What products are involved?


Rheonics density meter and viscometer using sensor electronics SME-DRM, SME-TRD, and SME-TR.


What devices are involved in getting sensor measurements to the Factory SCADA, MES, IIoT system?

Rheonics sensors have natively inbuilt digital interfaces that support industry standards like Modbus, Ethernet/IP, Profinet, HART, etc. Rheonics sensors also are available with native 4-20 mA channels. 4-20 mA communication is still the most widespread communication protocol used on plant floors, driven by the simplicity and popularity of the interface. When a 4-20 mA signal from the sensor is used, the digital measured value from the sensor is converted to the analog current signal and then transmitted to a PLC or IPC. The PLC makes the sensor measurement accessible to the factory or machine SCADA system. 

This article focuses on how to use the 4-20 mA signals from Rheonics sensors, which carry process fluid measurements such as density, viscosity, and temperature to a PLC while ensuring data integrity and resolution are maintained.

Is my 4-20mA signal correctly transmitting the measured value to the PLC?

As the sensor measurement is a digital value, the first conversion that happens on the sensor is the transfer of the digital value to an equivalent current. This is done by digital to analog converters built into the sensor electronics. The mechanism of this conversion is simple, it uses 4 mA for the lowest value and 20 mA for the highest value of the measurement referred to as the 4-20mA channel range. Rheonics sensors provide the capability for users to adjust the range of the 4-20 mA channels.

The next step is the transmission of the current over a 2-wire cable from the sensor to the PLC. Transmission of current is generally error-free till standard industry protocols for 4-20 mA current loops are followed. However, there are devices to check that the current loop between the sensor and PLC is functioning correctly.

Once PLC receives the current, it converts the analog current signal into a digital value. This is done using an Analog-to-Digital converter (ADC) built into the PLC 4-20 mA acquisition card. This conversion is done by using the current measured at PLC and interpolating it against the 4-20mA range and corresponding digital values set in the PLC.

Identifying issues in 4-20mA signal transmission

To check if the sensor measurement is correctly transmitted to the PLC, compare the digital value output directly by the sensor (over USB or Ethernet or other digital interface like Bluetooth or RS485) against the value that the PLC1. shows maintaining time sync for the two measurements. If the two values are same within the tolerance of the system then our settings are all correct. If you have a large discrepancy between the values that is outside the expected tolerance band or otherwise does not meet your precision requirement then an investigation of the cause is required.

The article now continues to investigate each of these aspects of the current transmission system, highlight issues that can pop up in each of the legs of the overall system, and how to diagnose and fix when there is a mismatch in the measured value as seen on the sensor and as received at the PLC.

Issue 1: Range Consistency

Ensure that the 4-20mA range settings on both the sensor and PLC are correctly matched, as mismatched settings are a common cause of discrepancies.

Fix 1: A common method for setting the 4-20mA signal range in the RCP software is through the Service Tab, where default values for each analog output channel can be viewed and adjusted as needed.


Figure 1. Service Tab-4-20mA settings

Fix 2: It’s crucial to correctly configure the 4-20mA output from the sensor in the PLC analog input. If these settings do not match, the PLC may misinterpret the sensor signal, resulting in inaccurate readings, process control errors, or missed alarms. Proper configuration ensures the PLC accurately captures the full range of data from the sensor, leading to reliable monitoring and control of your system. Rheonics has a few articles that describe the procedure to set the 4-20mA signals:  Analog signal Integration with Micro820 

Issue 2: Calibration Check

 RCP supports setting the 4-20mA channels to calibration modes for validation with a multimeter or a PLC.  

Figure 2. Calibration Check

Fix 1: Use the calibration check mode in RCP to validate the 4-20mA signals. Follow the instructions in the support article Analog signals setup with the RCP to confirm that the values sent by the SME are accurately received by the PLC. This step is crucial for troubleshooting and ensuring signal integrity.


Issue 3: Signal Resolution

Poor signal resolution due to incorrect scaling of the 4-20mA signal can result in undetected fine changes, leading to inaccurate data and potentially incorrect process control decisions.

Figure 3. Signal scaling and resolution.

Fix 1: Ensure that the 4-20mA signal is scaled correctly to avoid a range that is too wide. Proper scaling improves resolution and accuracy, helping to detect fine changes and ensure reliable process control decisions. 

We can summarize this investigation into 4-20 mA signal transmission issues in the following flowchart, which highlights the key steps involved in diagnosing and resolving discrepancies between sensor measurements and PLC readings.