Gas Analysers Questions and Answers

Answer: C

Oxygen has two unpaired electrons and is strongly paramagnetic, hundreds of times more so than any common gas, so it is drawn into a magnetic field while nitrogen and carbon dioxide are weakly repelled. That contrast is what makes a magnetic measurement almost specific for oxygen. It has no electric dipole and so no infrared absorption, and its thermal conductivity is unremarkable. Nitric oxide and nitrogen dioxide are the only common gases paramagnetic enough to interfere.

Answer: D

Two nitrogen filled glass spheres on a torsion suspension sit in a non-uniform magnetic field, and oxygen drawn into the strong field region pushes the dumbbell aside by an amount proportional to its concentration. A feedback coil usually restores it and the restoring current is the output. Hot wire methods belong to the katharometer and the pellistar, combustion belongs to a different class of analyser, and no ionisation is involved. The measurement is direct, highly linear and does not consume the oxygen.

Answer: A

Because no other common gas is appreciably paramagnetic, the measurement is close to specific, and being physical rather than chemical it neither consumes the sample nor wears out. Electrochemical cells are far cheaper but have a limited life, a zirconia probe is the one that works hot and in situ, and every analyser needs periodic zero and span checks. Longevity and freedom from interference are what justify the higher price in laboratory and medical service.

Answer: B

Above about 600 degrees, yttria stabilised zirconia conducts oxygen ions, so a difference in oxygen partial pressure between the two faces develops a logarithmic Nernst potential against a reference of air. It is the sensor in a car exhaust lambda probe and in most combustion control systems, because it can sit directly in the hot flue with no sample handling at all. Its weaknesses are the logarithmic scale, which loses resolution at high oxygen, and its response to unburnt fuel.

Answer: C

The probe is hot and catalytically active, so carbon monoxide and hydrocarbons reaching it burn on the electrode and locally deplete the oxygen, giving a reading below the true bulk value. In combustion control that is often useful, since it warns of incomplete burning, but for an accurate oxygen figure it is an error. Coating is a slow fouling failure rather than this effect, the sensor does not respond to hydrocarbons as such, and the reference side is sealed from the sample.

Answer: D

Oxygen diffusing through a membrane is reduced at the cathode, and the resulting current is proportional to the oxygen partial pressure. In a galvanic cell a lead or zinc anode is consumed, which is why such cells have a finite life measured in months and eventually simply stop. They are cheap, small and linear, which suits portable and safety instruments, but they are temperature sensitive and are poisoned by some acid gases. The consumable anode is real but is not where the signal is generated.

Answer: A

Both statements are true and the second is the reason. The cell has no off switch: as long as it sits in air, oxygen diffuses in and consumes the anode, so shelf life and service life run together and cells are supplied sealed in a foil pouch. Planning for replacement is part of owning the instrument. The contrast with a paramagnetic analyser, which has nothing to consume, is the clearest illustration of what the lower purchase price is buying.

Answer: B

A heated filament loses heat to the surrounding gas at a rate set by the gas thermal conductivity, so its temperature and hence its resistance changes with composition. Hydrogen and helium conduct far better than air and are measured very sensitively; gases close to air in conductivity are measured poorly. It is not a combustion measurement, which is the pellistor, and neither heat capacity nor viscosity is the governing property. Simplicity and universality are its virtues, selectivity its weakness.

Answer: C

Hydrogen has about seven times the thermal conductivity of nitrogen, so a hydrogen in nitrogen mixture gives a large, easily measured change and is the classic application, used for generator cooling and for hydrogen purity. Carbon monoxide and nitrogen are almost identical in conductivity, oxygen and argon differ only modestly, and nitrogen in air is a measurement with almost no contrast at all. The technique is only as good as the difference between the two components.

Answer: D

Thermal conductivity is a single number for the whole mixture, so a reading cannot distinguish more of one gas from some of another, and a third component makes the result ambiguous. In practice the technique is used where the background is fixed and only one component varies. The filament, the bridge and the reference cell impose no such limit themselves. This is the general difficulty of any non-selective analyser: it needs the matrix to be known in advance.