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:
- inside the dryer in the vicinity of the web
- in an exhaust or recirculation duct
The first method can produce the highest readings provided the probe is placed properly at the highest peak of the concentration profile along the dryer. However, it is susceptible to errors from shifts in the concentration profile, as can occur during an upset.
Placement in an exhaust duct can give a good average solvent reading, provided the duct does not have its concentration diluted by exhaust flow from another zone as through the joining of two zone’s exhaust flows.
An audit of the dryer can be of some use in locating the final position of the sample probe and in determining if there are pockets of poor ventilation which should be corrected.
In general, however, extra consideration should always be given to the ways in which the upset condition will vary from the normal coating condition, and also the ways in which different web widths, dryer temperatures, ventilation rates, and other configurations will affect sampling.