4–20 mA, 0–10 V or Resistance | Sensor Output Signal?

Differences, Advantages and Applications of Output Signals for Level Sensors Explained Simply.

4–20 mA, 0–10 V or Resistance? Comparing Output Signals of Level Sensors

A level sensor detects the height of a liquid in a container. In order for this measured value to be processed by a controller, display or other evaluation electronics, it must be converted into a suitable electrical output signal. Common variants include resistance signals, 0–10 V and 4–20 mA. There is therefore no universally best solution. The decisive factor is always the overall system consisting of the sensor, cable, power supply and evaluation electronics.

Which signal is best suited depends, among other things, on the cable length, interference environment, existing controller and the desired diagnostic capabilities. Depending on the version, our transmitters can be implemented with resistance, current or voltage outputs and adapted to the respective application.

What Is the Function of the Output Signal of a Level Sensor?

With continuous level measurement, the aim is not only to detect whether a specific limit level has been reached. Instead, the current level is measured across a defined measuring range.

With our transmitters, a float moves along a guide tube. The magnet in the float activates the reed contacts arranged inside. A level-dependent electrical signal is generated via a resistor chain.

This signal can either be output directly as a resistance value or electronically converted into a standardized current or voltage signal. Which variant is suitable depends on how the measured value is subsequently processed.

You can find more information about the underlying measuring principle on our page about continuous level measurement.

Resistance, 0–10 V and 4–20 mA in Direct Comparison

The three output variants basically perform the same task: They electrically represent the measured level. The main differences lie in signal conditioning, wiring, interference immunity and evaluation.

Comparison

Widerstand (min.-max. Ohm)

Advantages:
simple, passive design; direct evaluation possible

Particularly Suitable for:
applications with existing resistance evaluation and short signal paths
Voltage (0-10V)

Advantage:
easy to evaluate; can be used directly with many controllers

Particularly Suitable for:
short to medium cable runs and environments with manageable interference
Current (4-20mA)

Advantages:
high interference immunity; well suited for longer cable runs; diagnostic capabilities

Particularly Suitable for:
industrial systems and demanding signal transmission

Hall Sensors as Mini Transmitters

Compact level measurement with Hall sensors: precise signal generation, high vibration resistance and integration into 4 to 20 mA systems.

Hall Sensors as Mini Level Sensor

Resistance Output: Simple and Passive

With a resistance output, the electrical resistance changes depending on the fill level. The operating principle is comparable to a potentiometer.

A key advantage is the simple design without additional electronic signal conversion directly at the sensor. If the downstream controller or display is already designed to evaluate a variable resistance, this can be an economical and technically straightforward solution.

With longer cable runs, however, it must be taken into account that cable and contact resistances can influence the measurement result. For greater distances or electrically demanding industrial environments, a standardized current signal is therefore often more advantageous.

0–10 V Signal: Straightforward Voltage Transmission

With a 0–10 V output, the measuring range is proportionally converted into a voltage between 0 and 10 V.

In simplified terms, for example:

0 V = empty
5 V = 50% fill level
10 V = full

The main advantage is simple processing. Many PLCs, controllers and measuring devices have corresponding analog voltage inputs.

However, voltage signals are more sensitive to voltage drops and electrical interference than a current loop. The longer the cable and the more demanding the electromagnetic environment, the more important suitable cabling and signal routing become.

4–20 mA Signal: Robust for Industrial Applications

The 4–20 mA signal is one of the established standards in industrial process measurement technology. Unlike a voltage output, it is not the voltage but the current flowing through the measuring loop that is evaluated as the measured variable.

Typically, the measuring range is represented as follows:

4 mA = start of measuring range or 0%
12 mA = 50%
20 mA = end of measuring range or 100%

A sensor with a 4–20 mA output is particularly suitable for systems where the signal must be transmitted reliably over longer cable runs.

Since the same current flows at all points connected in series within a current loop, moderate voltage drops across the cable resistance have no direct influence on the transmitted measured value. This makes the 4–20 mA signal particularly robust against cable effects and electrical interference.

How Does a 4–20 mA Signal Work?

A 4–20 mA transmitter converts the detected measured value into a proportional current. The connected controller measures this current and calculates the corresponding fill level from it.

For a linearly scaled tank, for example:

4 mA = 0 mm or empty
8 mA = 25%
12 mA = 50%
16 mA = 75%
20 mA = 100% or full

How the fill level is subsequently displayed or processed further depends on the scaling of the controller.

For correct operation, the current loop must be considered as a complete system. Supply voltage, cable resistance and input resistance of the evaluation unit must be matched to each other so that the sensor can supply the required current across the entire measuring range.

Why Does the Signal Start at 4 Instead of 0 mA?

The fact that the measuring range starts at 4 mA rather than 0 mA is an important advantage of the current loop.

A regular measured value of 0% still corresponds to a current of 4 mA. If the current is completely absent, this can therefore indicate a fault such as a cable break or power supply failure.

This principle is referred to as “Live Zero”. This allows the measured value and a basic fault condition to be distinguished from each other.

With a classic 0–10 V signal, this distinction is less clear: 0 V can represent either an actual measured value of 0% or an electrical fault.

Distinguishing Measured Values and Fault Signals

Another advantage of analog current signals is that, in addition to the actual measuring range, defined values outside 4–20 mA can be used for diagnostic purposes.

With a customer-specific version, for example, an additional fault condition can be provided. A signal of 21 mA would be conceivable if the float is no longer detected correctly or has fallen off.

The controller can therefore distinguish between a normal measured value and a defined fault condition and, for example, issue a warning message.

Which diagnostic values, signal ranges and electrical characteristics are appropriate depends on the respective system. For this reason, we can also individually adapt our transmitters in terms of signal conditioning.

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Which Output Signal Is Suitable for Which Application?

The selection should not be based solely on which signal type appears to be more powerful in principle. What matters is which requirements exist in the specific system.

Cable Length and Electromagnetic Interference

For short cable runs in a controlled electrical environment, resistance or voltage signals can be completely sufficient.

However, as the cable length increases, the influence of cable resistances, electromagnetic interference and potential differences also increases.

In industrial systems with longer cable runs, motors, frequency converters or other potential sources of interference, the 4–20 mA signal is therefore often preferred. It was specifically established for reliable analog signal transmission under such conditions.

Even with a current loop, however, the permissible load and available supply voltage must be taken into account. “Insensitive to cable resistance” therefore does not mean that arbitrarily long cables are possible without electrical design considerations.

Controller, Analog Input and Existing Evaluation Electronics

The existing controller often already determines which signal is particularly suitable.

If a PLC, for example, has a free 4–20 mA analog input, a corresponding transmitter can be integrated directly. The same applies to an existing 0–10 V input or resistance evaluation.

During planning, the following questions should therefore be answered, among others:

Which analog inputs are already available?
How far is the sensor from the controller?
Which supply voltage is available?
How much electrical interference is present in the environment?
Is additional diagnostic information required?
Should the evaluation be as simple as possible or particularly robust?

Especially with new systems, it is worthwhile to coordinate the output signal with the overall automation concept already during sensor selection.

Diagnostic Options and Operational Safety Requirements

If only a fill level value needs to be transmitted, all three variants can generally be suitable.

However, if the controller should additionally be able to detect whether the sensor is operating correctly, a 4–20 mA output offers additional possibilities. The measuring range and fault conditions can be clearly separated electrically. Optionally, a customer-specific 22 mA fault signal can be provided, for example, to detect a detached float.

Continuous level measurement does not automatically replace an independent limit switch. Depending on the safety requirements, separate switching points or additional level limit sensors may be required.

Depending on the version, our transmitters can also be combined with additional switching functions and adapted to the respective application.

Unique Applications Require Individual Measurement Systems

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Converting 4–20 mA to 0–10 V: What Needs to Be Considered?

If a sensor with a 4–20 mA output already exists while the controller only has a 0–10 V input, both systems can be connected using a suitable signal converter.

The converter not only performs the electrical conversion, but also ensures the correct scaling:

4–20 mA → 0–10 V

A simple current-to-voltage conversion using a resistor does not automatically result in this scaling. If, for example, a 500 Ω resistor is used, Ohm’s law gives:

at 4 mA: 2 V
at 20 mA: 10 V

The result would therefore be 2–10 V rather than 0–10 V.

If the complete 4–20 mA range is actually to be converted to 0–10 V, the 4 mA offset must be taken into account accordingly. This can be achieved with a signal converter designed for this purpose.

However, if it is already known during the design phase that the controller requires a voltage signal, it is often easier to specify the sensor directly with the appropriate output.

Individually Configuring Output Signals and Level Measurement

The choice between resistance, 0–10 V and 4–20 mA is not limited to a few rigid standard variants.

At Reed Electronics, we develop and manufacture custom transmitters that can be tailored both mechanically and electrically to the respective application.

In addition to different materials, dimensions and connections, the signal output can therefore also be specifically adapted to the existing system environment.

Custom Voltage and Current Signals

Not every controller works with exactly the same electrical specifications. Depending on the project, the voltage input, supply and output signal can therefore be designed to customer-specific requirements, for example.

Special voltage outputs outside a classic standard configuration can also generally be implemented according to the technical requirements.

This is particularly useful when an existing system is to be retained and the sensor must be integrated into an already defined electrical environment.

Compact variants such as our Mini Transmitter are also available with different electrical output signals.

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Additional Fault Signals and Switching Functions

In addition to the actual measurement signal transmission, additional electrical functions can be integrated.

One example is a defined output value of 21 mA as a fail signal when a specific fault condition is detected. This allows the higher-level controller not only to evaluate the current fill level, but also to respond to a sensor fault.

Depending on the application, additional customer-specific signal ranges or switching functions are also possible.

This allows the level sensor system to be specifically adapted to the existing control logic instead of having to adapt the entire system to a rigid standard sensor.

Conclusion: 4–20 mA, 0–10 V or Resistance – Which Signal Is the Best Choice?

For simple applications with existing resistance evaluation, a passive resistance output can be the most straightforward solution. A 0–10 V output is suitable when a corresponding analog input card is available and the signal paths are manageable.

For longer cable runs, industrial interference conditions and higher diagnostic requirements, a 4–20 mA signal is often the most robust solution.

However, which output is actually best suited always depends on the overall system. Our transmitters are therefore available with different output signals and can be electrically and mechanically adapted to the specific application if required.

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FAQ: Frequently Asked Questions About Output Signals of Level Sensors

What Is the Difference Between 4–20 mA and 0–10 V?

With 4–20 mA, the measured value is transmitted as a current, while with 0–10 V it is transmitted as a voltage. 4–20 mA is particularly robust for longer cable runs and industrial interference, while 0–10 V is often easier to evaluate directly.

Why Does a 4–20 mA Signal Start at 4 mA?

The 4 mA serves as a so-called live zero. This makes it possible to distinguish an actual measured value of 0% from certain fault conditions, such as a failure of the current loop.

What Does 12 mA Mean in a 4–20 mA Signal?

With linear scaling from 4 to 20 mA, 12 mA corresponds exactly to the midpoint of the measuring range, i.e. 50% fill level.

When Is a Resistance Output Suitable?

A resistance output is particularly suitable for simple applications where a suitable resistance evaluation system is already available. It does not require additional signal conversion directly at the sensor.

Is 4–20 mA Better Than 0–10 V?

Not generally. For longer cable runs and demanding industrial environments, 4–20 mA is often advantageous. With short signal paths and existing voltage inputs, 0–10 V can be the simpler solution.

Can a 4–20 mA Signal Be Converted to 0–10 V?

Yes. With a suitable signal converter, 4–20 mA can be scaled to 0–10 V. A simple resistor, however, does not automatically generate the full range from 0 to 10 V.

Can Fault Conditions Be Transmitted via a 4–20 mA Signal?

Yes. In addition to the normal measuring range, defined current values outside 4–20 mA can be used as fault signals. In a customer-specific solution, for example, 21 mA can indicate a defined fault condition.

Can Reed Transmitters Provide Customer-Specific Output Signals?

Yes. In addition to resistance, 4–20 mA and 0–10 V outputs, transmitters can be adapted to the respective application in terms of supply, output signal and additional electrical functions.

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