| Facility | Area (m²) | Detectors | Radius (m) | Raw Coverage | Final Score | Rating |
|---|---|---|---|---|---|---|
| Warehouse 3 | 8,000 | 14 | 13 | 92.6% | 88.1 | Good |
| Packaging Line | 2,500 | 6 | 10 | 71.8% | 62.4 | Fair |
| Tank Farm Zone | 12,000 | 24 | 14 | 98.3% | 95.7 | Excellent |
- Coverage per detector = π × r² (radius mode) or the provided area (area mode).
- Gross covered area = detectors × coverage_per_detector.
- Effective covered area = gross × overlap_factor × access_factor.
- Raw coverage (%) = (effective / total_area) × 100, capped at 100%.
- Final score = raw_coverage × reliability × redundancy × availability × nuisance_penalty.
- Enter the total protected area for the zone or building.
- Choose radius or area mode based on your data source.
- Set overlap and access to reflect layout constraints.
- Use reliability, redundancy, maintenance, and nuisance values for operational realism.
- Click Calculate Coverage. Results appear above the form under the header.
- Download CSV or PDF for sharing and recordkeeping.
Coverage drivers in real facilities
Alarm coverage is shaped by geometry and process conditions, not only device quantity. Tall racks, ducting, partition walls, and wind loads can shadow sensing paths and reduce usable area. Treat overlap as an effectiveness factor: 0.70–0.80 for congested layouts, 0.80–0.90 for mixed spaces, and 0.90–1.00 for open, verified zones. Accessibility reflects mounting height, line-of-sight, and airflow; values below 0.85 usually indicate the need for relocation or additional points.
Translating radius to protected area
In radius mode, coverage per detector is computed as π × r2. A 12 m radius corresponds to about 452 m² of theoretical area. With 10 detectors the gross covered area becomes roughly 4,520 m². After applying overlap 0.85 and access 0.95, effective coverage is about 3,651 m². Against a 5,000 m² target area, raw coverage equals 73.0%, highlighting how spacing assumptions materially change results.
Reliability and redundancy multipliers
Coverage can look high while performance is poor, so the score applies operational multipliers. Reliability aggregates power stability, self-test success, and historical fault rates; a plant with 92% pass rate may use 0.92. Redundancy rewards independent backup detection such as cross-zoning or mixed sensor technologies. Values of 1.05–1.20 are typical when a single point failure does not eliminate detection in a critical zone; keep 1.00 when coverage is strictly single-layer.
Availability from maintenance planning
Maintenance cadence influences availability because systems spend time in test, bypass, or repair states. Using a simple proxy of one downtime day per cycle, a 30-day interval yields availability near 0.97, while 90 days approaches 0.99 but risks longer undetected faults. If your program includes frequent functional tests, lockouts, or harsh environments, shorten the interval and record downtime minutes. Use consistent intervals to compare scenarios across buildings and contractors.
False alarms and practical readiness
False alarms drain response capacity and can cause operators to delay escalation. The calculator converts the false alarm rate into a nuisance penalty so that cleaner systems score higher. For example, 0.8% produces a mild reduction, while 5% can reduce the final score substantially even when raw coverage is strong. When nuisance rises, prioritize threshold tuning, environmental shielding, and device placement changes. Export results to document tradeoffs and to support safety reviews and commissioning sign-offs. A monthly trend chart of alarms per zone helps pinpoint problem areas quickly.
1) What does the raw coverage percentage represent?
It represents estimated physical area covered after overlap and accessibility adjustments, divided by total protected area. It is capped at 100% and is reported separately from the final score.
2) When should I use radius mode versus area mode?
Use radius mode when a device provides an effective radius from testing or guidance. Use area mode when vendor documentation specifies an approved coverage area per device for your environment.
3) Why can my final score be lower than raw coverage?
The score also applies reliability, redundancy, availability, and nuisance penalties. A system can cover area well but still perform poorly if it is unreliable, often bypassed, or generates frequent false alarms.
4) How do I choose an overlap factor?
Start with 0.85 for typical layouts. Reduce toward 0.70 for obstructions, irregular spaces, or highly directional sensing. Increase toward 0.95 only when spacing is validated by walkthroughs and functional tests.
5) How is availability estimated from maintenance days?
The calculator uses a simple proxy: one downtime day per maintenance cycle, capped to avoid extreme values. Shorter intervals can improve fault detection but may increase planned downtime.
6) What should I export the PDF/CSV for?
Use exports to record assumptions, compare design options, and attach evidence to reviews, commissioning packages, and compliance documentation. Keep dated reports to show improvement over time.