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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.

Each drying process has a unique rate of solvent increase in the normal and upset conditions. The potential maximum rate of solvent concentration increase should be estimated, so that the analyzer has time to generate an alarm, and the method of corrective action has time to reduce the solvent concentration, before a flammable or explosive limit is reached in the dryer. 

Some of the factors include: 

Most analyzer specifications clearly indicate the time needed for the analyzer reading to reach 63% (or 90%) of a final reading, in response to a sudden increase in concentration. 

These times are based on the response of the analyzer alone, and do not take into account the following that may be present in the complete analyzer system as installed: 

Sample Transport Time:

• sample tubing 
• filtration

Alarm System: 

Authorities require that an analyzer makes “continuous” measurements. 

Portable devices used for occasional checks are not recognized. Sequential sampling systems, which take samples from several locations in the dryer, and multiplex the sample stream to a single analyzer, are not recognized. 

A reasonable definition of a “continuous” analyzer is that it can detect a sudden increase in solvent concentration in time to make an effective alarm, at any time during the dryer’s operation.