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Gas Detection System – Definition

A gas detection system is a safety device or network of devices designed to monitor the presence and concentration of hazardous gases in an environment. It continuously measures the air for toxic, combustible, or oxygen-deficient gases and provides alerts (alarms, signals, or automatic shutdowns) when unsafe levels are detected, helping prevent accidents, health risks, fires, or explosions.

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✅ Key Points:

• Purpose: Protect human health, workplace safety, and property.

• Types:

  • Fixed gas detectors (installed permanently in a location, like industries, plants).

  • Portable gas detectors (handheld, used by workers for personal safety).

• Applications: Oil & gas, chemical plants, mining, refineries, laboratories, confined spaces, and public safety.

• Gases Monitored: Carbon monoxide (CO), methane (CH₄), hydrogen sulfide (H₂S), oxygen (O₂), ammonia (NH₃), LPG, SO2, etc.

• Catalytic Bead (Pellistor) Detectors

  • Detect combustible gases (methane, propane, hydrogen).

  • Work by oxidation of gases on a heated sensor bead.

• Infrared (IR) Gas Detectors

  • Detect hydrocarbons & CO₂.

  • More stable, not affected by oxygen levels.

• Electrochemical Gas Detectors

  • Detect toxic gases (CO, H₂S, Cl₂, NH₃).

  • Work via chemical reaction producing current proportional to gas concentration.

• Photoionization Detectors (PID)

  • Detect volatile organic compounds (VOCs).

  • Use UV light to ionize gases.

• Semiconductor (MOS) Gas Detectors

  • Detect a wide range of gases.

  • Affordable but less selective.

• Common gases detected:

• Combustible gases: Methane, Propane, Hydrogen

• Toxic gases: CO, H₂S, NH₃, Cl₂

• Asphyxiant gases: CO₂, Nitrogen, Oxygen depletion

• 🔹Range of Combustible Gas Detectors (LEL – Lower Explosive Limit)

  • Typical Range: 0 – 100% LEL

  • Example: Methane, Propane, Hydrogen

  • Sometimes expressed in %Vol (e.g., 0–100% Vol for specific gases).

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  🔹 Toxic Gas Detectors

  1. Carbon Monoxide (CO): 0 – 300 ppm (sometimes up to 1000 ppm)

  2. Hydrogen Sulfide (H₂S): 0 – 100 ppm (extended range up to 200 ppm)

  3. Chlorine (Cl₂): 0 – 10 ppm

  4. Ammonia (NH₃): 0 – 100 ppm (sometimes up to 1000 ppm for industrial use)

  5. Sulfur Dioxide (SO₂): 0 – 20 ppm

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  🔹 Oxygen (O₂) Detectors

  • Typical Range: 0 – 25% Vol

  • Alarm points often set at 19.5% (low) and 23.5% (high).

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  🔹 Carbon Dioxide (CO₂) Detectors

  • Typical Range: 0 – 5000 ppm (0–0.5% Vol)

  • Industrial ranges: up to 5% or 10% Vol depending on application.

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  ✅ Summary (General Ranges):

  • LEL (Combustibles): 0–100% LEL

  • Toxic Gases (CO, H₂S, NH₃, etc.): 0–10 / 100 / 1000 ppm (depends on gas)

  • O₂: 0–25% Vol

  • CO₂: 0–5000 ppm / 0–5% Vol

• 🔹 Supply Options

  1. 24V DC (most common for industrial detectors)

     • Stable, safe, widely used in control systems.

  2. 230V AC (some models available)

     • Used when DC power is not available.

     • Less common for field detectors.

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  🔹 Output Types

  1. Analog Output

     • 4–20 mA signal (industry standard)

     • Proportional to gas concentration.

     • Used for connecting to DCS / PLC / smart transmitter panels.

  2. Digital Output

     • RS-485 (Modbus RTU) communication.

     • Allows multiple detectors on one communication line.

     • Provides gas reading + diagnostic info.

  3. Relay Output (Optional)

     • Alarm relays (SPDT) for local hooter, siren, or shutdown.

     • Example: Alarm 1 (Low), Alarm 2 (High), Fault.

  4. Wireless Output (Optional in advanced models)

     • For remote monitoring when wiring is difficult.

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  ✅ So, a typical industrial gas detector (FLP or WP) will have:

  • Power Supply: 24V DC (sometimes 230V AC)

  • Output Signals:

    • Analog 4–20 mA

    • Digital RS-485 (Modbus)

    • Optional relay contacts for alarms/faults

• FLP Gas Detector (Flameproof)

  • Housed in a flameproof enclosure (cast aluminum / stainless steel).

  • Designed for hazardous areas (Zone 1 & Zone 2) where explosive gases may be present.

  • Can withstand internal explosion and prevent it from spreading outside.

  • Commonly used in oil & gas, refineries, chemical plants.

  • Looks rugged, often metallic with threaded cable entries.

  🔹 WP Gas Detector (Weatherproof)

• Housed in a weatherproof enclosure (IP65 / IP66 rated).

• Suitable for safe areas or outdoor environments exposed to dust, rain, or humidity.

• Used in factories, warehouses, boiler rooms, process plants.

• Usually lighter design (polycarbonate, aluminum), not as heavy-duty as FLP.

• Alarm & Monitoring Panel – Key Features

  1. Power Supply

     • 230V AC input with 24V DC output for detectors

     • Battery backup (optional)

  2. Inputs

     • Accepts signals from detectors:

       • 4–20 mA analog

       • RS-485 Modbus

  3. Display & Monitoring

     • Digital LCD/LED display

     • Shows real-time gas concentration values for each connected detector

     • Can monitor multiple channels (commonly 4, 8, 16, 32, or more)

  4. Alarms

     • Audio Alarm (buzzer/hooter inside panel)

     • Visual Alarm (LED indicators: Power, Alarm, Fault)

     • Alarm levels: Low, High, Fault

  5. Outputs

     • Relay contacts for external hooter, siren, exhaust fan, or shutdown system

     • RS-485 Modbus output for integration with PLC/DCS/SCADA

  6. Enclosure Types

     • Wall-mounted panel (indoor, safe area, weatherproof)

     • FLP type panel (for hazardous area use)

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  🔹 Typical Connections

  • Detectors (FLP/WP) → Panel via 4–20 mA / RS-485

  • Panel → Alarm Hooter/Flasher (external)

  • Panel → PLC/DCS/SCADA (for central monitoring)

  • 230V AC → Panel power input

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  ✅ So in your system diagram, the Smart Gas Transmitter/Panel would:

• Show detector values

• Have alarm LEDs + buzzer

• 
  Be connected to external hooter/flasher

• Be powered by 230V AC

• 📖 Calibration with Standard Gas

  • Definition: Calibration means adjusting the gas detector to ensure it gives accurate readings when exposed to known concentrations of a target gas.

  • Standard Gas (Calibration Gas): A precisely prepared cylinder containing a known concentration of a gas (or gas mixture) certified by an authority (e.g., 50 ppm CO, 100 ppm H₂S, 2.5% CH₄, etc.).

  • Process:

    1. Connect the detector to the calibration gas cylinder via tubing and regulator.

    2. Expose the sensor to the gas.

    3. The detector reading is compared with the known concentration.

    4. Adjust the detector (zero and span) until its output matches the standard gas value.

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  ⏱️ Calibration Frequency

  • Manufacturer’s Recommendation: Usually every 6 months or as specified in the manual.

  • Industrial Practice:

    • Bump Test (functional check using test gas): Before each day/shift (quick test, not full calibration).

    • Calibration (full adjustment): Every 3–6 months depending on usage, environment, and safety requirements.

  • Harsh Environments (dust, humidity, chemical vapors): More frequent calibration (every 1–3 months).

  • Regulatory Standards (e.g., OSHA, ISO, IS standards): Often mandate calibration at least twice per year.

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  ✅ Example:

  • Portable CO Detector → Bump test daily, calibration every 30–90 days.

  • Fixed H₂S Detector in refinery → Calibration every 6 months, or sooner if exposed to extreme conditions.

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  👉 In short:

  • Standard gas = certified known concentration used for calibration.

  • Frequency = Daily bump test + Full calibration every 3–6 months (or per manufacturer/standard).

• if you need support for gas detection system.

• Selection guide/Make → how to choose the right FLP/WP detector, panel, and accessories.

• System design → detector placement, wiring, zoning, alarm logic.

• Installation guidelines → mounting, cabling, safety considerations.

• Commissioning & testing → calibration, alarm checks, acceptance tests.

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