Smart Pressure Transmitter Calibration - Sensor Trim Basics

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In pressure transmitter calibration, sensor trim can be performed using either sensor or zero trim functions. Both trim functions alter the transmitter’s interpretation of the input signal. Also analog output trim is required to calibrate the output section of the transmitter.

Zero Trim

Zero trim is a single-point adjustment. It is useful for compensating for mounting position effects and is most effective when performed with the transmitter installed in its final mounting position. Zero trim should not be used in place of a sensor trim over the full sensor range. When performing a zero trim,

Basics of Smart Pressure Transmitter Calibration:

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Calibration is the process of optimizing transmitter accuracy over a specific range by adjusting the factory sensor characterization curve located in the microprocessor. Calibrating a smart transmitter is different from calibrating an analog transmitter. The one-step calibration process of an analog transmitter is done in several steps with a smart transmitter. These calibration steps involved are:

(a) Re-ranging - Re-ranging involves setting the lower and upper range points (4 and 20 mA) points at required pressures. Re-ranging does not change the factory sensor characterization curve.

(b) Analog Output Trim - This process adjusts the transmitter’s analog  characterization curve to match the plant standard of the control loop.

(c) Sensor Trim - This process adjusts the position of the factory characterization curve to optimize the transmitter performance over a specified pressure range or to adjust for mounting effects. Trimming has two steps, zero and sensor trims.

Factory Characterization Curve of Pressure Transmitter.

The characterization of a smart transmitter allows for permanent storage of reference information. In the factory setup, known pressures are applied and the transmitter stores information about these pressures and how the pressure sensor reacts to these pressure changes. This creates a transfer function of applied pressures versus output shown below:

How to Setup a Smart Transmitter Using a HART Communicator:

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Setting up a smart transmitter with a HART communicator is fairly simple and straightforward. However, one requirement is that you have to get yourself familiarized with the HART communicator if you are new to the device by reading the manual or better still get a tutorial from someone who has used the device previously.

The transmitter can be setup and commissioned before or after installation in the plant. In all cases, it is advisable to do a bench commissioning of the smart transmitter before installation to ensure proper operation, to familiarize yourself with transmitter functionality and to avoid exposing the transmitter electronics to a plant environment especially hazardous locations.

Commissioning the transmitter involves

HART Communication Commands

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The HART Command Set provides uniform and consistent communication for all field devices. The command set includes three classes: Universal, Common Practice, and Device Specific. Host applications may implement any of the necessary commands for a particular application.

Universal Commands

All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units).

Common Practice Commands

Common Practice commands provide functions implemented by many, but not necessarily all, HART communication devices.

Digital Communication Modes & Network Configurations in the HART Protocol

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The HART digital communication signal has a response time of approximately 2‐3 data updates per second without interrupting the analog signal. A minimum loop impedance of 230 Ω is required for communication though 250 Ω is typically used in practice. HART communication occurs in two modes:

(a) Request – Response Mode

(b) Burst Mode

Request-Response Mode

During normal operation (2‐3 data updates per second), each field device (slave) communication is initiated by a host (master) communication device. Two hosts can connect to each HART loop. The primary host is generally a distributed control system (DCS), programmable logic controller (PLC), or a personal computer (PC). The secondary host can be a handheld terminal or another PC. Field devices include transmitters, actuators and controllers that respond to commands from the primary or secondary host.

Basics of the HART Communication Protocol -Working principle

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The worldwide accepted standard for analog signals in the measurement industry is the 4 to 20mA signal. One key drawback of this signal standard is that it can only transmit one parameter or measured value.

A two-way or bidirectional communication protocol, which addresses this drawback, has been developed with which additional information can be transmitted using an alternating current signal superimposed on the 4 – 20mA analog signal. This system is called the HART communication protocol. HART stands for: Highway Addressable Remote Transducer. 

The HART communication protocol has become a widespread solution, allowing for convenient and efficient parameterization of smart (intelligent) measuring devices. Additionally, device-specific diagnostic data can be read which provides information about the device's physical health and allow for predictive maintenance. Monitoring various device parameters is also possibility with the HART protocol.