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


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, NEMA 4X
IP65, NEMA 4X (/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, NEMA 4X (/84Ex, /85Ex)
IP66/68, NEMA 4X (/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 721 (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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