Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2019-09-12.
Buyers of emission monitoring sometimes ask for redundancy by default: two systems, one running and one on standby, on the assumption that a spare is the only way to keep data flowing. The article argues that the choice of system type matters more than the number of systems. An extractive CEMS draws sample through a probe, a heated line, a pump, valves and a measuring cell — every one of those can clog, corrode or fail, and a second identical system duplicates exactly the same failure points. An in-situ system measures across the duct or the open air instead: no sample line, no filters, no pump, no cell to contaminate, so there is much less to maintain. OPSIS reports the longest maintenance intervals on the market for this type of system, a long record of reliable hardware, and service through its representatives worldwide when something does go wrong.
In Chinese projects redundancy usually appears in one of two forms: a complete second system, or spares for critical parts. The requirement more often comes from the owner's concern about data assessment, from group rules or from tender clauses than from a mandatory rule, so we suggest converting the question into a comparable number first — data availability — and then deciding where the budget goes. The failure points of an extractive system are concentrated and predictable: probe filter clogging, insufficient heat tracing that lets condensate into the sample path, pump diaphragm wear, cell and line contamination, peristaltic drain failure. Duplicating the system duplicates those points, and if spares, response time and calibration practice are weak, both systems degrade together while maintenance hours and calibration gas consumption double. In-situ systems remove the sampling and conditioning stages, but they are not maintenance-free: window fouling on dusty duties such as cement, sintering and waste incineration, alignment drift, lamp ageing, condensation in rain and fog, platform vibration and thermal movement all need scheduled checks and purge air. If two systems really are required, define the switching logic, the validity flags and the comparison period in advance, so that disagreement between them can be resolved. One boundary is worth stating: open-path DOAS suits fence-line, industrial-area and ambient path averages, while stack emission monitoring still follows the methods required by HJ 75 and HJ 76.
No Need for Redundant Systems — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2019-09-12. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
Send the gases, ranges, duct or path dimensions and required certification — we will respond with a technical proposal.