The speed at which the sample is drawn from the process and reaches the detector is critical.
It is known that in many cases the response time of the entire sensing system must be not more than a few seconds. The following add crucial seconds & should be avoided:
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.
The desensitizing effects of several substances common to industrial processes are well known.
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.
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.
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.
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.
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.
In verifying the analyzer's range of response to different solvents, it’s best to obtain calibration data based on the response to solvent concentrations expressed in terms of LFL.
Do NOT use the following:
An analyzer should respond uniformly to all solvents that might be used. Any differences between individual solvent response factors should not reduce the margin of safety. Typically, this means that the calibration is based upon that solvent producing the lowest response, so the analyzer indicates the true concentration of this one solvent and it indicates readings higher than actual concentration for all others in use.
