D-LX 721

Compact flame monitor with fibre optic system

Self-monitoring and fail-safe compact flame monitor for the monitoring of gas, oil and coal flames with integrated UV or IR flame sensor.

Version for application with


  • Wide sensitivity range
  • For ambient temperatures from -40°C up to +85°C
  • Highest safety combined with highest availability
  • Ideal Support for Functional Safety
  • Selective to individual burners and fuels
  • Wide range of certifications and standards covered
  • Low maintenance requirements
  • Local display for status and flame intensity
  • Optional flame stability analysis in real time

Your Benefit

  • Safe process control
    Even for strong and fast load changes of the combustion the process can be controlled safely with the flame stability analysis, reactions are possible in due time
  • Increased load and fuel flexibility
    Excellent selective monitoring and information about stability allow operation of the plant in less stable regimes like, e.g., one mill operation
  • Protection from unscheduled shut downs
    Burner specific information about the stability allows decisions for preventive maintenance
  • Fulfilment of special requirements
    Flexible pre-settings permit preparation for varying combustion conditions and fuels
  • Same technology for differing projects
    The same device technology can be used for plants in many territories and based on the most varying systems of standards


  • Burners with difficult installation conditions for conventional flame sensors or on those whose ambient temperature near the sighting tube is too high
  • Power stations
  • Chemical industry
  • Refineries
  • Cement plants
  • Waste incinerators
  • Steam generators
  • Heating plants

Technical Data

Operational modeIntermittent operation and continuous operation
SafetySelf-monitoring and fail-safe
Electrical connection24 VDC, 5 W, PELV
Flame relay, Status relay1x NO contact, 24 VDC, 0.5 A each
Flame Failure Detection Time
1, 2, 3, 5 s
(separate setting per range)
Analogue output0/4...20 mA, 750 Ohm max.
Optional real-time informationFlame stability analysis
Local displayFlame intensity, Flame ON/OFF, Thresholds,
Range selection
Spectral rangesUV, IR
Threshold switchesFlame intensity and flicker frequency
CommunicationLED, Modbus RTU, IrDA
Ambient temperature range-40...+85°C
Protection classIP66/68
IP65 for version /MP3
Dimensions85 x 85 mm, length approx. 175 mm (housing P2)
100 x 100 mm, length approx. 180 mm (housing M5)
Weight (without cable)approx. 0.9 kg (housing P2)
approx. 1.2 kg (housing M5)
Ex protection
D-LX 721 ../M4/84ExII 2G Ex d IIC T6 or T5 Gb
II 2D Ex tb IIIC T85°C or T100°C Db
D-LX 721 ../M4/85ExCl. I, Div. 1, Gr. A, B, C, D T6/T5
Cl. II, Div. 1, Gr. E, F, G T6/T5; Class III
Type 4
Hazardous locations: indoor/outdoor
D-LX 721 ../M5/86ExCl. I, Div. 2, Gr. A, B, C, D T6/T4
Cl. II, Div. 2, Gr. E, F, G T6/T4; Class III
Type 4
Hazardous locations: indoor/outdoor
D-LX 721 ../M5/87ExII 3G Ex nAnC IIC T6 or T4 Gc
II 3D Ex tc IIIC T85°C or T135°C Dc
Max. ambient temperature+85°C (/84Ex, /85Ex)
+65°C (/86Ex, /87Ex)
Protection classIP66 (/84Ex, /85Ex)
IP66/68 (/86Ex, /87Ex)
DimensionsØ120 mm, length appr. 230 mm
(/M4/84Ex, /M4/85Ex)
100 x 100 mm, length approx. 180 mm
(/M5/86Ex, /M5/87Ex)
Weight (w/o cable)approx. 2.8 kg (/M4/84Ex, /M4/85Ex)
approx. 1.2 kg (/M5/86Ex, /M5/87Ex)


  • LED bar graph display for the flame intensity (D-ZS 129)
  • Terminal box for connecting the flame monitor (D-ZS 140-12)
  • Installation flange for D-LL 703 fibre optic system (D-ZS 703)
  • Welding flange for D-LL 704 fibre optic system (D-ZS 704)
  • Wide range power supply unit for supply of several D‑LX 720 (D-NG 24/25a)
  • D-LX 200 Test kit for software supported tests of the compact flame monitors D-LX 200/720 and D-LX 201/721

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