September 2016

For multi-solvent, multi-zone drying processes, it is not always proper to assume uniform evaporation of solvents in each zone.

For an analyzer with wide variation in response factors to the solvents of interest, there could be a considerable error from the assumption that the solvent mixture is uniform in each zone of the dryer. Potentially, the relatively volatile solvents could exist in greater proportion in the first zone(s) of the dryer, and the less volatile solvents could exist in greater proportion in the later zone(s) of the dryer. 

Testing by the authorities has shown that at elevated temperatures the Lower Flammable Limit decreases, that is, the mixture increases in flammability. 

This means that extra care is required in the calibration of analyzers used in heated industrial process above and beyond that typically performed for ambient temperature leak detectors. 

The calibration data obtained for a particular solvent should always include the temperature correction, or give some basis for making proper temperature correction.

Unfortunately, the process of preparing accurate solvent mixtures for the precise calibration of the analyzer under the actual operating conditions is sufficiently difficult and unreliable in many cases, to generally prevent calibration with solvent mixtures. 

The practical difficulty in accurately preparing and storing exact solvent concentrations often precludes their use. Multiple solvent mixtures are even more difficult to adequately control. 

As for the sampling system, the sensor, and all its components in contact with the sample, must be heated to prevent condensation. 

The analyzer must be heated to at least the highest flash point of all the solvents, with perhaps a 10°C margin, so that the solvent mixture produced in the heated process does not condense in the analyzer tubing and detector, causing a significant, possibly a complete, loss of reading.