Principles Of Operation - Emerson Rosemount 975MR Reference Manual

Reference Manual
00809-0100-4975
• Exde: integral junction box for easy wiring.
• SIL-2: TÜV approved.
• Hazardous area certification: ATEX, IECEx, FM, and CSA.
• Functionality approval:
—
—
• Accessories are approved as part of ATEX and IECEx approval.
1.4

Principles of operation

1.4.1 Hydrocarbon fire detection
The Rosemount 975MR uses four infrared sensors, each sensitive to its own wavelength
range. Two of the sensors are sensitive to wavelengths within the emission peak of hot
CO . The other two sensors are sensitive to wavelengths above and below this peak.
2
In the event of fire, the signal measured in the first sensor is significantly higher than those
measured in the other two sensors. In order to issue a fire alarm, the detector requires that
this occurs, as well as other conditions (for example, radiation is flickering in frequencies
typical of flames). If exposed to non-fire radiation sources, the specific conditions required
do not occur, and the detector does not react.
1.4.2 Heated optics
The flame detector uses heated optics. The heater increases the temperature of the
optical surface by 5 to 8 °F (3 to 5 °C) above the ambient temperature to improve
performance in icing, condensation, and snow conditions.
The heated optics can be set to one of the following:
• Off, not operating.
• On continuously.
• Automatic, per temperature change (default): the operator can define the start
temperature below which the window is heated. The default is 41 °F (5 °C). The
operator can define this temperature between 32 °F (0 °C) and 122 °F (50 °C). The
heating stops when the temperature is 27 °F (15 °C) above the start temperature.
For more information, see
1.4.3 HART
The flame detector uses the HART protocol.
HART communication is a bi-directional industrial field communication protocol used to
communicate between intelligent field instruments and host systems. HART is the global
standard for smart process instrumentation, and the majority of smart field devices
installed in plants worldwide are HART-enabled. HART is available in output configurations
1A, 2A, and 3A (see
Rosemount 975MR
EN54-10 approved by VdS
FM approved per FM3260
Configure the
®
protocol
Table 1-1).
detector.
Introduction
August 2019
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