Back to the Basics: Accuracy Requirements for Steady-State Conditions

Common practice is to assume complete accuracy in the LFL values from whatever authority is recognized at the time. But, as we have seen, one can reasonably assign an uncertainty of 10% LFL to the initial LFL values, as indicated by the precision of the published LFL values and the amount of agreement between the various competent authorities. Temperature effects can account for perhaps an additional 10% LFL, if one chooses to use the lesser amount of correction and in a particular case, the greater amount of correction is appropriate. Is it probably true that the authorities include in the margin of safety, a factor for this uncertainty in LFL value. Thus one should recognize that not all the margin of safety can be allowed as compensation for analyzer error, and an amount equal to at least 10% LFL should be allowed for margin of error in LFL values. 

Seen in this light, the analyzer must alarm reliably at a concentration of not more than 90% LFL under steady state conditions. The combination of initial accuracy, especially calibration accuracy, and analyzer drift, should not account for more than 90% LFL minus the upper alarm point. Where the alarm level is 50% LFL, the analyzer error could be as much as 40% LFL (-44% of a 90% LFL actual gas concentration) without constituting a hazard. If the alarm level is raised, the accuracy required is greater.

The attempt is often made to make all analyzer inaccuracies “safe-side error.” That is, the error produced by the analyzer, for example from calibration to different solvents with varying response factors, should result in exaggerated high readings, but never false low readings. This approach can be valid, as long as false alarms from exaggerated readings do not shut down the process.

In single solvent systems with proper calibration and maintenance, such accuracy is possible to achieve with several analyzer types. In multi-solvent systems the relative ratios of calibration factors becomes very important, and the number of analyzer types which are appropriate is very small.