Many analytical instruments are cyclic, or sampled data devices, such as the chromatograph. These automatically take a sample of the process stream, analyze it, and transmit the results to the desired device. Since most analysis cycles take from one to 20 minutes to complete, considerable “dead-time” is introduced into a control loop using this type of measurement. Dead-time compensation should be included in the control scheme for proper control response. Process stream sampling is an item of vital concern in good chromatography or with any analysis technique. Some points of consideration are:
• Sample probe:
Must be located at a point in the process where the material to be analyzed is in the desired phase (vapor or liquid) at sufficient pressure and flow. • Sample lines: Should be kept as short as possible for minimum transport time. May need to be heat-traced to keep vapor samples from condensing. Sample flow must be great enough to completely flush the line between analysis cycles.
• Sample filters:
Used to keep any particulate matter out of the analyzer. Knock-out pots or other devices may be required to remove liquid condensate.
• Sample pumps:
Required for certain low pressure processes. Vacuum aspirators may be used for low sample flows.
Other cyclic or batch type instruments determine such quantities as vapor pressure, dew point, end point, flash point, Btu content, and sulfur content.