Gas Analysers Questions and Answers

Answer: A

Organic carbon burning in a hydrogen air flame produces ions in numbers roughly proportional to the mass of carbon entering, and the collected current is the signal. That makes it very nearly a carbon counter, which is exactly what a total hydrocarbon measurement wants. It is not a calorimetric or an optical measurement, and the small temperature change is incidental. Its blindness to water, carbon dioxide and the permanent gases is another virtue in exhaust and stack work.

Answer: B

The detector counts carbon that can still be oxidised, and fully oxidised carbon produces essentially no ions in the flame, so carbon dioxide, carbon monoxide, water and the permanent gases are all invisible. That selectivity for organic carbon is what allows a total hydrocarbon reading to be taken from a wet, carbon dioxide rich exhaust without any conditioning beyond a heated line. Mass transport, quenching and scrubbing are not the mechanism.

Answer: C

Higher hydrocarbons condense readily, and anything that condenses in the line is not measured, so the reading is low and drifts as the deposit builds and re-evaporates. Lines are typically held at 180 degrees for engine and stack work. The flame does not need help to stay alight, the detector chemistry is fast at any line temperature, and the water problem is real but secondary. The general rule is that a sample must reach the analyser unchanged or the best instrument in the world will report the wrong number.

Answer: D

Ozone oxidises nitric oxide, and a fraction of the nitrogen dioxide formed is electronically excited and emits light in the red and near infrared as it relaxes, the intensity being proportional to the nitric oxide present. To measure total oxides of nitrogen the sample is first passed through a converter that reduces nitrogen dioxide back to nitric oxide, and the difference between the two modes gives the dioxide. The other options describe the sulphur dioxide analyser, the flame ionisation detector and a wet chemical method.

Answer: A

The chemiluminescent reaction only detects nitric oxide, so a heated molybdenum or carbon converter turns nitrogen dioxide into nitric oxide so that total oxides of nitrogen can be measured. Running alternately with and without the converter gives nitric oxide and the total, and their difference is the dioxide. Ozone is destroyed in a separate scrubber before the sample is vented, and signal linearisation is done in the electronics. Converter efficiency is a specified quantity that must be checked periodically.

Answer: B

Sulphur dioxide absorbs in the ultraviolet and re-emits at a longer wavelength, and the fluorescence intensity is proportional to concentration. Measuring emitted rather than absorbed light gives a far better detection limit, because a small signal is seen against a dark background rather than as a small difference between two large numbers. Hydrocarbons quench the fluorescence and are removed by a scrubber upstream. Ozone chemiluminescence is the nitrogen oxides method rather than this one.

Answer: C

Aromatics such as xylene absorb the excitation and fluoresce themselves, or take energy from excited sulphur dioxide, so both a positive and a negative interference are possible. A selective permeation scrubber removes them while passing sulphur dioxide. Window fouling is a slow maintenance issue rather than the reason for the scrubber, no significant reaction occurs at cell conditions, and complete absorption of the excitation would be a far grosser effect than is observed.

Answer: D

A pellistor holds a catalysed bead and a matched inert bead in a bridge; flammable gas burns on the catalysed one, warming it and unbalancing the bridge. It is the standard sensor for lower explosive limit monitoring in industry. Electrochemical cells handle toxic gases, paramagnetism is the oxygen method, and thermal conductivity is the katharometer, which the inert bead is there to cancel. Being a combustion measurement, it needs oxygen to work and can be poisoned by silicones and by sulphur compounds.

Answer: A

The measurement is a combustion, so with too little oxygen the gas does not burn and the sensor under-reads, dangerously, in exactly the atmosphere where flammable gas is most concentrated. Above the upper explosive limit the same failure occurs. This is why instruments for confined spaces read oxygen alongside flammable gas, and why infrared sensors, which need no oxygen, have displaced pellistors in many applications. The catalyst is not dissolved, and the bridge balances regardless.

Answer: B

A reading of fifty per cent LEL means the gas is at half the concentration at which the mixture would become flammable, so for methane, whose limit is five per cent by volume, that is two and a half per cent. The scale is used because the hazard, not the concentration, is what matters to the user, and alarms are conventionally set at ten and twenty per cent LEL. It is not a volume percentage, a range fraction or an oxygen measurement, and confusing the two scales by a factor of twenty has caused real accidents.