Why is 250 ohm resistance using for transmitter calibration?
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To develop enough impedance in the loop to get the HART protocol to work. HART requires a minimum loop resistance of 230 Ohms for HART’s FSK signal to be ‘seen’ by a HART modem circuit.
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To provide a means of checking loop current by reading the voltage drop across a precision resistor
The picture shows a typical configuration for calibrating a differential pressure (DP) transmitter supporting HART communication. This configuration comprises a 24V DC power supply, a 250-ohm resistor, a multimeter set in milliampere (mA) mode, a HART communicator, a pressure calibrator, and the differential pressure transmitter itself. Every part has a certain function that helps correct calibration and digital connection with the transmitter. Essential for both loop integrity and HART protocol operation, the 250-ohm resistor is at the heart of this system.
The 250-ohm resistor in a 4–20 mA analog loop produces a voltage drop ranging from 1V at 4 mA to 5V at 20 mA. The HART communicator’s detection of Frequency Shift Keying (FSK) signals superimposed on the analog current depends on this voltage drop. Lacking this resistor, the HART communicator would not operate since there would not be enough voltage differential, which would cause communication failures. For efficient communication, HART protocol calls for at least 230 ohms of loop impedance; 250 ohms has been the industry standard to guarantee compliance with this criteria.
The circuit’s multimeter measures the transmitter’s output current, which should change in accordance to the pressure calibrator’s applied pressure. This enables the checking and correction of the accuracy of the transmitter. During calibration, the HART communicator connects in parallel with the transmitter and resistor to set range, damping, and trimming. With the 250-ohm resistor inserted in series with the loop to permit both current measurement and digital transmission, the picture clearly shows how each component is linked. Industrial automation makes great use of this configuration to guarantee correct integration of smart transmitters into control systems as well as accurate calibration.