Gas sampling from a distance

Our AccuSafe sensor units can often be installed close to the point source of the toxic gas sample, or in the area where workers might be exposed.  Still, it is sometimes necessary to transport sample from some distance to the analyzer (or sensor unit). There is no inherent problem in doing this, unless the sample is subject to condensation (see “Sample Conditioning” article); the analyte is unstable; or the interconnect tubing material might exhibit chemisorption properties.

Recommended tubingBev-A-Line IV (Polyethylene liner with ethyl vinyl acetate shell)     and      PTFE (Polytetrafluoroethylene)   [PTFE has superior properties, but is more expensive]

System lag time, reflecting the time for gas sample to travel through interconnect tubing, from a remote point back to the analyzer, can be estimated as follows, with the familiar πr2h (volume of a cylinder) formula:

Consider this example:   100 feet (30.48 m) of typical 0.250 inch OD x 0.125 inch ID tubing (6.4 x 3.2 mm) contains a volume of 0.2415 liters. Assuming a flow rate of 0.5 liter/minute, it will take 0.483 minutes (29 seconds) for the remote sample to traverse the tubing.  (Spreadsheet of this calculation.)

Various methods are available to decrease lag time (also known as transit time), including sampling at a higher flow rate and bleeding off to the analyzer, and incorporating heavier duty pumps and wider bore tubing.

For further information and applications assistance, please contact the factory.

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