Oxygen gas analyser: where O₂ control is needed and how to choose

An oxygen analyser is chosen by answering three questions: where you measure (a drawn sample or directly in the process), which range you need (ppm or percent), and what else is in the sample. According to Cambridge Sensotec, a zirconia sensor suits ppm in inert gas, an electrochemical sensor suits samples with hydrogen, helium and hydrocarbons, and an optical sensor suits percent measurements with low drift.

Where oxygen control is needed

Oxygen is monitored in two different situations, and they need different instruments.

Process gas. Nitrogen or argon is fed into a chamber, furnace or vessel to displace oxygen. Cambridge Sensotec puts it this way: the key to nitrogen blanketing is to make sure the critical oxygen level is not exceeded. The manufacturer lists these nitrogen blanketing applications:

• modified atmosphere packaging;
• welding stainless steel and titanium;
• flow soldering and solder reflow ovens;
• semiconductor manufacturing;
• protecting flammable cargoes;
• avoiding oxidation of metals;
• additive manufacturing (metal 3D printing).

The Rapidox 1100 application list on the manufacturer's site also includes heat treatment, electrical switchgear, hydrogen production, seal integrity and medical gas analysis. A separate power-sector task, oxygen in a transformer gas blanket, is covered in Oxygen in the transformer headspace.

Workplace air. Nitrogen, argon and SF₆ displace oxygen in enclosed rooms. Under OSHA 29 CFR 1910.146:

O₂ in air, % by volume	Atmosphere under OSHA 1910.146
below 19.5	oxygen deficient
above 23.5	oxygen enriched

People are protected with gas detectors approved under the site's safety rules, not with a process analyser. Monitoring in rooms with SF₆ switchgear is described in Oxygen displacement in enclosed GIS rooms.

Types of oxygen sensor

Cambridge Sensotec uses three sensor types in Rapidox analysers.

Sensor	Strengths (manufacturer data)	Limitations	When to choose
Zirconia	Fast response, accuracy at ultra-low O₂ levels, works in vacuum and with pressurised gas, lifespan over 35,000 hours	Operates at a very high temperature, so less suited to hazardous areas	ppm O₂ in nitrogen or argon: furnaces, chambers, vacuum
Electrochemical	Suitable for samples with hydrocarbons and VOCs, performs well in helium and hydrogen	The manufacturer names zirconia as the main choice for ppm	Samples with flammable gases, hydrogen, helium; hazardous areas
Optical	Range 0.01–100 %, very low drift, life up to 10 years	More expensive than electrochemical	Percent measurements where infrequent calibration is wanted

The most common selection mistake is fitting a zirconia sensor where the sample contains hydrogen or hydrocarbons. For such samples the manufacturer recommends an electrochemical sensor.

Rapidox models and their ranges

Model	Sensor and O₂ range	Sampling	Outputs
Rapidox 1100	Zirconia 0.5 ppm – 30 %; electrochemical L 0.1 ppm – 1 %; electrochemical E 0.1–100 %; optical 0.01–100 %	Diaphragm pump 0–1.2 l/min or vacuum ejector; sample temperature up to 50 °C	0–10 V, 4–20 mA, RS232 (RS485 optional), Modbus RTU, Ethernet; two programmable alarm relays
Rapidox 1100 OPT-MAX	Optical + electrochemical, 0.1 ppm – 100 %	Sampling inside the instrument	0–10 V, 4–20 mA, RS232/Modbus
Rapidox 2100	Zirconia on a remote cable, 10⁻²⁰ ppm – 30 %; electrochemical 0–100 %	Sensor in the process, up to 650 °C; 2 m cable, up to 25 m with extensions; type K thermocouple 0–1250 °C	0–5 V, 4–20 mA, RS232 (RS485 optional), Modbus RTU, Ethernet; two alarm relays

Rapidox 1100 and 2100 data are from the manufacturer's pages. The dual-sensor Rapidox 1100 switches between sensors with solenoid valves: the optical sensor handles percent levels, the electrochemical one handles ppm.

How to choose an analyser, step by step

1. Define the task. Process gas (inerting, nitrogen blanketing, furnace) needs a process analyser. Breathing air for personnel needs a gas detector under the safety rules.
2. Define the range. Residual oxygen in inert gas is ppm; purging, gas mixtures and air are percent; if the process passes through both, use a dual-sensor instrument.
3. Check the sample composition. Hydrogen, helium, hydrocarbons, VOCs: electrochemical. Pure nitrogen or argon, vacuum, pressure: zirconia.
4. Decide where the sensor sits. A pumped sample (Rapidox 1100, sample temperature up to 50 °C) or a sensor directly in the hot zone (Rapidox 2100, up to 650 °C).
5. Check integration. Do you need analogue outputs, Modbus, alarm relays to control gas supply? Cambridge Sensotec notes that controlling nitrogen flow from Rapidox readings reduces nitrogen consumption.
6. Check whether people are at risk in the room. If inert gas can escape into the room, personnel protection is needed in addition to the process analyser.

Which instrument to choose

• General-purpose benchtop or portable analyser with sample draw: Rapidox 1100, zirconia for ppm in inert gas or electrochemical for samples with hydrogen and hydrocarbons.
• Sensor directly in a furnace, chamber or high-temperature zone: Rapidox 2100 with a remote zirconia sensor.

KEG TRK supplies Cambridge Sensotec Rapidox gas analysers in Kazakhstan. Price on request.

Sources

• Cambridge Sensotec. The Difference Between Zirconia and Electrochemical Oxygen Gas Sensors.
• Cambridge Sensotec. Rapidox 1100 Oxygen Analyser.
• Cambridge Sensotec. Rapidox 2100 Oxygen Analyser.
• Cambridge Sensotec. Nitrogen Blanketing in Industrial Applications.
• OSHA. 29 CFR 1910.146 Permit-required confined spaces.