There are three options.
1. Fuel cell devices, like the Delta F. These are sensitive and reliable. They require some periodic maint. Not very expensive. Better for medium and high O2 levels.
2. Hot cell sensors. Similar technology to auto O2 sensors. Have a small furnace that keeps the sensor at the correct elevated temp. These are very good for low O2 (10ppm and lower) since the signal increases as O2 decreases. They are a bit pricey.
3.Glow discharge. Very stable and reliable. Require more support equipment but little attention. For large sites (refinery) this is what I would look at. More expensive but very durable.
Of course there is a fourth, you could use a mass spectrometer (like an RGA). This would give you the option of measuring any gas within the mass range of the device. These are not outrageously expensive but you do have to maintain a delicate vacuum pump system.
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Plymouth Tube
