Understanding the Impact of Pressure on Electrochemical Oxygen Sensors

Understanding Pressure Effects on Electrochemical Oxygen Sensors

Many galvanic electrochemical oxygen sensors are designed to function effectively at typical atmospheric pressures. These sensors accurately measure the oxygen partial pressure in both ambient and flowing gases, accommodating a range of pressures as long as they remain stable and changes are introduced gradually. For instance, these sensors can operate reliably even in environments with higher pressures, such as within underwater diving bells where the atmospheric pressure can gradually increase to 30 atmospheres, ensuring the safety and comfort of those inside.

When managing pressure to reduce emissions released into the atmosphere, installing a backpressure regulator downstream of the oxygen sensor allows for controlled venting of sample gases to a flare stack at pressures above atmospheric levels. For example, if the flare stack operates at 7.5 psig, gradually adjusting the backpressure regulator to 8.0 psig ensures efficient venting of the sample from the analyzer. This approach helps optimize environmental management practices while maintaining operational effectiveness.

Many oxygen analyzers can safely handle inlet pressures of up to 2 bar g (maximum), with careful attention needed to avoid pressure shocks. The inlet pressure of the gas flowing into the analyzer is constrained by the pressure limits of components like flowmeters (up to 125 psig) or other sample conditioning devices such as H2S scrubbers (up to 30 psig). To ensure precise control and accuracy, the analyzer typically requires a regulator that can maintain pressures within the range of 5-30 psig or 20-50 psig (with a maximum of 100 psig), optimizing the flow rate to the oxygen sensor. This setup enhances performance and reliability in analytical operations.

¹ Calibrate the oxygen analyser as near to the temperature and pressure of the sample gas as possible.

² Set the regulator at the lowest pressure anticipated for the sample gas before the flow control valve is set.

Managing Pressure Pulses for Enhanced Signal Output

Pumps can occasionally generate pressure pulses that affect oxygen readings by increasing or decreasing the partial pressure of oxygen in a sample. This fluctuation stabilizes once the pulses cease. Using a reservoir (such as a filter housing) between the pump and sensor helps mitigate these pulses.

Diaphragm pumps, especially when paired with valves in the sample system handling span and zero gases, can momentarily increase pressure by 1.6 psig and introduce stagnant air, causing brief spikes in oxygen readings.

Recovery time is exponential due to oxygen gas forced into the sensor, which slowly dissolves and moves through the electrolyte before oxidation at the anode. Recommendations include using a high-quality pump, positioning the pump downstream of the sensor with a maximum draw of less than 8.5”Hg, and installing a bypass valve immediately before the sensor after the inlet valves.

These steps optimize sensor performance and minimize disruptions from pressure fluctuations in the sampling system.

Physical damage

Sudden pressure changes can physically damage sensors, potentially puncturing the diffusion membrane and cathode layer, leading to electrolyte leakage. This corrosive electrolyte (alkaline or acidic for XLT sensors) poses risks to both equipment and operators.

Sudden changes in pressure at the sensor can cause physical damage to the sensor despite the presence of needle valves, pressure regulator.

While electrolyte leaks into sample tubes or processes are rare and require unusually high pressures, please refer to Material Safety Data Sheets for handling precautions.

Influence on flow rates

Sample gas pressure affects flow rate: Ensure the sample vent pressure is lower than the inlet pressure for proper flow through the oxygen sensor housing. Ideally, the sample should be vented to atmosphere or into a tube at atmospheric pressure. When measuring flare gas, operate the unit up to 0.5 bar g, calibrating it accordingly. Use a back-pressure regulator to maintain constant pressure.

For emissions control, install a backpressure regulator downstream of the oxygen sensor to vent sample gas to a flare stack at pressures above atmospheric. Example: Increase the regulator from 7.5 psig to 8.0 psig for venting

A.I.I. Galvanic electrochemical oxygen sensors tolerate flow rate variations (1-5 SCFH), while A.I.I. Pico-Ion sensors require a flow controller and restrictor instead of a valve due to flow sensitivity. Excessive flow rates and ⅛” diameter tubing cause backpressure and erroneous high readings, potentially damaging the sensor.

Note: With ¼" diameter tubing, flow rates up to 50 lpm maintain accuracy.

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