Properly operating instruments are critical to plant safety and product quality. To calibrate process instruments, it is essential that controlled conditions and measurement signals present in the actual process installation are simulated.
Input and Output Measurement Standards
For an analog electronic pressure transmitter a variable signal source provides a pressure input at the same values as used in the process, and a precision input standard measures the calibration input. A regulator is used to precisely control the input pressure. The range of the transmitter determines the input measurement standard. The output of the transmitter is connected in a series circuit that furnishes the 24 VDC transmitter power and measures the output signal from the transmitter. With the output signal range of 4-20 mA, the most appropriate output measurement standard would be a milliammeter. The instrument should be mounted in the same position as it is installed in the process.
The input is connected so the precision gage indicates the pressure to the instrument under test. Locate the precision gage on a tee between the regulator and the transmitter. A bleed valve connected between the regulator and the transmitter releases pressure from the pneumatic input calibration circuit and allows the system to return to zero. The output of the transmitter is connected in series with the milliammeter and the power source.
Five Point Check
Calculate the input test points for the upscale and downscale check and the expected output values. The recommended five test points are typically 10%, 30%, 50%, 70%, and 90% of input span. Because 0% and 100% are extremes of the transmitter range, they are not recommended as test points.
Accuracy of the Instrument
With the five point check complete, use the resulting values to determine if any errors are present and calculate the accuracy.
Accuracy = (Deviation / Span ) * 100
Deviation = Expected Value - Actual Value
The zero shift is corrected first, since adjusting the span requires an accurate base point, or zero. Zero is typically adjusted at 10% input until 10% output is produced. Next, adjust the span with 90% of input pressure applied. With the input pressure at 90%, the span is adjusted to provide 90% of the output. Zero and span adjustments interact, continue correcting until both zero and span are correct.
Smart Pressure Transmitter Calibration Procedure
Smart transmitters can communicate with a hand held configuration instrument. Smart transmitters are calibrated at the factory. It is usually necessary to configure the transmitter to meet your process requirements.
Configuration enables the user to select the appropriate range for a specific process within the transmitter’s wide span. Reranging the pressure transmitter may be necessary to meet measurement requirements when the application of the transmitter changes or when a smart pressure transmitter is replaced. At the beginning of the configuration procedure it is usually necessary to enter information about the instrument and its function in the process. Once the upper and lower range values are entered and verified they can be downloaded directly to the transmitter.
Changing the Inputs
Enter the applicable measurement units by pressing the UNITS key on the interface device. Then, press ENTER and PROCEED, to confirm the units selected. Next, enter the upper and lower range values. Press the LRV key to select the lower range value. Then key in the value and press ENTER. The interface device will display the lower range value and is represented in the process as a 4 mA signal. The upper range value is set the same way, using the URV key. This selected value will now be represented in the process by a 20 mA signal