. Before adjusting blending procedures, the analyzer should be evaluated.
Can an analyzer pass calibration and still be inaccurate?
Yes. Calibration in air only confirms performance at atmospheric partial pressure. An aging sensor may respond correctly at 21% oxygen yet fail to perform linearly across higher oxygen partial pressures.
Should production continue if readings seem unstable?
No. If the analyzer cannot provide a stable and repeatable reading, the gas mixture is not verified. Production should pause until the issue is resolved.
Is replacing the sensor usually the first step?
Not necessarily. If a reading seems inaccurate, first recalibrate the analyzer using fresh air and verify the result. If possible, cross-check with a second calibrated analyzer. If the sensor fails calibration, responds slowly, or cannot reach expected oxygen levels after verification, then replacement is the appropriate next step.
