Technical Specifications:

• Measurement Principle: Near-Infrared (NIR) absorption spectroscopy
• Wavelength Range: 1650 nm to 1750 nm (C-H overtone region) and 2100 nm to 2400 nm (C=O, C≡C regions)
• Optical Path Length: 50 mm (S50 variant)
• Measurable Gases: H₂, CH₄, C₂H₆, C₂H₄, C₂H₂, CO, CO₂
• Measurement Range:
  • H₂: 0–2000 ppm
  • CH₄: 0–5000 ppm
  • C₂H₂: 0–500 ppm
  • CO: 0–5000 ppm
  • CO₂: 0–50000 ppm
• Accuracy: ±10% of reading or ±5 ppm (whichever is greater) for H₂
• Response Time (T90): < 15 minutes
• Operating Temperature: 0°C to 50°C (sensor head), -20°C to 60°C (electronics)
• Power Supply: 24 V DC (±10%)
• Output Signal: 4–20 mA DC (per gas), RS-485 Modbus RTU
• Protection Rating: IP65 (sensor head), IP54 (electronics box)
• Certification: Ex ia IIC T4 Ga (intrinsically safe for Zone 1)
• Dimensions (Sensor Head): 120 × 80 × 60 mm (W × H × D)
• Weight: Approximately 0.8 kg (sensor head)

Functional Features:

• Simultaneous multi-gas measurement (up to 7 gases)
• Automatic baseline correction and drift compensation
• Self-diagnostic function (light source intensity check, optical window contamination alert)
• Alarm outputs for each gas (configurable setpoints)
• Data logging with timestamp
• Remote configuration via Modbus RTU or HART

Performance Parameters:

• Light Source: Broadband NIR LED (1650–1750 nm) + NIR LED (2100–2400 nm)
• Detector: InGaAs photodiode
• Optical Window Material: Sapphire (scratch-resistant, chemically inert to transformer oil)
• Sample Flow Rate: 100–500 mL/min (recommended)
• Sample Pressure: 0.1 to 0.5 MPa (gauge)
• Linearity: ±2% of full scale
• Long-Term Drift: < 5% per year (with auto-calibration)

Material Composition:

• Sensor body: 316L stainless steel
• Optical windows: Sapphire (both sides)
• Flow cell: 316L stainless steel with PEEK seals
• Cable: PTFE-insulated, stainless steel braided shield
• Electronics enclosure: Aluminum alloy with powder coating

Structural Features:

• Inline flow-through design for continuous oil sampling
• Quick-connect fittings for sample inlet and outlet (Swagelok-compatible)
• Front-panel LCD display (optional) showing gas concentrations
• DIN rail mounting for electronics module
• Modular design: sensor head separates from electronics box via cable

Working Principle:
Transformer oil flows continuously through the 50 mm optical cell. The NIR light source emits broadband radiation through the oil. Dissolved gas molecules absorb specific wavelengths corresponding to their molecular overtone vibrations (C-H stretch for hydrocarbons, C≡C stretch for acetylene, C=O stretch for CO/CO₂). The InGaAs detector measures the transmitted light intensity at each wavelength. The microprocessor applies Beer-Lambert law to calculate gas concentration from the absorbance. Multi-wavelength analysis with chemometric algorithms separates overlapping absorption bands to quantify each gas independently.

Advantages & Highlights:

• Online real-time monitoring eliminates the need for periodic off-line DGA sampling
• Multi-gas detection from a single sensor reduces installation cost
• Sapphire optical windows resist fouling from transformer oil contaminants
• Intrinsically safe for installation in substations
• Automatic drift correction maintains accuracy without frequent recalibration
• No consumable gases or carriers required (unlike gas chromatography)

Applicable Industries:

• Electric power utilities (transformer monitoring in substations)
• Power generation plants
• Railway traction power systems
• Industrial facilities with large power transformers
• Renewable energy (wind farm, solar farm transformers)

Installation Requirements:

• Install in a clean, temperature-controlled environment (0–50°C)
• Connect the sensor in a bypass loop on the transformer oil circulation system
• Ensure sample flow rate is 100–500 mL/min (use a sample pump if natural circulation is insufficient)
• Purge the sensor with clean transformer oil for 30 minutes before first measurement
• Mount the electronics box in a control room, connected to the sensor head via shielded cable (maximum 50 meters)

Usage Precautions:

• Do not allow air bubbles in the optical cell (they cause measurement errors)
• Replace the sapphire windows if optical transmission drops by more than 20%
• Perform a zero-gas calibration (nitrogen-purged oil) every 6 months
• Do not expose the sensor head to temperatures above 50°C
• Filter the oil sample (5 µm filter recommended) to prevent window fouling
• Verify the sample flow rate daily; low flow causes stale readings