Today we expect all instruments, including the flue gas analyzer, to be fully automated and do all the work for us. To a certain extent this is also true. A modern flue gas analyzer performs all relevant calculations and presents the results in a simple fashion. There is no need to carry out measurements and take samples for processing at home. In the early days, a sample would be taken of the gas and either tested using liquid chemicals or sent off to a laboratory for analysis. The temperature and oxygen measurements would be compared with a set of charts and the efficiency, excess air and other factors would be read from the chart. Obviously, this cannot exactly be described as an interactive process and gives no opportunity to use the results for a direct influence on the burner system.
The modern flue gas analyzer gives real-time results, to use a phrase from the computer branch. And why not use computer terminology? The flue gas analyzer is, indeed, a miniature computer, that is specialized for the purpose of measuring the components in flue gas and calculating the relevant results from the temperature and gas readings. With these results, adjustments can be carried out on the burner system and the effects can be seen immediately. A bit too much? No problem, just back it off half a turn and see what happens! The flue gas analyzer and related equipment have become a standard part of the boiler technician's baggage. Nobody is now prepared to accept results based on rules of thumb or plain experience with no numerical results to be seen.
One can argue that this is not really an improvement, there are many people with a high level of experience working in the field, who can get a burner setting nearly right without a flue gas analyzer, just using experience. The operative phrase is "nearly right". In these days of high fuel costs, nearly right can be a very expensive option and the "completely right" supplied by the flue gas analyzer is essential to keep costs in acceptable boundaries. The flue gas analyzer may be expensive, but fuel is also expensive, and getting more so.
One other reason for the increase in use of the electronic flue gas analyzer is the relative decrease in the cost over the last few years and the fact that it can be used with very little direct training. Most people can take a measurement, even if some expertise is needed to interpret and use the results. The methodology is also required by law for some purposes and in some countries. If you have to have a flue gas analyzer, you might as well use it regularly and gain some direct benefit from the investment. EPA compliance testing may also be done in this way, although the standards still refer to the old chemical testing, and merely mention other, equivalent systems.
The flue gas analyzer in this form is not exactly a modern invention. There were electronic flue gas analyzers over twenty years ago, but the size, weight and relative cost have decreased to the point that they are within the reach of every freelance boiler maintenance operative. The old argument that you cannot carry it up the ladder to the measuring point is also now history, since there are instruments available weighing less than a bag of sugar.