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).
A major delay in the response time of the analyzer comes from the length and inner diameter of the sample tubing.
The sample transport delay is increased by the square of the sample tube’s inside radius, and in direct proportion to its length. Doubling the inside diameter of the sample tube will quadruple the time delay it causes. Although the maintenance interval can be increased by the use of a large inner-diameter tube, such delays are usually unacceptable.
Because the analyzer response time is critical, large pre-conditioning filters in the sample line are to be avoided. The delay in a filter can be estimated by dividing the filter volume by the sample flow rate through the filter.
When choosing the appropriate the sampling location in multi-zone drying you must consider what happens during fault conditions.
The first or second zone might normally show the highest solvent concentration. But during an upset, it is not necessarily the zone where the peak concentration will appear. Excess solvent from an upset may very well tend to "carry over" into the next zone and produce a peak concentration there rather than in the first zone.
A key element in analyzer performance is the sampling location. Failure to install the sample probe in a "representative" location can lead to inaccurate or greatly delayed readings. A location representing the average solvent concentration of a single zone is desired.
There are two basic methods of probe location:
