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12. Watch for changes in the appearance of the Dräger tube. Before the
front starts to smear, note and record the maximum darkened length in
units marked on the tube. Continue flowing for a total of 15 minutes, even
though the front may attain a diffuse and feathery coloration. In the high
range tube, an orange color caused by SO
2
may appear ahead of the
black front if sulfites are present in the sample. The orange SO
2
region
should be ignored when recording the darkened length. For more ac
-
curate results, the darkened length should fill more than half of the tubes
total length. Therefore, the filtrate sample volume must be carefully se
-
lected.
