Three forms, three engineering questions
| Form | How gas reaches the sensor | What the project must control | Typical question |
|---|---|---|---|
| Diffusion | Gas reaches an opening or diffusion structure from the surrounding environment. | Exposure, transport delay, pressure response, fouling and environmental conditions. | Is the surrounding gas representative and stable enough for this decision? |
| Extractive | A pump draws gas through a sample line into the module or cell. | Pump, filters, condensate, line material, flow, pressure and maintenance. | Can the host system deliver a clean, controlled sample? |
| Flow-through | Gas moves through a defined window or cell placed in the process path. | Insertion, fitting, pressure, flow direction, sealing, materials and access. | Can the measurement point sit in the pipe without creating an unverified boundary? |
Why the form changes response and accuracy
Reported response includes more than the optical algorithm. Gas arrival, cell exchange and averaging all contribute. A diffusion opening, a long sample line and a direct flow-through window do not have the same transport path.
The same applies to accuracy. A target value is meaningful only with the gas, pressure, temperature, flow, background gas, warm-up state and configuration that make it valid.
Put Line A and Line B in the right boxes
Inside the host instrument
Line A assumes a pumped or conditioned sample path supplied by the host. The module does not supply the pump, filter, condensate management or complete analyzer.
Line B · flow-throughAt the drainage pipeline
Line B carries a direct flow-through design intent with a 40 mm window. The actual pipe, pressure, fitting, installation and host route still belong in the project specification.
If the only evidence is that a sensor is mounted near a pipe, the gas path is still an open question.
FAQ
Questions engineers ask next
Why does sampling form change response time?
The reported response includes gas arrival, cell exchange and algorithm averaging. A diffusion opening, a long sample line and a direct flow-through window do not have the same transport path.
Can a diffusion sensor be mounted in a pipe?
It can be mounted near or in a pipe, but that does not prove a controlled flow-through measurement. The actual path and pressure behaviour must be assessed.
Is extractive measurement always the easier option?
Not necessarily. It can make the optical node easier to place inside an instrument, but it adds a pump, sample line, filtration, condensate and maintenance boundary.
What does Line B change?
Line B places the intended measurement window in the drainage pipe and can avoid a separate bypass sampling train. It does not remove the need to confirm pressure, sealing, materials, installation and host-system evidence.