Which Output Should You Choose for a Level Transmitter: 4–20 mA or RS485?

Choosing between 4–20 mA and RS485 output for a level transmitter directly affects signal stability, wiring costs, and system expandability. Technical evaluators should consider not only the interface type, but also the site distance, electromagnetic interference, control system type, number of measurement points, and future maintenance methods.

4–20 mA is a mature analog output method widely used in industrial applications. Its wiring is straightforward and it offers strong compatibility, making it particularly suitable for single-point measurement and conventional PLC control systems. RS485, centered on digital communication, allows multiple devices to be connected on the same bus and is better suited for centralized monitoring, long-distance data acquisition, and digital upgrade projects.

Xi'an Shenghongchuang Instrumentation Co., Ltd. and its subsidiary Shaanxi Qinkong Sensor Technology Co., Ltd. can provide matching recommendations based on the liquid medium, measuring range, installation position, power supply conditions, and signal protocol. Before selection, the actual site conditions should be clearly identified to avoid future retrofit difficulties caused solely by initial purchase price or interface preferences.

I. First Understand the Signal Logic of the Two Output Methods

4–20 mA represents level changes through current magnitude. For example, 0 meters corresponds to 4 mA, full scale corresponds to 20 mA, and intermediate levels are converted proportionally. Compared with 0–20 mA, 4 mA serves as a “live zero.” When the circuit is disconnected or the device malfunctions, the control end will normally detect a current below the normal range, facilitating fault identification.

RS485 itself is a physical communication interface, and level data are typically transmitted using protocols such as Modbus RTU. The controller can read the level value, temperature compensation value, fault status, device address, and parameter information. Register definitions may vary among manufacturers, so the communication protocol, baud rate, and data format should be verified in advance during project implementation.

Neither option has an absolute advantage over the other. 4–20 mA emphasizes simplicity and reliability, while RS485 emphasizes information capacity and networking capability. If the level transmitter needs to connect to an older analog input card, 4–20 mA should be evaluated first; if connection to a SCADA platform, IoT gateway, or multi-point monitoring system is required, RS485 generally offers greater room for expansion.

On-Site Characteristics of 4–20 mA Output

In industrial environments, the core advantage of 4–20 mA is its relatively strong interference resistance. Within a certain range, current signals are insensitive to voltage drops along the cable. When used with shielded cables, proper grounding, and isolation modules, it is suitable for level control applications in pump rooms, water tanks, storage tanks, wastewater pits, and general production lines.

For projects in which one level transmitter is connected to one PLC analog input channel, 4–20 mA wiring is generally easier to understand. Common systems use 24VDC power supplies. Two-wire products can complete both power supply and signal transmission through the same loop, and a multimeter can be used directly during commissioning to measure loop current.

However, the limitations of analog output also require attention. Each additional measurement point generally requires an additional cable set and analog channel. If there are 20 level measurement points on site, wiring quantity and PLC module costs will increase significantly. For projects requiring remote access to diagnostic information, the data provided by 4–20 mA are also relatively limited.

On-Site Characteristics of RS485 Output

RS485 uses differential transmission and can support relatively long communication distances when wired properly. In engineering applications, multiple level sensors are often connected in series to the same communication bus, with measurement points distinguished by different device addresses. For level monitoring projects distributed across multiple storage tanks, pools, or equipment areas, this can reduce part of the cable installation workload.

The value of digital communication lies not only in reading level values, but also in parameter management. Based on the product protocol, the system can read the measuring range, unit, status code, or abnormal information. Some models also support address setting, zero calibration, and communication parameter configuration. These capabilities facilitate centralized maintenance and equipment record management at later stages.

RS485 has higher requirements for installation quality. The bus should use a daisy-chain topology as far as possible, avoiding arbitrary star branches. For long-distance lines, terminal matching, shield treatment, and separation of power and signal circuits should be considered. Unstable communication may result from address conflicts, reversed A/B wires, improper grounding, inconsistent protocols, or on-site variable-frequency interference.

II. Select the More Suitable Output Based on Site Conditions

When selecting an output signal, first confirm what signals the control end can receive. If the on-site PLC is equipped only with analog input modules, or if the existing system already uses standard 4–20 mA loops, continuing to use the same type of output can reduce hardware replacement, program modification, and commissioning time.

If the project needs to monitor multiple level points simultaneously and upload data to a host computer, configuration software, or industrial gateway, RS485 is more convenient for centralized acquisition. Especially in storage tank areas, environmental water treatment, chemical auxiliary systems, and automated equipment, multi-point communication can reduce the use of analog modules inside the control cabinet.

The on-site interference environment must also not be overlooked. Large motors, variable-frequency drives, welding equipment, and high-power cables can easily generate electromagnetic interference. Regardless of the output selected, signal cables should be laid separately from power cables, and isolators, lightning protectors, or surge protection devices should be configured when necessary.

Situations Where 4–20 mA Should Be Prioritized

The first type is level control for a single tank, single pool, or single piece of equipment. Examples include pump start/stop control, high- and low-level alarms for water storage tanks, and hydraulic oil tank monitoring. In these applications, the control logic is clear and the level signal only needs to be sent to one PLC or display instrument, making 4–20 mA highly efficient to implement.

The second type is legacy equipment upgrade projects. Many conventional control cabinets already reserve 0–20 mA or 4–20 mA analog channels. Current-output level transmitters can directly replace the original equipment. For production sites with limited downtime, this retrofit approach is generally easier to manage in terms of risk.

The third type is applications where operators need to quickly identify faults. Under normal loop conditions, the signal generally remains within the 4 mA to 20 mA range. If a value close to 0 mA or a clearly abnormal value is detected, priority can be given to checking for broken wires, power supply issues, loose terminals, and sensor faults. Of course, specific alarm thresholds should be set by the control program according to the actual measuring range.

Key Points for 4–20 mA Installation Verification

Before installation, verify the supply voltage, load resistance, and input module type. Some two-wire level transmitters have requirements for total loop load. Excessive cable length, too many connected devices, or insufficient power supply may result in inadequate full-scale output, causing low or fluctuating displayed values.

For submersible level transmitters, the end of the vented cable should be kept dry and unobstructed and should not be immersed in water or blocked. If reference pressure compensation is obstructed, atmospheric pressure changes may affect measurement results. The installation position should also avoid areas with strong liquid flow impact, stirring vortices, and concentrated sediment.

During commissioning, the current values at empty level, intermediate level, and near full-scale level can be checked separately. Taking a 0–5 meter range as an example, 2.5 meters theoretically corresponds to approximately 12 mA. If site process conditions permit, comparison can be performed using standard level points. Calibration operations should be carried out according to the specific product instructions, and sealed structures should not be dismantled without authorization.

Situations Where RS485 Should Be Prioritized

The first type is centralized monitoring of multiple storage tanks. For example, when a project needs to collect data from 8, 16, or more level sensors, RS485 can distinguish each measurement point through device addresses. The control system only needs to be configured with the corresponding communication interface and protocol parsing program, and future device additions are also easier to plan.

The second type is applications requiring detailed data management. Digital level sensors can transmit richer information to the host computer, facilitating trend recording, anomaly traceability, and remote inspection. For environmental monitoring, intelligent equipment, logistics and warehousing, and unattended pumping stations, these functions help improve operation and maintenance efficiency.

The third type is projects with limited control cabinet space or high analog module costs. RS485 does not eliminate the need for power cables, but it can reduce the need for multiple devices to separately return analog signals. The actual savings depend on measurement point distribution, cable routing, and controller configuration, and the solution should be evaluated as a whole during design.

Key Conditions for Stable RS485 Communication

Communication cables should preferably be shielded twisted-pair cables meeting site requirements, with A and B wires kept consistent throughout the entire link. Device addresses must be unique. Common communication parameters include 9600bps and 19200bps, but the final settings should be based on the level transmitter protocol documentation and control system configuration.

When the bus is long or there are many devices, attention should be given to end-of-line termination resistor settings. A 120-ohm matching resistor is commonly used in engineering, but whether it should be installed, as well as its location and quantity, should be determined according to the actual communication architecture. Blindly adding termination resistors to every device may instead affect signal quality.

Communication fault troubleshooting can proceed in the sequence of “power supply, wiring, address, parameters, protocol, and interference.” First confirm that sensor power supply is normal, then check A/B polarity and terminal crimping, followed by verification of slave address, baud rate, parity bit, and register address. This can improve on-site troubleshooting efficiency.

III. Liquid Medium and Product Structure Also Determine Reliability

The output method addresses signal transmission, while sensor materials and structure determine medium compatibility. Clean water, wastewater, diesel, acid and alkali solutions, high-temperature liquids, and slurry containing impurities have different requirements for diaphragm materials, sealing components, probe structures, and installation methods.

For corrosive liquids, whether a ceramic capacitive level sensor is suitable cannot be determined solely based on a “corrosion-resistant” description. The medium name, concentration, temperature, pressure, crystallization characteristics, particle content, and wetted-part materials should be specified, after which technical personnel can confirm the compatibility of the diaphragm, housing, and sealing materials.

For mining areas, flammable and explosive environments, or areas containing combustible gases or dust, the explosion-proof type, rating, certificate number, and applicable environment of the product must be confirmed according to project requirements. Ordinary industrial level sensors must not be used as substitutes for explosion-proof products, nor should the explosion-proof rating be judged solely from the model name.

Application Limits of Intrinsically Safe Level Sensors

Intrinsically safe level sensors are generally used in areas with explosion hazards, but whether the complete system is compliant depends not only on the sensor itself. Safety barriers, power supply equipment, wiring methods, grounding measures, and installation area classification must all meet the requirements of applicable standards and project design documents.

For petrochemical auxiliary equipment, fuel storage, solvent tanks, and certain mining applications, project parties should provide suppliers with the hazardous area classification, medium characteristics, installation position, and control system requirements. Based on this information, Xi'an Shenghongchuang can assist in confirming the level sensor model, output method, and matching isolated safety barrier solution.

After installation, intrinsically safe products still require loop verification, including safety parameter matching, shield grounding, and line isolation. Especially when RS485 communication is also used, it should be confirmed whether relevant isolation equipment is compatible with the communication rate and protocol, to prevent improper non-intrinsically-safe-side configuration from affecting data acquisition stability.

IV. How to Reduce Selection Rework from Procurement to After-Sales Service

Before purchasing a level transmitter, it is recommended to provide complete parameters at one time: measurement medium, range, accuracy requirements, installation method, probe length, output signal, supply voltage, process connection, ambient temperature, and protection requirements. The more complete the parameters, the more accurate the model matching and the lower the probability of subsequent rework.

For example, 0–5V level sensors can be used for some equipment integration applications or specific control board input scenarios, but their signal stability can be affected by the power supply reference, cable length, grounding quality, and controller sampling accuracy. If the transmission distance is long or on-site interference is obvious, 4–20 mA and RS485 solutions should generally be compared at the same time.

The supply voltage range of wide-voltage level sensors should also be confirmed according to the specific model. Common designs may cover 12–36VDC or a narrower range, but general descriptions must not be used directly as the basis for a project. Particularly in battery-powered, solar-powered, or highly fluctuating voltage applications, sufficient power supply margin should be reserved.

What Should Be Considered for After-Sales Maintenance?

The after-sales response efficiency for submersible level transmitters is generally related to whether fault information is complete. When reporting a fault, maintenance personnel should provide the product model, factory serial number, installation photos, supply voltage, current output value, actual liquid level, and time of failure. This information helps quickly determine whether the issue is related to installation, wiring, or an abnormal product condition.

The service life of level sensors for corrosive liquids is closely related to medium conditions. Even with the same acid or alkali medium, a concentration change from 10% to 40%, or a temperature increase from ambient temperature to 80 degrees Celsius, may alter material resistance performance. A regular inspection system should be established to check the diaphragm, cables, sealing areas, and signal fluctuations.

When abnormal data occur with a digital level sensor, it is recommended to retain communication logs or host computer screenshots first. Compared with simply stating “no data,” providing the device address, register reading, error code, and communication parameters allows technical support personnel to identify the issue more quickly and shorten on-site downtime for troubleshooting.

Final Selection Recommendations

For projects prioritizing single-point measurement, conventional PLC connection, and easy maintenance, a 4–20 mA level transmitter is recommended. Its loop structure is mature and can be easily checked by on-site personnel, making it suitable for most conventional industrial automation control requirements.

For multi-point monitoring, platform integration, remote operation and maintenance, and digital upgrade projects, RS485 level transmitters should be evaluated first. The prerequisite is that the control system has protocol processing capability and that the installation team can complete wiring, address configuration, and interference suppression in accordance with communication bus specifications.

If it is necessary to determine whether to use 4–20 mA, RS485, 0–5V, or another output method, the medium, measuring range, site photos, control system model, and number of measurement points can be submitted to Xi'an Shenghongchuang technical personnel, who can provide specific level sensor selection, wiring, and matching recommendations based on the installation environment and application objectives.