Gas Analysers Questions and Answers

Answer: C

The gas diffuses to a working electrode where it is oxidised or reduced, and the resulting current is proportional to concentration; carbon monoxide, hydrogen sulphide, chlorine and many others each have a cell tuned by electrode material and electrolyte. Catalytic heat is the pellistor, resistance change is the metal oxide semiconductor sensor, and infrared is a separate technique. These cells are compact and sensitive at parts per million but have a limited life and are cross sensitive to related gases.

Answer: D

Cross sensitivity is the response, usually expressed as a percentage, that another gas produces in a sensor intended for a different one: a carbon monoxide cell may read a substantial fraction of any hydrogen present. It is stated in the data sheet for the likely interferents and must be considered when choosing a sensor for a particular atmosphere. Temperature coefficient, unit to unit variation and zero drift are all separate specifications, each with its own line in the same document.

Answer: A

A sealed lamp emits photons of a fixed energy, commonly 10.6 electronvolts, and any molecule whose ionisation potential is below that is ionised and the current collected. Aromatics and many solvents respond strongly while methane, at 12.6 electronvolts, does not, which makes the detector a good volatile organic monitor and a poor total hydrocarbon one. Flame ionisation, electron capture and thermionic detectors all work by the other mechanisms listed and belong to different instruments.

Answer: B

Ionisation is a threshold process, so a molecule needing 12.6 electronvolts cannot be ionised by a 10.6 electronvolt lamp however intense. Fitting a higher energy lamp extends the range but such lamps are shorter lived and less stable. Molecular size is not the criterion, nothing is destroyed, and methane has four carbon hydrogen bonds. The threshold behaviour is what gives the detector its selectivity and is the first thing to check before choosing a lamp for a given survey.

Answer: C

A diode laser is swept across a single rotational vibrational line of the target gas, and because the line is narrower than any interference the measurement is extremely selective. It is used in situ across a duct for ammonia slip, hydrogen chloride, oxygen and moisture at high temperature, where extracting a sample would be difficult. It scans a fraction of a wavenumber rather than the whole spectrum, measures no particulate, and separates nothing. Selectivity through spectral resolution is the guiding idea.

Answer: D

Measuring across the duct itself removes the probe, filter, line, cooler and pump, which between them cause most of the faults and most of the sample alteration in a continuous monitor. The price is difficult access for maintenance, exposure of optics to a hot dirty environment, and awkward calibration, usually done with a sealed cell or by a comparison measurement. In situ instruments are not cheaper, still need calibration, and typically measure fewer components rather than more.

Answer: A

A sintered or ceramic element at the probe stops dust before it can block the line, foul the optics or wear the pump, and it is usually blown back periodically with compressed air. It must not remove the analyte, which is why filters for acid gas service are chosen from inert materials, since a filter cake can adsorb sulphur dioxide or ammonia and give a low reading. Pressure reduction and heating are done by other components of the conditioning system.

Answer: B

Cooling to a fixed low temperature condenses water out and fixes the residual moisture at a known small value, putting the measurement on a stable near dry basis. The condensate must be removed continuously by a peristaltic pump, and soluble gases such as sulphur dioxide are partly lost with it, which is why some methods use a heated wet system instead. Flow rate, ignition and density are not what the chiller is for. Handling moisture is the central problem of stack sampling.

Answer: C

A dry basis figure excludes water vapour from the total, so it is always higher than the wet basis figure for the same gas, by a factor of one over one minus the water fraction. Since most analysers measure a dried sample while emission limits may be set on either basis, the conversion, using a separate moisture measurement, is a routine and frequently mishandled step. It has nothing to do with where the sample came from, with purging or with the calibration gas.

Answer: D

Adding the water back dilutes the measured gas by the water fraction:

\[ 200 \times (1 - 0.10) = 180 \]

so the wet basis value is 180 ppm. The dry figure is always the larger of the two, which is a useful sanity check on any conversion. Emission limits state the basis and often a reference oxygen level as well, and applying the two corrections in the wrong order is a common source of reported values that cannot be reconciled between laboratories.