Oxygen analyzer

01

Set the oxygen limits the analyzer must protect

An IChemE paper on inerting sets four levels: the limiting oxygen concentration (LOC) at operating temperature and pressure, the maximum permissible oxygen concentration (MPOC) used as the automatic shutdown level, a first alarm level and normal operating conditions. It is usual to subtract 2 percentage points from the LOC to give the MPOC, sometimes more depending on how fast oxygen can rise. Where the LOC is below 5% and the plant is continuously monitored, the plant should not operate above 60% of the LOC; without continuous monitoring the limit is 60%, or 40% if the LOC is below 5%. [1]

Linde's inerting white paper says the LOC must be determined by experiment and lists, with nitrogen as the inert gas, 6.9 vol.% for ethylene, 9.0% for benzene and 10.0% for hexane; LOC values with carbon dioxide can be up to 25% higher. [2]

Editorial calculation: with a 2-point margin, the MPOC would be 4.9% for ethylene, 7.0% for benzene and 8.0% for hexane under nitrogen. The analyzer range and accuracy must resolve the alarm and shutdown levels set below these values. [1] [2]

Give the supplier the gas or vapour being inerted, inert gas, LOC basis and test conditions, shutdown and alarm levels, range, background gas and contaminants, sample pressure and temperature, number of sample points, area classification and output signals.

02

Choose the sensor technology for the gas

Panametrics' white paper compares sensors used for purge and blanket applications: paramagnetic sensors are not consumed ("With proper care, the sensor will last for many years"), but liquid carry-over can damage them, and the sensor "typically is used for percent oxygen measurements"; galvanic fuel cells are lower cost and background-gas insensitive but are consumed ("Typical life time for the sensor is a couple of years"); non-depleting electrochemical sensors give stable, fast ppm or percent measurement where liquid carry-over is very unlikely. [3]

Zirconium oxide sensors are the fastest responding and measure from percent to sub-ppm, but they operate hot and combust hydrocarbons with available oxygen, reading falsely low; Panametrics limits them to gas where hydrocarbons are orders of magnitude below the oxygen, such as glove boxes or nitrogen before injection. [3]

The sample system must deliver a continuous flow without changing the sample, and remove condensed hydrocarbon liquids before the sensor. [3] The IChemE paper adds that the analyzer should preferably sample from multiple parts of the plant, at points not close to inert-gas inlets or outlets, alarm on lost or reduced sample flow, and not show a fault as a safe reading, for example a zero on power loss. [1]

03

Know the oxygen limits for entering an inerted vessel

An inerted vessel becomes a hazard to people once it is opened for entry. OSHA's permit-required confined spaces rule (29 CFR 1910.146) defines an oxygen-deficient atmosphere as less than 19.5 percent oxygen by volume and an oxygen-enriched atmosphere as more than 23.5 percent, and counts an oxygen concentration below 19.5 or above 23.5 percent, flammable gas above 10 percent of its lower flammable limit, or toxic concentrations above published exposure limits as a hazardous atmosphere. When testing for atmospheric hazards, the rule requires testing first for oxygen, then for combustible gases and vapours, then for toxic gases, and monitoring during entry as needed to confirm that acceptable entry conditions are maintained. [4]

The inerting limits above keep oxygen low enough to prevent combustion inside the process; the confined-space limits apply once people enter, so the same vessel may need an analyzer range covering both a few percent and normal air. [1] [2] [3]

Editorial calculation: normal air holds about 20.9% oxygen, so the 19.5% limit is a drop of 1.4 percentage points; an analyzer with ±1% of full-scale accuracy on a 0–25% range has ±0.25 percentage points uncertainty at that point.

If the vessel will be entered after inerting, ask for an analyzer range that covers both the process limit and the 19.5–23.5% entry band, and its accuracy at 19.5%.

04

Compare what suppliers offer

Suppliers linked to this page list gas oxygen analyzers; each card links to the page reviewed.

Chang AI lists electrochemical oxygen analyzers with other gas analyzers; Yongan lists zirconia oxygen analyzers with gas detection systems; BCST lists fixed and portable oxygen analyzers; AmcareMed and Canta list oxygen analyzers for oxygen generators and concentrators.

05

Trace the analyzer components

This is a functional reference built from the sources cited; it is not one maker's parts list.

06

Fill the specification fields with values for your process

An oxygen analyzer quotation needs values for your gas and limits. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the limits, gas and sample data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Process: [material; inert gas; LOC and test conditions]
Levels: [normal; alarm; shutdown]
Gas: [background; hydrocarbons; liquids; acids]
Sample: [points; pressure; temperature; flow]
Outputs: [signals; fail-safe state; flow alarm]
Site: [area classification; enclosure; power]

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