Gas Analysers Questions and Answers

Answer: A

Without an oxygen correction, a plant could meet a concentration limit simply by admitting excess air, since dilution lowers every concentration without reducing the mass emitted. Correcting to a stated reference level, such as three or six per cent oxygen, removes that loophole. Analysers do not consume enough oxygen to matter, the wet and dry conversion uses moisture rather than oxygen, and temperature is handled separately. The correction is about the integrity of the limit rather than about instrument error.

Answer: B

Zero gas contains none of the target component, typically nitrogen or purified air, and sets the baseline of the instrument. Span gas at a known concentration near the top of the working range then sets the sensitivity, and the two together define the calibration line. Using ambient air as a zero is acceptable only when the component is genuinely absent from it, which is not true for carbon dioxide. The zero and span pair is the minimum calibration for almost every continuous analyser.

Answer: C

Injecting at the probe puts the calibration gas through the same filter, line, cooler and pump as the sample, so losses and leaks anywhere in the chain show up in the result. A calibration at the analyser inlet validates only the last few centimetres and can look perfect while the system loses half the analyte. It uses more gas rather than less, and the pressure and access arguments are conveniences rather than reasons. The principle is to calibrate the measurement, not the detector.

Answer: D

A sealed permeable tube holds the liquefied compound and lets it diffuse out at a rate fixed by temperature, giving a steady trace concentration when swept with a known flow. It is the standard way of making parts per billion standards of sulphur dioxide, nitrogen dioxide or ammonia, which cannot be stored in a cylinder because they would react with the walls. Drying, filtration and pressure control are done by quite different devices. A permeation tube must be held at constant temperature to a tenth of a degree.

Answer: A

Two mass flow controllers blend a concentrated certified gas with zero air in an accurately known ratio, generating any concentration in the range from a single cylinder. That allows a multi point linearity check without a shelf full of cylinders and is standard practice for ambient air monitoring networks. Sample transport, scrubbing and gas phase conversion are all other functions, though a calibrator often includes an ozone generator for the nitrogen dioxide measurement by titration.

Answer: B

T90 is the time from a step change at the probe until the reading has covered ninety per cent of the difference, and it includes transport delay in the line as well as the response of the detector itself. Regulations often specify a maximum, because a slow analyser cannot demonstrate compliance during short excursions. Long sample lines and large cell volumes are the usual culprits when the figure is poor, and reducing line diameter or raising flow is the usual cure.

Answer: C

Drift is a gradual movement of the zero or the span with time and temperature, and it is quantified over a stated interval, typically twenty four hours or the calibration period. It is the reason automatic zero and span checks are built into regulatory monitors. Noise is fast and random rather than slow and systematic, unit to unit variation is a manufacturing spread, and the lag is the response time. Distinguishing the four is essential when reading a specification sheet.

Answer: D

A leak on the suction side pulls in ambient air, which shifts the zero for anything present in air and dilutes everything else, and it is the commonest fault in an extractive system. A genuine change in gas concentration would not affect a check made on zero gas, barometric change affects some analysers slightly and is usually compensated, and mains frequency is irrelevant. Leak testing under vacuum should therefore be the first response to an unexplained zero shift.

Answer: A

Both statements hold and the second explains the first. Since a pump downstream draws the sample along, the whole line is below atmospheric pressure and a leak admits air rather than losing sample. Testing by pressurising can therefore pass a system that leaks inwards at working conditions. The test is done by blocking the probe and confirming the vacuum holds. It is one of the few checks that finds a fault which every calibration at the analyser would miss.

Answer: B

Keeping the pump body hot prevents condensation inside it, which would corrode the diaphragm and strip soluble gases from the sample. The pump is placed after the filter but before or after the cooler depending on the design philosophy. It does not usefully compress the sample, does not meter volume, which is done by a flowmeter, and removes no dust. Every component in a conditioning system is chosen with the dew point in mind, and that is the thread linking the heated line, the heated head and the chiller.