Emerson Rosemount, Reference Manual

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124
Reference Manual
00809-0100-4021, Rev GC
Section 3: HART commissioning
April 2014
HART® commissioning
3.15.2 Active calibrator and EMF compensation
The transmitter operates with a pulsating sensor current to allow EMF compensation and
detection of open sensor conditions. Because some calibration equipment requires a steady
sensor current to function properly, the “Active Calibrator Mode” feature should be used when
an active calibrator is connected. Enabling this mode temporarily sets the transmitter to provide
steady sensor current unless two sensor inputs are configured. Disable this mode before putting
the transmitter back into the process to set the transmitter back to pulsating current. “Active
Calibrator Mode” is volatile and will automatically be disabled when a Master Reset is performed
(through HART) or when the power is cycled.
EMF compensation allows the transmitter to provide sensor measurements that are unaffected
by unwanted voltages, typically due to thermal EMFs in the equipment connected to the
transmitter, or by some types of calibration equipment. If this equipment also requires steady
sensor current, the transmitter must be set to “Active Calibrator Mode.” However, the steady
current does not allow the transmitter to perform EMF compensation and as a result, a
difference in readings between the active calibrator and actual sensor may exist.
If a reading difference is experienced and is greater than the plant’s accuracy specification
allows, perform a sensor trim with “Active Calibrator Mode” disabled. In this case, an active
calibrator capable of tolerating pulsating sensor current must be used or the actual sensors
must be connected to the transmitter. When the field communicator or AMS Device Manager
asks if an active calibrator is being used when the sensor trim routine is entered, select No to
leave the “Active Calibrator Mode” disabled.
In temperature measurement loops using RTDs, small voltages, called EMFs, can be induced on
the sensor wires, increasing the effective resistance and causing false temperature readings. For
example, a 12 mV reading equates to 390F or 60ohm error for a PT100 385 RTD. The Emerson
EMF Compensation detects these externally induced voltages and eliminates the erroneous
voltages from the calculations performed by the transmitters. Externally induced voltages
come from motors, calibration devices (dry block calibrator), etc.
How it works: Our transmitter provides RTD measurement updates at a rate of less than one
second for a single sensor. This measurement update consists of a series of smaller
measurement scans. A part of these smaller measurement scans is a check for EMF induced
voltage, up to 12 mV, on the sensor loop. The transmitter is designed to compensate out the
induced voltage up to 12 mV and provide a corrected temperature value. Beyond 12 mV, the
transmitter will notify the user that “Excess EMF” is present and warn them of possible
inaccuracies in the temperature measurement due to excessive induced voltage on the RTD
sensor loop. In the case of excessive EMF in the transmitter, it is recommended that the user
identify the external sources of electromagnetic interference and isolate them from the
transmitter and RTD sensor wiring.
HART 5 Fast Keys
3, 4, 4, 4
HART 7 Fast Keys
3, 4, 4, 4