120
Reference Manual
00809-0100-4021, Rev JA
HART Commissioning
April 2017
HART Commissioning
3.16.2 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 Protocol) 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 390 °F or 60 Ω 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
To Next Page
Loading...
