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:
|Factory Characterization Curve of a Smart Pressure Transmitter|
To understand why several calibration steps are required for a smart pressure transmitter, let us see how data flows within the transmitter.
How Data Flow Inside the Smart Pressure Transmitter.
Smart transmitters operate differently than analog transmitters. A smart transmitter uses a microprocessor that contains information about the sensor’s specific characteristics in response to pressure and temperature inputs. A smart transmitter compensates for these sensor variations. The diagram below shows the flow of data within the smart pressure transmitter in four basic steps:
|Smart Pressure Transmitter block diagram|
- A change in pressure is measured by a change in the sensor output (Sensor Signal)
- The sensor signal is converted to a digital format that can be understood by the microprocessor. This conversion is done by the Analog-to-Digital Signal Converter section(A/D) within the transmitter circuitry. Sensor trim functions affect this value.
- Corrections are performed in the microprocessor to obtain a digital representation of the process input (Digital PV).
- The Digital PV is converted to an analog value by the Digital-to-Analog Signal Converter section(D/A). Re-ranging and Analog trim functions affect this value.
Choosing the Right Trim Procedure for the Pressure Transmitter
To decide which trim procedure to use, you must first determine whether the analog-to-digital section(A/D) or the digital-to-analog section(D/A) of the transmitter electronics need calibration. To do so, perform the following procedure:
|Choosing the right sensor trim procedure|
- Connect a pressure source, a HART communicator, and a digital readout device to the pressure transmitter.
- Establish communication between the transmitter and the communicator.
- Apply pressure equal to the upper range point pressure (150 inH20, for example).
- Compare the applied pressure to the Process Variable (PV) line on the Communicator on-line display menu. IF the PV reading on the communicator does not match the applied pressure (with high-accuracy test equipment), then the transmitter requires a SENSOR TRIM
- Compare the Analog Output (AO) line on the communicator on-line menu to the digital readout device. IF the AO reading on the communicator does not match the digital readout device, then the pressure transmitter requires an OUTPUT TRIM. See How to calibrate Smart Transmitters for a detailed equipment setup on choosing the right sensor trim