with help of a sketch desirable the principles of operations of FID analyser ?

 

🔍 Flame Ionization Detector (FID) – Principle of Operation with Sketch

The Flame Ionization Detector (FID) is a device used to measure the concentration of unburned hydrocarbons (UHC) in engine exhaust gases. It is highly sensitive and widely used in emission testing labs for Spark Ignition (SI) engine analysis.

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✅ Basic Principle:

• Hydrocarbons, when burned in a hydrogen flame, produce ions and electrons.

• These ions are collected by electrodes and produce a small current.

• The current is directly proportional to the concentration of hydrocarbons in the sample.

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🧪 Working Process:

1. Fuel (H₂) and Air are supplied to the burner to create a clean flame.

2. Exhaust gas sample is injected into this flame.

3. Hydrocarbons in the sample ionize in the flame:

   $$\text{CH}_x + O₂ \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{ions/electrons}$$

4. A polarizing voltage is applied between electrodes.

5. The generated ions/electrons are collected, resulting in an electric current.

6. The current (picoamperes to nanoamperes) is amplified and displayed as HC concentration (ppm).

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✍️ Sketch of FID Analyzer:

Here's a labeled conceptual diagram:

          Exhaust Gas Sample
                   ↓
            ┌────────────┐
            │   Sample   │
            │   Injector │
            └────┬───────┘
                 ↓
        ┌─────────────────────┐
        │     Hydrogen + Air  │
        │      Burner (Flame) │
        └────────┬────────────┘
                 ↓
          ┌────────────┐
          │   Ionization│
          │   Region    │
          └────┬───────┘
     (Electrode + Collector Plates)
        (+)           (–)
          │           │
         [Current measured by]
        ┌────────────┐
        │ Amplifier  │
        └────┬───────┘
             ↓
       HC Reading (ppm)

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📌 Key Features:

Feature	Description
Sensitivity	Detects HC levels as low as 1 ppm
Response Time	Fast – in milliseconds
Selectivity	Responds mainly to hydrocarbons
Limitation	Does not detect CO, CO₂, NOx, or O₂

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✅ Applications:

• Engine exhaust HC measurement

• Environmental monitoring

• Fuel system leak detection

• Emission certification and regulatory compliance

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🧠 Summary:

The FID operates by burning hydrocarbons in a hydrogen flame, ionizing them, and measuring the resulting current. It is highly accurate for hydrocarbon measurement, making it essential for engine emission diagnostics.