This selection is too many times done by a seat of the pants….”we did it this way before” selection method. The task is to make sure that the sample we are measuring is truly representative of the process. In the case of “timed interval” sampling (GC’s commonly use this) we should utilize a fast loop (a continuous flow of sample either back to the process and/or to a flare). By tapping a fast loop stream with the smallest possible diameter tubing will greatly reduce the resonance time. These considerations are especially critical in the instances whereby the sample extracted is liquid and is subsequently vaporized. Due to the volumetric increase in the phase change it is possible to be measuring a sample that is many days old!
Since most sample systems control the sample flow to approximately 1 liter/minute you must also consider the volumetric expansion of gas samples. A two times pressure drop will result in twice the volume.
| Simple formulas for calculating the lag time in minutes:
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Rule of Thumb Tubing Volumes:
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Lag (min) = Line Length (ft) / Velocity (ft/min)
Lag (min) = Line Volume (cc) / Flow Rate (cc/min)
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0.125” diameter tube 1cc/ft
0.250” diameter tube 5cc/ft
0.500” diameter tube 30cc/ft
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