4–20 mA vs. Modbus Water Sensors: Which Is Better?

4–20 mA vs. Modbus Water Sensors: Which Is Better?

4–20 mA is simple, robust and extremely well established in industrial automation. Modbus, on the other hand, allows sensors to become intelligent devices that can provide significantly more information than just a measurement.

For modern, multi-sensor water monitoring systems, Modbus is often the more flexible architecture, while 4–20 mA remains an excellent choice for simple and highly standardized installations.

What Is 4–20 mA?

A 4–20 mA sensor communicates its measurement using an analog current signal.

Typically:

• 4 mA = minimum measurement

• 20 mA = maximum measurement

For example, a dissolved oxygen sensor configured for 0–20 mg/L could output:

The controller measures the current and converts it back into the corresponding measurement.

The important characteristic is that the sensor is essentially sending one analog value to the controller.

Why 4 mA instead of 0 mA?

The 4 mA "live zero" provides an important diagnostic capability.

For example:

• 4 mA → valid measurement at the bottom of the range

• 12 mA → measurement somewhere in the middle

• 20 mA → measurement at the top

• 0 mA → potentially broken wire, disconnected sensor or power failure

This makes 4–20 mA particularly useful in industrial environments.

What Is Modbus?

Modbus is a digital communication protocol. For water sensors, one of the most common implementations is Modbus RTU over RS485.

Instead of representing the measurement as an electrical current, the sensor sends digital data to the controller.

A sensor might expose registers containing:

Register → Value

DO → 9.4 mg/L
Temperature → 22.6 °C
Sensor status → OK
Calibration status → Valid

Register → Value

DO → 9.4 mg/L
Temperature → 22.6 °C
Sensor status → OK
Calibration status → Valid

Register → Value

DO → 9.4 mg/L
Temperature → 22.6 °C
Sensor status → OK
Calibration status → Valid

The controller can request this information from the sensor whenever it is needed.

Multiple devices can also share the same RS485 bus, with each Modbus device having a unique address.

4–20 mA vs. Modbus: The Fundamental Difference

The simplest way to think about the two technologies is:

4–20 mA = a measurement represented by an analog current

Modbus = digital communication with an intelligent device

This distinction becomes increasingly important as water monitoring systems become more sophisticated.

1. Number of Parameters

This is one of the biggest differences.

A 4–20 mA output generally represents one measurement.

A water sensor might therefore have:

• 4–20 mA output → dissolved oxygen

• another 4–20 mA → temperature

If you want pH, ORP, conductivity and turbidity, you may need additional outputs or additional devices.

A Modbus sensor can potentially expose many parameters through the same communication connection.

For example, one multi-parameter probe could provide:

• pH

• ORP

• temperature

• conductivity

• salinity

• dissolved oxygen

• turbidity

All through a single digital communication interface.

Advantage: Modbus

2. Modbus Sensors Can Perform Calculations

One of the less obvious advantages of Modbus sensors is that they can contain their own processing and computing capabilities.

A 4–20 mA output generally communicates the final analog value. More sophisticated processing normally has to happen elsewhere, such as inside the PLC or controller.

A digital sensor, however, can perform calculations internally before sending the result.

For example, a sensor may perform:

• temperature compensation

• pressure compensation

• salinity compensation

• conductivity compensation

• pH compensation

• sensor-specific linearization

• averaging and filtering

• conversion between measurement units

• calculated parameters

• sensor diagnostics

This means the sensor isn't simply a measurement device. It can act as an intelligent measurement node.

For example:

Raw sensor data
       ↓
Sensor processing
       ↓
Temperature compensation
       ↓
Calibration correction
       ↓
Measurement calculation
       ↓
Modbus
       ↓
PLC / Edge computer

Raw sensor data
       ↓
Sensor processing
       ↓
Temperature compensation
       ↓
Calibration correction
       ↓
Measurement calculation
       ↓
Modbus
       ↓
PLC / Edge computer

Raw sensor data
       ↓
Sensor processing
       ↓
Temperature compensation
       ↓
Calibration correction
       ↓
Measurement calculation
       ↓
Modbus
       ↓
PLC / Edge computer

This can simplify the controller and, importantly, keep sensor-specific algorithms inside the sensor where the manufacturer can maintain and validate them.

3. Sensor Diagnostics and Information

A major limitation of a basic 4–20 mA interface is that the controller primarily receives the measurement.

Suppose a dissolved oxygen sensor outputs 12 mA.

The controller knows the measurement, but the analog signal alone doesn't tell it much about why that measurement is what it is.

A Modbus sensor can expose considerably more information:

• measurement

• temperature

• sensor status

• error codes

• calibration status

• calibration date

• operating hours

• sensor serial number

• firmware information

• diagnostic values

• measurement quality

• maintenance information

This can be extremely valuable for remote monitoring and predictive maintenance.

4. Calibration History and Compliance

This is particularly important in professional water monitoring applications.

An intelligent digital sensor can potentially store information such as:

• when it was calibrated

• who performed the calibration

• calibration values

• calibration standards used

• previous calibration

• calibration status

• calibration due date

• sensor serial number

The exact functionality depends on the sensor manufacturer, but the underlying architecture makes this kind of information possible.

This becomes important when systems need to demonstrate traceability and compliance.

For example, instead of simply seeing:

DO = 8.7 mg/L

a monitoring platform could also know:

DO = 8.7 mg/L
Sensor: SN123456
Last calibrated: 12 August 2026
Calibrated by: Technician A
Calibration status: Valid

That creates a much stronger audit trail than a simple analog value.

5. Wiring

A basic 4–20 mA installation is conceptually simple:

Sensor
  │
  │ 4–20 mA
  │
PLC / Controller

Sensor
  │
  │ 4–20 mA
  │
PLC / Controller

Sensor
  │
  │ 4–20 mA
  │
PLC / Controller

However, every analog output requires an appropriate analog input channel.

With multiple sensors, the number of wires and I/O channels can grow quickly.

Modbus works differently:

              ┌── Sensor 1
              │
PLC ─ RS485 ──┼── Sensor 2
              │
              ├── Sensor 3
              │
              └── Sensor 4

              ┌── Sensor 1
              │
PLC ─ RS485 ──┼── Sensor 2
              │
              ├── Sensor 3
              │
              └── Sensor 4

              ┌── Sensor 1
              │
PLC ─ RS485 ──┼── Sensor 2
              │
              ├── Sensor 3
              │
              └── Sensor 4

Multiple devices can share the same RS485 network.

Advantage: Modbus for larger systems

6. Accuracy

It is not correct to say that Modbus sensors are inherently more accurate than 4–20 mA sensors.

The sensor itself is usually the dominant factor in measurement accuracy.

However, 4–20 mA introduces an additional analog conversion chain:

Sensor
  ↓
Measurement
  ↓
DAC
  ↓
4–20 mA
  ↓
PLC analog input
  ↓
ADC
  ↓
Digital value

Sensor
  ↓
Measurement
  ↓
DAC
  ↓
4–20 mA
  ↓
PLC analog input
  ↓
ADC
  ↓
Digital value

Sensor
  ↓
Measurement
  ↓
DAC
  ↓
4–20 mA
  ↓
PLC analog input
  ↓
ADC
  ↓
Digital value

With Modbus, the measurement can be transmitted digitally:

Sensor
  ↓
Measurement
  ↓
Digital data
  ↓
RS485
  ↓
Controller

Sensor
  ↓
Measurement
  ↓
Digital data
  ↓
RS485
  ↓
Controller

Sensor
  ↓
Measurement
  ↓
Digital data
  ↓
RS485
  ↓
Controller

This eliminates the need to convert the measurement to an analog current and then convert it back into a digital value at the controller.

Advantage: Modbus for digital architectures

7. Cable Distance and Industrial Robustness

4–20 mA is extremely well established for industrial applications and is highly resistant to many common electrical interference problems.

RS485 is also designed specifically for robust industrial communication.

Both technologies can support long cable runs when properly installed.

With Modbus, however, greater attention must be paid to:

• cable type

• shielding

• grounding

• bus topology

• termination

• baud rate

• communication parameters

The result is that both can be extremely reliable, but 4–20 mA is often simpler to deploy, while Modbus provides considerably more functionality.

8. Failure Detection

4–20 mA has a useful built-in concept of a live zero.

For example:

0 mA       → possible broken wire / power failure
<4 mA      → possible fault / out-of-range condition
4–20 mA    → normal measurement
>20 mA     → possible fault / out-of-range condition

0 mA       → possible broken wire / power failure
<4 mA      → possible fault / out-of-range condition
4–20 mA    → normal measurement
>20 mA     → possible fault / out-of-range condition

0 mA       → possible broken wire / power failure
<4 mA      → possible fault / out-of-range condition
4–20 mA    → normal measurement
>20 mA     → possible fault / out-of-range condition

Modbus provides a different and much richer diagnostic mechanism.

The controller can detect:

• no response

• communication timeout

• CRC errors

• invalid data

• sensor error codes

• diagnostic flags

• sensor status

• calibration problems

Therefore, Modbus can provide considerably more information about the health of the measurement system, rather than just the measurement itself.

9. Scalability

This is where Modbus becomes particularly attractive.

Imagine a water monitoring system containing:

• dissolved oxygen

• pH

• ORP

• conductivity

• turbidity

• temperature

• ammonia

• dissolved ozone

Using 4–20 mA, you could require a large number of analog input channels.

With Modbus, many devices can communicate through RS485 networks.

The architecture becomes:

                    ┌── DO
                    ├── pH
                    ├── ORP
Controller ─ RS485 ─┼── EC
                    ├── Turbidity
                    ├── Temperature
                    └── Other sensors

                    ┌── DO
                    ├── pH
                    ├── ORP
Controller ─ RS485 ─┼── EC
                    ├── Turbidity
                    ├── Temperature
                    └── Other sensors

                    ┌── DO
                    ├── pH
                    ├── ORP
Controller ─ RS485 ─┼── EC
                    ├── Turbidity
                    ├── Temperature
                    └── Other sensors

This makes Modbus particularly suitable for systems expected to grow over time.

10. Cost

The answer depends heavily on the sensor and system architecture.

A basic 4–20 mA transmitter can be inexpensive.

However, the complete system also requires:

• analog input hardware

• additional wiring

• one input channel per analog signal

• potentially additional I/O modules

A Modbus sensor may have a higher purchase price, but it can reduce the amount of I/O hardware required and provide substantially more information.

For a small system with one or two measurements, 4–20 mA can therefore be very economical.

For a larger monitoring platform, Modbus can become more cost-effective.

11. Modbus Enables Dynamic Sensor Management

This is an important distinction between a traditional PLC-based architecture and a modern software-defined control system.

In a conventional PLC implementation, adding a Modbus device generally means configuring the device's address, registers, communication jobs and associated PLC data structures in the project. Siemens' own Modbus RTU example, for instance, requires communication jobs and slave information to be added when additional slaves are introduced.

At Waboost, we take a different approach with the Waboost PLC.

Instead of hard-coding every sensor into the control software, we maintain a map of Modbus sensors.

The map defines things such as:

Sensor
├── Manufacturer
├── Model
├── Modbus ID
├── Register map
├── Parameters
├── Units
├── Calibration
└── Configuration

Sensor
├── Manufacturer
├── Model
├── Modbus ID
├── Register map
├── Parameters
├── Units
├── Calibration
└── Configuration

Sensor
├── Manufacturer
├── Model
├── Modbus ID
├── Register map
├── Parameters
├── Units
├── Calibration
└── Configuration

The PLC software can then use this information dynamically.

This means sensors can be added or removed without modifying the core control software.

For example, a system could contain:

Sensor Map

ID 1 → Dissolved Oxygen
ID 2 → pH
ID 3 → ORP
ID 4 → Conductivity

Sensor Map

ID 1 → Dissolved Oxygen
ID 2 → pH
ID 3 → ORP
ID 4 → Conductivity

Sensor Map

ID 1 → Dissolved Oxygen
ID 2 → pH
ID 3 → ORP
ID 4 → Conductivity

A customer could then add another sensor:

ID 5 → Turbidity

ID 5 → Turbidity

ID 5 → Turbidity

without requiring a new PLC program specifically written for that machine configuration.

Multiple Sensors of the Same Model

This architecture also makes it much easier to deploy multiple sensors of the same type.

For example:

ID 1 → DO Sensor
ID 2 → DO Sensor
ID 3 → DO Sensor
ID 4 → DO Sensor

ID 1 → DO Sensor
ID 2 → DO Sensor
ID 3 → DO Sensor
ID 4 → DO Sensor

ID 1 → DO Sensor
ID 2 → DO Sensor
ID 3 → DO Sensor
ID 4 → DO Sensor

The software doesn't need four different implementations of a dissolved oxygen sensor. It has one definition of the sensor model and multiple instances with different Modbus addresses.

This is particularly useful for machines containing multiple tanks, columns or treatment stages.

What about traditional PLCs like Siemens?

Traditional PLCs absolutely can communicate with multiple Modbus devices and use different slave addresses. The Modbus protocol requires unique addresses, and Siemens exposes the slave address as part of its Modbus communication configuration.

The difference is architectural.

A traditional PLC application typically has the device configuration, communication jobs and data structures incorporated into the PLC project.

Traditional PLCs can be programmed to support dynamic Modbus device configuration, but this typically requires considerably more custom PLC engineering. Waboost is designed around a configuration-driven architecture where sensor definitions, Modbus addresses and register maps are treated as data rather than being hard-coded into the control application

The Waboost Approach: Sensors as Software-Defined Devices

This allows the Waboost PLC to function more like a sensor platform than a traditional fixed PLC program.

Instead of:

PLC Program
│
├── DO Sensor 1
├── DO Sensor 2
├── pH Sensor
├── ORP Sensor
└── Conductivity Sensor

PLC Program
│
├── DO Sensor 1
├── DO Sensor 2
├── pH Sensor
├── ORP Sensor
└── Conductivity Sensor

PLC Program
│
├── DO Sensor 1
├── DO Sensor 2
├── pH Sensor
├── ORP Sensor
└── Conductivity Sensor

the architecture becomes:

                 Waboost PLC
                      │
                Sensor Manager
                      │
                Sensor Map
                      │
       ┌──────────────┼──────────────┐
       ↓              ↓              ↓
   Sensor Model   Sensor Model   Sensor Model
       │              │              │
      ID 1           ID 2           ID 3

                 Waboost PLC
                      │
                Sensor Manager
                      │
                Sensor Map
                      │
       ┌──────────────┼──────────────┐
       ↓              ↓              ↓
   Sensor Model   Sensor Model   Sensor Model
       │              │              │
      ID 1           ID 2           ID 3

                 Waboost PLC
                      │
                Sensor Manager
                      │
                Sensor Map
                      │
       ┌──────────────┼──────────────┐
       ↓              ↓              ↓
   Sensor Model   Sensor Model   Sensor Model
       │              │              │
      ID 1           ID 2           ID 3

The same software can therefore support different machine configurations without creating a completely different PLC program for every machine.

This is especially powerful when building a platform intended to integrate with many different industrial applications.

Comparison

Which Should You Choose?

There isn't a universal winner.

Choose 4–20 mA when:

• You have a simple installation.

• You only need one measurement per channel.

• You are connecting directly to a PLC.

• Maximum simplicity is important.

• You need a traditional industrial interface.

• You don't need extensive sensor diagnostics.

For example:

One dissolved oxygen transmitter → one PLC analog input.

4–20 mA is an excellent solution.

Choose Modbus when:

• You have multiple sensors.

• You need multiple measurements from one device.

• You want sensor diagnostics.

• You want calibration and maintenance information.

• Sensors perform compensation or calculations.

• You want remote configuration.

• You are building an IoT/cloud-connected system.

• You expect the system to expand.

• You want sensors to be dynamically managed by software.

For a modern water monitoring platform, Modbus is generally the more powerful architecture.

The Best Approach May Be Both

It doesn't have to be 4–20 mA vs. Modbus.

A robust industrial system can support both.

For example:

                    Water Sensors
                         │
              ┌──────────┴──────────┐
              │                     │
          Modbus RS485           4–20 mA
              │                     │
              ▼                     ▼
       Modbus Interface       Analog Input
              │                     │
              └──────────┬──────────┘
                         ▼
                    Waboost PLC
                         │
                         ▼
                 Waboost Platform

                    Water Sensors
                         │
              ┌──────────┴──────────┐
              │                     │
          Modbus RS485           4–20 mA
              │                     │
              ▼                     ▼
       Modbus Interface       Analog Input
              │                     │
              └──────────┬──────────┘
                         ▼
                    Waboost PLC
                         │
                         ▼
                 Waboost Platform

                    Water Sensors
                         │
              ┌──────────┴──────────┐
              │                     │
          Modbus RS485           4–20 mA
              │                     │
              ▼                     ▼
       Modbus Interface       Analog Input
              │                     │
              └──────────┬──────────┘
                         ▼
                    Waboost PLC
                         │
                         ▼
                 Waboost Platform

This provides compatibility with traditional industrial equipment while allowing modern digital sensors to take advantage of their full capabilities.

For our new multi-sensor water-monitoring system, Modbus/RS485 makes a strong primary interface for water sensors, while 4-20 mA sensors are used inside the machine, for things like oxygen concentration, pressure sensors and so on.

Check out our Sensor Box, which manages data collection for you from start to finish.