Gas Analysers Questions and Answers

Answer: A

Carbon monoxide, carbon dioxide, methane and many other gases absorb at characteristic infrared wavelengths because their vibrations change the molecular dipole moment, and the analyser measures how much of a beam survives passage through the sample. Emission would require a hot source in the cell, refraction is not a compositional measurement, and scattering belongs to particulate monitors. The dependence on a changing dipole is also why oxygen, nitrogen and hydrogen are invisible to the technique, which is a limitation and occasionally a convenience.

Answer: B

A vibration only absorbs infrared if it changes the dipole moment, and a molecule of two identical atoms has no dipole to change, so oxygen, nitrogen, hydrogen and chlorine are transparent. Their absorption is absent rather than merely inconveniently placed, and they are chemically harmless to the optics. This is precisely why oxygen has to be measured by some other principle altogether, most often its paramagnetism or an electrochemical or zirconia cell.

Answer: C

Wavelength selection is done by an interference filter or by the absorption of the detector's own filling gas, rather than by dispersing the spectrum with a prism or grating. That makes the instrument simple, rugged and cheap enough for permanent field installation, at the cost of measuring only the gases it was built for. The term says nothing about the sample handling, the thermal design or the beam geometry, all of which are separate matters in the instrument.

Answer: D

A rotating blade interrupts the beam at a few hertz, turning a slowly drifting direct signal into an alternating one that can be amplified narrowly and rejected of drift and stray light. It is the same trick used throughout optical instrumentation. It does not handle the gas flow, is not a calibration shutter, and does nothing to the detector filling. Removing the modulation would leave amplifier drift indistinguishable from a change in gas concentration.

Answer: A

The detector chamber contains the very gas the analyser is built for, so it absorbs at exactly the wavelengths that gas absorbs and is blind to everything else. The absorbed energy heats the filling, the pressure rises and a diaphragm moves, which is sensed capacitively. This gas filled correlation is what makes the classical NDIR so selective without any dispersing element. Interference filters are used in solid state designs, cooling belongs to some semiconductor detectors, and no source emits a single frequency.

Answer: B

Passing a matched beam through a sealed cell of non-absorbing gas gives a reference against which source ageing, window fouling and detector drift largely cancel, since both beams suffer them equally. Some designs achieve the same end by switching one beam between two paths. It adds no range, performs no sample conditioning, and holds no calibration gas. Ratio measurement of this kind is the standard defence against slow drift in any optical analyser.

Answer: C

Water absorbs broadly through the infrared and overlaps the carbon monoxide band enough to give a false reading unless it is removed or corrected. The usual remedies are a chiller that condenses moisture out, a permeation dryer, or an optical or mathematical cross interference correction. Nitrogen and argon have no infrared absorption at all, and helium's thermal conductivity affects a katharometer rather than an infrared instrument. Water is the standard nuisance in almost every stack gas measurement.

Answer: D

Removing moisture cures two problems at once: the infrared interference disappears and the reading is placed on a dry basis, free of the varying dilution that changing humidity would cause. It also protects the optics from condensation. The source is outside the sample path, no reaction with carbon monoxide occurs at cell conditions, and thermal conductivity is not the measured property. Whether a result is on a wet or a dry basis must always be stated, because the two differ by the moisture fraction.

Answer: A

Absorbance is the product of absorptivity, path length and concentration, so a longer cell gives more signal for the same gas and the response is linear in absorbance rather than in transmitted intensity. That is why trace analysers use folded paths of several metres. Transmittance falls exponentially rather than linearly, path length matters a great deal, and no square law appears. Real analysers depart from strict linearity at high concentration and are linearised in the instrument.

Answer: B

Absorbance is proportional to path length, so doubling it doubles the reading:

\[ A \propto l \Rightarrow l = 2 \times 10 = 20 \]

giving 20 cm. Trace analysers exploit this by folding a path of several metres into a small cell with mirrors. The limit is set by the loss at each reflection and by the volume, which lengthens the response time because the cell takes longer to sweep out. Sensitivity and speed pull against each other here as they do throughout instrumentation.