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.
Several analyzer types are of sufficiently sound design and manufacture to meet the industry-standard requirements for general purpose gas detection. Many of these also maintain this accuracy under normal conditions for a reasonable period of time following calibration.
It is much less common, however, for the accuracy to be maintained for the following, that are routinely found in industrial processes:
In monitoring the solvent concentration in industrial processes, the accuracy of the analyzer directly affects the safety and economy of the process.
There are two types of errors to look out for:
The ability of the analyzer to measure the solvent concentration depends on the delivery of the solvent vapor to the analyzer through a sampling system in which all elements in contact with sample are above the dew points and flash points of all substances in the process.
Each solvent has a characteristic “flash point.” This is the minimum temperature at which the solvent can produce sufficient vapors to form a flammable concentration in air at its surface. The flash point is determined by several test methods, giving some small variation in the published values.
Any tendency for leaking in the sample tubing or analyzer is to be avoided, as it will directly affect the accuracy of the reading. For this reason stainless steel or a similarly strong and durable material should be used for sample tubing.
Leaks can be detected by a "probe injection" of calibration gas. In this method calibration gas is injected close to, or directly at, the very end of the sample tube, and the reading is compared to that reading obtained from the routine calibration method (injection directly at the analyzer).
