How are Electrochemical Oxygen Sensors Affected by Pressure?

The majority of galvanic electrochemical oxygen sensors operate at nominally atmospheric pressure. In ambient or flowing gas where the inlet pressure exceeds the outlet pressure, the electrochemical galvanic oxygen sensor actually measures the partial pressure of oxygen.

These sensors are accurate and can operate at elevated pressure as long as the pressure is constant and the adjustment is gradually applied. For example, pressure in an underwater diving bell’s ambient atmosphere can reach 30 atmospheres, which is slowly reached to protect the human inside.

A backpressure regulator installed downstream of the oxygen sensor allows the sample gas to be maintained at an elevated pressure and vented to a flare stack at a varying pressure above atmospheric pressure.

For example, if the pressure of the flare stack is 7.5 psig, the backpressure regulator should be increased gradually to 8.0 psig to vent the sample from the analyzer (see general rule (1) below). The inlet pressure to the oxygen analyzer can be as much as 2 bar g (maximum) but to prevent pressure shock, care must be taken.

The inlet pressure of a flowing gas stream to the oxygen analyzer is limited by the pressure rating of the flowmeter (125 psig) or other sample conditioning components (e.g. H2S scrubbers 30 psig) which are included in the gas path.

Usually, analyzer specifications for inlet pressure demand a regulator which generates 5 to 50 psig (with a maximum 100 psig) to create better accuracy from the valve controlling the flow rate to the oxygen sensor.

General Rules

(1) Calibrate the oxygen analyzer at the pressure and temperature of the sample gas.

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

Physical Damage

A sudden large change in pressure can result in physical damage to the sensor. For example, puncturing the diffusion membrane causing the electrolyte leak into the process. The electrolyte is extremely corrosive – either alkaline or acidic (XLT sensors) – which puts operators at risk and could also damage the pipework and contaminate to the process.

In order to mitigate any sudden pressure shocks, pressure relief valves, needle valves, or pressure regulators can be utilized to slowly ramp up and decrease the pressure in the sensor.

Even with a good sample system in place, it is still possible to damage a sensor by unforeseen blockages to the vent line.  Never place a finger over the vent as this could cause back pressure on the sensor and removing the finger quickly could cause a vacuum, tearing the diffusion barrier.

The electrolytes are corrosive, the handling of them is outlined in the Material Safety Data Sheets, should a sensor become damaged and leak.

Effect on Flow Rates

The sample gas pressure is influenced by the flow rate: the sample vent pressure has to be lower than the inlet pressure in order for sample gas to flow through the oxygen sensor housing.

Preferably, the sample should be vented to the atmosphere or into a vent line at atmospheric pressure. The analyzers can generally be operated at up to 0.5 bar g, however they should be calibrated at this pressure and a back-pressure regulator must be utilized to maintain a constant pressure.

Please note, with ¼ " diameter tubing flow rates of 50 l/hr are viable with accurate readings.

Pressure Pulses and Increased Signal Output

Occasionally, pumps can create pressure pulses that can decrease or increase the pressure, changing the partial pressure of oxygen and affecting the oxygen reading.

A reservoir (even a filter housing) between the sensor and pump can help to smooth out pulses. Pico-Ion sensors must be used in positive pressure applications. Note: 15 ” water column produces the recommended flow rate of 1 SCFH.

As well as diaphragm pumps, valves in the sample system (for example: sample/span/zero gas inlets) can create momentary increases in pressure of up to 1.6 psig and introduce air which has leaked in behind the valves, causing an upward spike in the oxygen reading.

Recommendations

(1) Source a pump of superior quality

(2) “Try” locating the pump, maximum draw less than 8.5 ” Hg, downstream of the sensor

This information has been sourced, reviewed and adapted from materials provided by Analytical Industries Inc.

For more information on this source, please visit Analytical Industries Inc.

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