Gas Analysers Questions and Answers

Answer: A

A stable reading tells you nothing about whether the gas is fresh, so a simple rotameter showing flow is the quickest confirmation that the sample is arriving and that no blockage has developed. It is the first thing an experienced technician looks at. Volume is not needed for a concentration measurement, regulation is done by a pressure controller, and calibration is triggered by a timer. The cheapest component in the system is often the most diagnostic.

Answer: B

Portable combustion analysers pair an infrared or electrochemical carbon dioxide and carbon monoxide measurement with an electrochemical oxygen cell, because oxygen has no infrared absorption and needs a different principle. The other three combinations each apply a technique to a gas it cannot measure. Being able to say quickly which principle suits which molecule is the practical skill this topic is really testing, and it follows entirely from the physics of each measurement.

Answer: C

Excess oxygen shows how much air is being supplied and carbon monoxide shows whether that air is being used effectively, so the pair locates the operating point between wasteful excess air and incomplete combustion. Efficiency is highest just before carbon monoxide begins to rise. Moisture, sulphur and calorific value are all measured or calculated by other means. This is the classic example of two simple measurements being far more informative together than either is alone.

Answer: D

Air that takes no part in combustion is still heated to flue temperature and thrown away, so efficiency falls steadily once enough air is present to burn the fuel. Too little air raises carbon monoxide and unburnt fuel, so the optimum sits just above the point where combustion is complete. Trimming to that point using an oxygen analyser is one of the most cost-effective uses of process instrumentation, which is why zirconia probes are so widely installed on boilers.

Answer: A

Cross sensitivity is quoted as a percentage response to a named interferent and appears in the data sheet for every electrochemical and many optical sensors. It is the reason a sensor must be chosen for the atmosphere it will meet rather than for the gas alone. Cross calibration and cross modulation are not standard terms here, and gas correlation refers to the gas filled detector technique that gives an infrared analyser its selectivity, which is a quite different idea.

Answer: B

Filling the detector with the target gas means it absorbs precisely where that gas absorbs, which is a spectral selection of extraordinary sharpness achieved without any optical component. That is the correlation the name refers to. The zero is set by a separate zero gas, water interference is handled by drying or correction, and no cooling is involved. It remains one of the most elegant ideas in analytical instrumentation, and it is why the classical NDIR outperforms a filter based instrument on selectivity.