It is critical to check the calibration of all field devices periodically. By design, the HART Protocol provides commands to easily enable this function. It is one reason why HART technology is so popular. Calibration ensures that a field device provides a control system with the correct value of a process variable. The controller can then use this information to take action.
The calibration process consists of three major steps, as shown in the diagram below. They involve transforming and scaling the process variable, followed by production of the signal.
These steps are necessary because of sensor physics, measurement techniques and process variable dynamics. The HART Protocol provides standardized trim and related commands to facilitate the process.
Transforming the Process Variable
The first calibration step is the transducer block (see diagram above left). It transforms a variable – such as pressure, temperature or flow – into a digital representation of that parameter.
In this step, the digital value is generated from properties like the transducer upper and lower limits, trim points, and characterization data. In general, the field device is calibrated by providing a value from a traceable reference near the upper and lower limits. Trimming the resulting internal mathematical calculations compensates for any error. Process calibrators are available commercially to perform this function using the HART Protocol.
Scaling the Process Variable
The second step is referred to as the range block or "zeroing and spanning the device." This provides values for the upper and lower limits, as well as points in-between.
Here, the upper and lower range values are used to produce digital values. These will eventually correspond to a 4mA signal for the lower range and a 20mA signal for the upper range. Percent range, which covers everything between these two extremes, is developed in this step.
In addition, an appropriate transfer function – such as linear, square root, quadratic, cubic spline, and so on – may be applied as part of this scaling. The transfer function corrects for non-linear relationships between a process variable and a transducer.
In some cases, this is needed because the transducer is measuring a related variable. Square root functions, for example, are used to approximate a flow measurement from a differential pressure measurement. Note that this ranging of a field device is not the same as calibrating it.
Producing the Signal
The third step creates the final signal. Performed by the DAC (digital to analog conversion) block, this step produces the 4-20mA output. It ensures that 0% equals exactly 4mA and 100% equals 20mA. The signal thus generated corresponds to the process value. From a system perspective, this signal – and the fact that it corresponds to a particular process value – is the most critical function of the field device.
Be sure to follow the recommended calibration procedures provided by your device supplier. This information may also identify additional capabilities provided in your device.