How to Set Up Analyzer Stream Switching
When one analyzer serves several sample points through a selector, the stream switching sequence decides whether each stream reads its own gas or a blend of itself and the last stream, and getting the purge timing wrong quietly corrupts every reading. This procedure is for the analyzer engineer or technician commissioning a multi-stream selector arrangement. It walks sizing the purge time so the shared path is fully cleared, sequencing the selector valve, and proving each stream reads its own composition with no carryover. The whole discipline turns on one number: the purge time has to exceed the time it takes fresh stream gas to sweep the shared volume before the analyzer takes its reading.
Set Up Analyzer Stream Switching in one line: To set up an analyzer stream switching sequence, size the purge time so fresh gas from the newly selected stream fully sweeps the shared sample path and analyzer volume before a reading is taken, sequence the stream selector valve to switch cleanly and dwell on each stream long enough to purge and measure, and prove each stream reads its own composition with no carryover from the previous one. Confirm the purge by switching between streams of clearly different composition and checking the reading settles fully before the analyzer samples.
Map the Shared Path and Its Volume
Stream switching carryover lives in the shared volume, so start by understanding what is shared. Trace the path from the selector valve to the analyzer cell that every stream passes through, because that shared volume is what must be purged of the previous stream before the next reading, while the individual stream lines up to the selector each carry only their own gas. The larger the shared volume and the lower the flow through it, the longer it takes to clear, which is the residence-time relationship covered in the note on sample loop residence time.
Confirm the selector valve itself is right for the job and sealing. A stream selector must switch cleanly and, importantly, isolate the unselected streams so they do not bleed into the analyzed sample, and a valve that does not seal injects a trace of another stream regardless of purge time. The valve and its role are described in the note on a stream selector valve. Confirm each stream reaches the selector with adequate, similar flow, because a weak stream that barely flows purges slowly and reads carryover even when the others are fine.
Size the Purge Time to Clear Carryover
The purge time is the heart of stream switching, so size it deliberately. After the selector switches to a stream, fresh gas from that stream has to displace the previous stream's gas out of the shared path and analyzer volume before the analyzer takes a reading, and the purge time must exceed the time that displacement takes. Sizing it from the shared volume and the flow gives a starting point, and giving margin beyond the bare residence time accounts for mixing rather than perfect plug flow. A purge time set too short leaves the previous stream partly in the cell when the reading is taken, the definition of carryover.
Balance purge time against how often each stream needs updating. A longer purge is safer against carryover but slows how often every stream is read, so the sequence has to purge long enough to be clean while still cycling through all streams often enough for the process. Where a stream matters more or changes faster, it may warrant more frequent selection in the sequence. The memory-effect mechanism that a good purge time defeats is described in the note on sample carryover and memory effect, which is exactly what an undersized purge produces.
Sequence the Selector and Dwell Correctly
Configure the sequence so each stream gets both its purge and its measurement within its dwell. The selector switches to a stream, the sequence waits out the purge time while fresh gas sweeps the shared path, and only then does the analyzer take its reading, before moving to the next stream. Confirm the analyzer samples after the purge, not during it, because a reading taken mid-purge captures the transition and is contaminated. Confirm the dwell on each stream covers purge plus the analyzer's own measurement time.
Handle the order and the special cases. Sequence the streams so any stream whose carryover would most corrupt the next is not immediately followed by a sensitive one where possible, and confirm what the analyzer reports for a stream between its updates, since a multi-stream analyzer holds each stream's last value until it is next selected. Confirm the sequence handles a stream that faults or reads out of range without stalling the whole cycle, so one bad stream does not stop the others from being read.
Prove Each Stream Reads Clean and Baseline It
Prove the purge works by switching between streams of clearly different composition and watching the reading. Select a stream, watch the reading settle to that stream's composition, and confirm it has fully settled before the point in the sequence where the analyzer samples, with no tail of the previous stream's value. The most revealing test is switching from a stream at one extreme to a stream at the other and confirming the reading reaches the new value cleanly within the purge, because if any carryover survives the purge it shows up most starkly there.
If a stream reads a blend of itself and the previous stream, the purge time is too short, the shared volume is larger than assumed, the stream flow is low, or the selector is leaking, and you extend the purge or fix the cause rather than accepting the carryover. Record the purge time, the sequence, and the proven clean readings as the commissioning baseline. When each stream's value and the switching behavior are trended in a monitoring platform such as Merobix, a stream that starts tracking the previous one over time signals a plugging stream, a failing selector, or a purge that is no longer adequate, so the sequence is corrected before the readings quietly blend. The same carryover concern in a chromatograph context is covered in the guide to troubleshooting GC carryover and ghost peaks.
Frequently Asked Questions
How long should the purge be when switching analyzer streams?
Long enough for fresh gas from the newly selected stream to fully displace the previous stream's gas from the shared path and analyzer volume before the reading is taken, which you size from the shared volume divided by the flow, with margin added for mixing. A purge set too short leaves the previous stream partly in the cell when the analyzer samples, producing carryover. Prove the chosen purge by switching between streams of very different composition and confirming the reading settles fully before the analyzer measures.
Why does one analyzer stream read like the previous stream?
That is carryover, and it usually means the purge time is too short to clear the shared path, the shared volume is larger than assumed, the stream flows too weakly to sweep quickly, or the selector valve is leaking an unselected stream into the sample. The fingerprint is a stream whose reading tracks whatever stream ran before it. The fix is to extend the purge, restore adequate stream flow, or repair the selector, then re-prove each stream reads its own composition cleanly.
Should the analyzer take its reading during or after the purge?
After the purge, never during it. A reading taken while the shared path is still being swept captures the transition between the previous stream and the new one and is contaminated by the blend. The sequence must dwell on each stream long enough to complete the purge and then take the measurement, so the analyzer only samples once fresh stream gas has fully displaced the previous stream. Confirming this timing is central to proving the stream switching is clean.
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