To address the discrepancy between the values shown by your PLC and HART communicator, it’s essential to understand the scaling and units involved. Let’s break down the process step-by-step, using the provided details:
- Understand the Transducer Range and Analog Output:
- LRV (Lower Range Value): 1.411 inH2O
- URV (Upper Range Value): 165.680 inH2O
- Analog Output Reading: 4.347 mA
- HART Communicator Reading:
- The HART communicator shows 5 inH2O.
- PLC Reading:
- The PLC reads around 700. We need to verify the units and scaling factors used by the PLC.
Step-by-Step Calculation:
1. Convert the Analog Signal (mA) to Pressure (inH2O)
The standard 4-20 mA signal corresponds to the range of the transducer:
- 4 mA = LRV = 1.411 inH2O
- 20 mA = URV = 165.680 inH2O
To find the pressure corresponding to 4.347 mA, use the following linear interpolation formula:
This matches closely with the 5 inH2O shown by the HART communicator, confirming that the HART reading is accurate.
2. Scaling the PLC Reading
To understand why the PLC displays 700, we need to determine its scaling settings. Here’s how to approach it:
- Verify the units used by the PLC. If the PLC is displaying a value of 700, it might not be in the same unit (inH2O) or it might be scaled differently.
- Ensure the PLC is correctly interpreting the 4-20 mA signal within the range of 1.411 to 165.680 inH2O.
If the PLC scaling is off, you can calculate the proper scaling factor:
- Determine the PLC Scaling Factor:
- Assume the PLC displays a raw value (e.g., counts from an ADC). To map this raw value to the pressure range, you need to use the transducer’s range.
- Calculate the Expected Raw Value for a Given Pressure:
- If the PLC uses an integer range (e.g., 0-1000, 0-4095, etc.), determine how this maps to the 4-20 mA signal.
For example, if the PLC raw value range is 0-1000 for 4-20 mA:
- 4 mA corresponds to raw value 0
- 20 mA corresponds to raw value 1000
Using the pressure calculated from the analog signal (4.347 mA), convert this to the PLC’s raw value:
If the PLC shows 700, this suggests either a scaling issue or a different interpretation of units. Double-check the PLC configuration for the input range and scaling factors.
Adjusting the Scaling Factor:
To properly scale the PLC readings to match the pressure in inH2O:
- Ensure the PLC input range is correctly set for 4-20 mA to correspond to 1.411-165.680 inH2O.
- Adjust the scaling in the PLC programming or configuration settings to map the raw input correctly.
Example Configuration:
- Set the PLC input to 4-20 mA.
- Configure the scaling such that 4 mA corresponds to 1.411 inH2O and 20 mA corresponds to 165.680 inH2O.
- Verify the linear interpolation matches the pressure calculated for various mA values.
By ensuring the correct scaling, the PLC reading should align closely with the HART communicator, and discrepancies should be minimized.
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Refer the below calculator to understand and calculations
1.Calculator for 4-20mA Signal to 1- 5 Volt and PLC 16-bit Raw Count Values
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Differences in the values displayed by a PLC (Programmable Logic Controller) and a HART (Highway Addressable Remote Transducer) communicator can arise due to several reasons, including calibration issues, signal interpretation, scaling factors, and data processing methods. Here’s a detailed explanation along with an example range in mmWC (millimeters Water Column) units:
1. Calibration Differences
- PLC Calibration: The PLC may not be calibrated as frequently or accurately as the HART communicator. Calibration drift over time can cause discrepancies between the two readings.
- HART Communicator Calibration: HART communicators often provide more precise calibration capabilities and can be used to perform field calibrations, potentially leading to more accurate readings.
2. Signal Interpretation and Scaling
- Analog Signal to Digital Conversion: The PLC interprets the 4-20 mA analog signal from the transmitter and converts it to a digital value. Any errors in this conversion process, such as incorrect scaling or signal noise, can cause discrepancies.
- HART Digital Signal: The HART communicator reads the digital signal directly from the transmitter, which can be more accurate because it avoids the analog-to-digital conversion step.
3. Data Processing
- Filtering and Averaging: PLCs often apply filtering and averaging to smooth out the readings, which can introduce slight differences compared to the real-time data shown by the HART communicator.
- Update Rates: The update rate of the PLC might differ from the HART communicator, leading to differences in displayed values at any given moment.
4. Communication Protocol Differences
- Analog vs. Digital Signals: The PLC primarily deals with analog signals, while the HART communicator uses digital communication, which can be more robust against noise and signal degradation.
Example Range in mmWC
Let’s assume we have a pressure transmitter configured to measure pressure in the range of 0 to 2000 mmWC, corresponding to a 4-20 mA output signal.
- PLC Reading: The PLC might display a pressure of 1000 mmWC.
- HART Communicator Reading: The HART communicator might display a pressure of 1005 mmWC.
Potential Reasons for the Difference:
- Calibration Offset: If the PLC or transmitter calibration is slightly off, this could cause a 5 mmWC discrepancy.
- Signal Noise: The analog signal read by the PLC might have noise or be affected by electrical interference.
- Scaling Error: The PLC might have an incorrect scaling factor set, leading to a small error in the displayed value.
- Data Refresh Rate: The HART communicator might show a more instantaneous value, while the PLC might display an averaged or filtered value.
To minimize discrepancies, it is crucial to regularly calibrate both the PLC and the transmitter, ensure correct scaling factors, and verify that the data processing methods align as closely as possible. Understanding these factors can help diagnose and correct any inconsistencies between the readings of a PLC and a HART communicator.
