Analogue Detector Sensitivity and Drift Compensation
Analogue addressable systems do something conventional systems cannot: each detector reports a measured value, and the panel decides what that value means. This opens up sensitivity settings, condition monitoring and drift compensation — powerful tools that keep detection accurate over time, provided they are understood and not misused. This guide explains how analogue detection, drift compensation and sensitivity work, and what engineers should check.
The key idea is that the panel, not the detector, makes the decision — which is what allows the system to manage sensitivity and compensate for gradual change.
Who this is for
This is for competent fire alarm engineers working with analogue addressable systems. The experience level assumed is competent engineer. Use it for the principles; the specific values, thresholds and settings come from the manufacturer's design and BS 5839-1.
How analogue detection works
An analogue addressable detector reports a measured value to the panel rather than making its own fire/no-fire decision. The panel interprets that value against thresholds to decide alarm, pre-alarm or fault, which lets the system apply sensitivity settings, monitor each detector's condition, and compensate for gradual drift. This is fundamentally different from a conventional detector that simply switches at a fixed point. How the values and thresholds are used follows the manufacturer's design and BS 5839-1, and understanding it is the basis for everything else in this topic.
Drift compensation
Detectors slowly accumulate dust and contamination, which tends to shift their baseline reading over time. Drift compensation is where the system gradually adjusts for this slow change so the detector keeps responding correctly, while still flagging when contamination reaches a level that needs attention. It maintains performance between services, but it has limits and is not a substitute for cleaning — a detector will eventually reach the point where compensation can no longer keep it in range. How it behaves and where its limits lie follow the manufacturer's design.
Sensitivity settings
Many analogue systems allow sensitivity settings, sometimes varying by time of day, so a detector can be made less sensitive during a known nuisance period and more sensitive otherwise. This is genuinely useful for managing false alarms, but it carries a clear condition: any setting must follow the fire risk assessment and the design and must never reduce protection below what is required. From field experience, the misuse to guard against is turning sensitivity down to silence a nuisance that should instead be fixed at source. Sensitivity is a design decision within BS 5839-1 and the manufacturer's capabilities.
Using analogue values in service
The analogue data is a gift to maintenance if you use it. Where the system reports it, review detector analogue values and any contamination or drift indication, so detectors approaching their limit can be cleaned or replaced before they fault or start misbehaving. From field experience, rising values that signal contamination, and detectors sitting near their compensation limit, are exactly what a good service catches early — turning a future unwanted alarm or fault into planned maintenance. Record readings where available and act on those trending towards their limit.
Common points to check
Recurring issues include detectors near their drift-compensation limit, contamination trends ignored until a fault or false alarm, and sensitivity turned down to mask nuisance rather than fix it. Confirming analogue values are healthy and acted on is the essential check.
When not to rely on this alone
When not to use this article: do not use it to set thresholds or sensitivity for a specific system. Those come from the manufacturer's design, the fire risk assessment and BS 5839-1, applied by competent professionals.
Relevant standards
Analogue behaviour, sensitivity and drift are matters of the manufacturer's design applied within BS 5839-1, a code of practice. The legal duty for fire precautions in most non-domestic premises sits under the Regulatory Reform (Fire Safety) Order 2005, with Building Regulations statutory guidance in Approved Document B applying to building work. Separate the legal duty from the recommended methods, and always work to current editions.
Professional disclaimer
This is an educational and workflow resource for competent engineers and does not replace the current British Standards, the manufacturer's design data, the fire risk assessment, or competent judgement. Verify sensitivity and threshold settings against current documentation.
Related documentation
Use this with the current BS 5839-1, the manufacturer's design and analogue data, and the fire risk assessment. Record detector analogue values and contamination trends where available, and act on detectors approaching their limit.