Emerson Rosemount, Справочное руководство

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80
Reference Manual
00809-0100-4021, Rev GC
Section 3: HART commissioning
April 2014
HART® commissioning
Sensor drift alert configuration
Field communicator
The Sensor Drift Alert command allows the transmitter to set a warning flag (through HART), or
go into analog alarm when the temperature difference between Sensor 1 and Sensor 2 exceeds
a user-defined limit. This feature is useful when measuring the same process temperature with
two sensors, ideally when using a dual-element sensor. When Sensor Drift Alert mode is
enabled, the user sets the maximum allowable difference, in engineering units, between Sensor
1 and Sensor 2. If this maximum difference is exceeded, a Sensor Drift Alert warning flag will be
set.
When configuring the transmitter for Sensor Drift Alert, the user also has the option of
specifying that the analog output of the transmitter go into alarm when sensor drifting is
detected.
Note
Using dual sensor configuration in the 3144P, the temperature transmitter supports the
configuration and simultaneous use of the Hot Backup
®
feature and Sensor Drift Alert. If one
sensor fails, the transmitter switches output to use the remaining good sensor. Should the
difference between the two sensor readings exceed the configured threshold, the AO will go to
alarm indicating the sensor drift condition. The combination of Sensor Drift Alert and the Hot
Backup
®
feature improves sensor diagnostic coverage while maintaining a high level of
availability. Refer to the 3144P FMEDA report for the impact on safety.
Problem description: Sensors often drift before they fail. This causes issues because during the
drift period, the sensor is not reporting as accurate measurement. In control loops, and
especially safety loops this can lead to improper process control and potential safety hazards.
Our solution: The Sensor Drift Alert continuously monitors two sensor readings to detect a
drifting sensor. The diagnostic monitors the difference between the two sensors, and when the
difference becomes greater than a value entered by the user, the transmitter sends an alert to
indicate a sensor drift condition.
How it works: Two sensors are connected to a dual-input transmitter where the difference in
sensor readings is continuously being measured. A threshold is set by the user to determine
when an excessive drift (i.e. a significant delta) occurs between the two sensors. The
temperature delta between the two sensors is calculated by taking the absolute value of the
difference between sensor 1 and sensor 2. The user configures the transmitter to send a digital
alert or analog alarm when the alert has been triggered. The Sensor Drift Alert does not indicate
which sensor is failing. Rather the diagnostic provides an indication of a sensor drifting. The user
should view the individual sensor output trends on the host to determine which sensor is
drifting.
Take away: “Sensor Drift Alert detects a degrading sensor.”
Target applications: Redundant measurements, critical measurements, severe applications.
HART 5 Fast Keys
2, 2, 4, 2
HART 7 Fast Keys
2, 2, 4, 2