Connecting a temperature probe with 4–20 mA: when a transmitter is useful

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Temperature probes such as Pt100, Pt1000 or thermocouples can be connected directly to a PLC, display or controller. In simple applications, this works well. In practice, however, many measurement problems are caused by long cable runs, electrical interference, incorrect wiring, unsuitable input cards or missing diagnostic functions.

A temperature transmitter can significantly reduce these problems. It converts the sensor signal directly at the measuring point or in the control cabinet into a robust standard signal, for example 4–20 mA, 0–10 V, HART, RS-485 / Modbus or IO-Link. This makes temperature measurement easier to integrate into PLCs, control systems, controllers or remote monitoring systems.

This article explains when a direct Pt100 connection is sufficient, when a transmitter is useful, which advantages 4–20 mA offers for long cable runs and how head-mounted transmitters, DIN rail transmitters and digital temperature transmitters are selected correctly.

You can find suitable devices in our category
accessories and transmitters.
For typical applications, WIKA type T15, WIKA type T32.xS, WIKA types T91.10 / T91.20, IPAQ R530, IPAQ R460, IPAQ R461 and IPAQ-4L are relevant, among others.

Direct Pt100 connection or transmitter?

The key question is: should the sensor signal be routed directly to the PLC, or should it already be converted into a standard signal near the measuring point? With short cable runs, little interference and suitable input cards, a direct Pt100 connection may be sufficient. With longer cable runs, harsh environments or multiple measuring points, a transmitter is often the better solution.

A direct connection means that the sensor is routed as a resistance signal to the evaluation unit. A transmitter, on the other hand, measures the sensor value locally and outputs a standardised output signal. 4–20 mA is especially widespread in industry because the signal is robust, easy to transmit and simple to monitor.

The decision therefore depends not only on the sensor, but on the entire measuring chain: sensor, cable length, EMC environment, PLC input, required diagnostics, accuracy, maintenance and documentation.

How does direct connection of a Pt100 work?

A Pt100 is a resistance thermometer. Its resistance changes with temperature. At 0 °C, a Pt100 ideally has 100 ohms. Evaluation is performed by a measuring circuit in the display, controller, transmitter or directly in the PLC input card.

With direct connection, the Pt100 is connected using a 2-wire, 3-wire or 4-wire circuit. The 2-wire circuit is simple, but cable resistances can cause significant measurement errors. The 3-wire circuit partially compensates cable resistance. The 4-wire circuit offers the best compensation, but requires more conductors and suitable input technology.

Connection type	Advantage	Limitation
2-wire	Simple and cost-effective.	Cable resistance distorts the measured value.
3-wire	Partial compensation of cable resistance.	Requires similar cable resistances.
4-wire	Very good compensation of cable resistance.	More conductors and suitable input card required.
Transmitter	Converts sensor value into a robust standard signal.	Additional device must be selected and parameterised.

Typical problems with direct sensor wiring

Many temperature measurements appear inaccurate even though the sensor itself is working correctly. The cause is often the wiring or signal transmission. Long cable runs increase resistance, interference couples into the signal or the PLC input card is not ideally suited to the sensor type.

Especially with Pt100 signals, the resistance value is small. As a result, additional cable resistances or contact resistances can have a relevant influence. With thermocouples, on the other hand, the signal voltages are very small. Incorrect extension cables, terminal transitions or EMC interference can cause errors here.

A temperature transmitter reduces these risks because it processes the sensitive sensor signal only over a short distance and then outputs a more stable output signal.

What does a temperature transmitter do?

A temperature transmitter receives the signal from a sensor, processes it and outputs a standardised signal. Depending on the device, Pt100, Pt1000, other RTD sensors or thermocouples can be connected. The output can be, for example, 4–20 mA, 0–10 V, HART, Modbus or IO-Link.

The transmitter therefore performs several tasks at the same time: sensor adaptation, linearisation, scaling, signal conversion and often diagnostics as well. This turns a sensitive sensor signal into an industrial-grade signal for automation, display or remote transmission.

It is particularly practical that the measuring range can be parameterised. A Pt100 can, for example, be scaled to 0…100 °C, -50…150 °C or 0…400 °C. The PLC then receives a clear 4–20 mA signal that directly corresponds to the configured temperature range.

Why 4–20 mA is often useful for temperature measurements

The 4–20 mA signal is a current signal and is therefore very robust for industrial applications. Voltage drops on cables have a much less critical effect than with simple voltage signals. In addition, 4 mA as the lower signal value is a so-called live zero. This makes cable breaks or device failures easier to detect than with a 0 mA zero point.

For the PLC, 4–20 mA is easy to evaluate. The measuring range is clearly scaled: 4 mA corresponds to the lower range value, 20 mA to the upper range value. The temperature value is transmitted linearly in between.

Signal	Example at 0…100 °C	Meaning
4 mA	0 °C	Lower range value.
8 mA	25 °C	25 % of measuring range.
12 mA	50 °C	50 % of measuring range.
16 mA	75 °C	75 % of measuring range.
20 mA	100 °C	Upper range value.

Long cable runs and cable resistance

Long cable runs are one of the most common reasons for using a transmitter. If a Pt100 is routed directly to the PLC over many metres, cable resistances and contact resistances influence the measurement result. This can lead to significant deviations, especially with 2-wire circuits.

A head-mounted transmitter can be installed directly in the connection head of the temperature probe. This means that the sensitive Pt100 signal is only routed over a very short distance. It is then transmitted as a 4–20 mA signal over the longer cable run. In many plants, this is significantly more stable.

A transmitter can also be useful for thermocouples because thermocouple voltages are very small and can be sensitive to interference, incorrect transitions or unsuitable extension cable over long cable runs.

EMC, interference and industrial environments

There are many sources of interference in industrial plants: frequency inverters, motors, contactors, long cable trays, switching power supplies, power cables and radio interference. Direct sensor signals can be affected by these, especially if cables are not properly shielded or are routed incorrectly.

A transmitter improves the situation because it provides a more robust output signal. 4–20 mA is often better suited to industrial environments than a sensitive resistance or thermoelectric voltage signal over long distances.

Nevertheless, proper wiring, shielding, grounding and separation of power and signal cables remain important. A transmitter does not solve every EMC problem, but it reduces many typical sources of error.

Galvanic isolation and safe signal transmission

In some applications, galvanic isolation is useful or required. It electrically separates input, output or supply from each other. This can reduce interference currents, potential differences and ground loops.

Galvanic isolation is particularly relevant with long cable runs, different potentials, large plants, multiple control cabinets or sensitive measurement signals. It can help achieve stable measured values and avoid interference in the signal path.

When selecting a temperature transmitter, it should therefore be checked whether galvanic isolation is required and which insulation requirements apply to the specific setup.

Sensor break, short circuit and diagnostic functions

A major advantage of modern temperature transmitters is diagnostics. Many devices detect sensor break, short circuit or measuring range violation. The output signal can then be set to a defined error value so that the PLC detects the fault.

Without a transmitter, fault detection is often more difficult. A direct sensor input may also offer diagnostics, but this depends heavily on the PLC card and parameterisation. A suitable transmitter often makes the behaviour clearer and easier to standardise.

Diagnostics are especially important for critical measuring points: heating circuits, cooling circuits, process vessels, machine monitoring, hazardous areas, food processes, chemical plants or power plants benefit from clear fault states.

HART, Modbus, IO-Link and digital communication

In addition to classic analog signals, digital communication options are available. HART enables digital communication over a 4–20 mA signal. This allows parameters, measured values, diagnostic information and device status to be read out.

RS-485 / Modbus is often used when several measuring points are to be digitally connected to a system. IO-Link is particularly interesting in machine building and automation when sensor information, diagnostics and parameterisation are to be transmitted in a standardised way.

Which communication type is useful depends on the plant. For simple temperature transmission, 4–20 mA is often sufficient. For diagnostics, parameterisation and digital integration, HART, Modbus or IO-Link can offer significant advantages.

Head-mounted transmitter or DIN rail transmitter?

Temperature transmitters are available in different designs. A head-mounted transmitter is installed directly in the connection head of the temperature probe. This keeps the path of the sensitive sensor signal particularly short. This is ideal when long cable runs to the PLC are required.

A DIN rail transmitter is installed in the control cabinet. It is easily accessible, simple to maintain and suitable for central signal processing. However, the sensor signal still has to be routed to the control cabinet.

Design	Advantage	Typical application
Head-mounted transmitter	Conversion directly at the measuring point.	Long cable runs, harsh environments, decentralised measuring points.
DIN rail transmitter	Good accessibility in the control cabinet.	Central signal conditioning, retrofit, plant modernisation.
Field transmitter	Robust design for industrial environments.	Process plants, outdoor areas, demanding measuring points.
Digital transmitter	Diagnostics, parameterisation and digital integration.	HART, Modbus, IO-Link or modern automation.

Connecting a temperature probe to a PLC

When connecting a temperature probe to a PLC, there are two basic approaches. Either the sensor is routed directly to a suitable temperature input card, or a transmitter converts the signal into 4–20 mA, 0–10 V or a digital signal.

Direct connection is useful if the PLC has suitable RTD or thermocouple inputs, the cable run is short and the environment is low in interference. A transmitter is useful if the PLC only has analog inputs, long cable runs are present, several signals are to be standardised or diagnostics are required.

With 4–20 mA, scaling must be set correctly. If the transmitter is scaled to 0…200 °C, the PLC must interpret the same range. Errors often occur when transmitter and PLC use different measuring ranges.

Suitable transmitters for temperature probes

The accessories and transmitters category includes solutions for analog and digital signal conversion. Depending on the device, output signals such as 4–20 mA, 0–10 V, HART, RS-485 / Modbus or IO-Link are available. Diagnostic functions such as sensor break and short-circuit monitoring are also relevant.

WIKA type T15 is suitable as a compact temperature transmitter for typical applications with resistance thermometers or thermocouples when a standardised output signal is required.

WIKA type T32.xS is particularly interesting when HART communication, extended diagnostics or more demanding process integration are required.

WIKA types T91.10 / T91.20 are relevant when temperature measured values are to be conditioned via analog standard signals and transmitted to a controller, display or control system.

IPAQ R530, IPAQ R460, IPAQ R461 and IPAQ-4L are suitable, depending on the version, for different temperature probes, output signals and communication requirements. They are particularly interesting when flexible parameterisation, modern signal processing or digital connection is required.

Product comparison: which transmitter is suitable?

The selection of the suitable transmitter depends on sensor type, installation location, output signal, diagnostic requirements and communication interface.

Product / category	Suitable for	Typical application
Accessories and transmitters	Overview of temperature transmitters, accessories and signal converters.	4–20 mA, 0–10 V, HART, Modbus, IO-Link, diagnostic functions.
WIKA type T15	Standardised temperature signal conversion.	Pt100, RTD or thermocouple to analog output signal.
WIKA type T32.xS	Temperature measurement with HART and extended functions.	Process plants, diagnostics, parameterisation and control system connection.
WIKA types T91.10 / T91.20	Analog temperature signal conditioning.	Control cabinet, display, PLC and simple automation tasks.
IPAQ R530	Modern temperature signal conversion with flexible application.	Industry, process measurement and digital integration.
IPAQ R460 / R461	Temperature measurement with suitable output and communication options.	Standardisation of sensor signals and PLC connection.
IPAQ-4L	Multi-channel or flexible temperature signal processing.	Multiple measuring points, plant modernisation and control cabinet integration.

Practical examples from plant engineering and maintenance

Example 1: Pt100 over a long cable run to the PLC

A Pt100 is routed to the PLC over 80 metres. The temperature display fluctuates and deviates from the reference value. By using a head-mounted transmitter, the Pt100 signal is converted into 4–20 mA directly at the measuring point. The long cable run now transmits a more robust standard signal.

Example 2: EMC interference from frequency inverters

In a machine, temperature cables run parallel to motor cables. The measured values jump sporadically. In addition to better cable routing and shielding, a transmitter is used to process the sensitive sensor signal locally and provide a more stable signal to the controller.

Example 3: PLC has no Pt100 input card

An existing PLC only has analog 4–20 mA inputs. Instead of installing a new temperature card, a temperature transmitter is used. The Pt100 is connected to the transmitter, and the PLC receives a scaled 4–20 mA signal.

Example 4: Sensor break must be clearly detected

In a heating process, probe failure must be detected safely. The transmitter is parameterised so that a defined error current is output in the event of sensor break. The PLC detects the fault and can trigger an alarm message.

Example 5: Digital diagnostics with HART or IO-Link

A plant is not only intended to transmit the temperature value, but also diagnostic information. A transmitter with HART or IO-Link enables parameterisation, status query and additional diagnostics via the communication interface.

Checklist: selecting a temperature transmitter

This checklist helps with selecting the suitable transmitter for Pt100, thermocouple and other temperature probes.

Check question	Why important?	Recommendation
Which sensor is used?	Transmitter must match Pt100, RTD or thermocouple.	Check sensor type and connection type.
How long is the cable run?	Long cable runs increase error and interference influence.	For long cable runs, check head-mounted transmitter or 4–20 mA.
Which PLC inputs are available?	Input card determines the required signal.	Select 4–20 mA, 0–10 V, RTD, TC or digital signal.
Is EMC an issue?	Motors, inverters and power cables can cause interference.	Consider transmitter, shielding and cable routing.
Is galvanic isolation required?	Potential differences can influence measured values.	Select transmitter with suitable isolation.
Which diagnostics are required?	Sensor break and short circuit should be detected.	Check diagnostic and error current behaviour.
Which communication is desired?	HART, Modbus or IO-Link enable parameterisation and diagnostics.	Select communication to match the plant.
Where should the device be installed?	Installation location influences signal quality and maintenance.	Define head-mounted transmitter, DIN rail or field transmitter.

Conclusion: 4–20 mA is often the more robust solution than a direct Pt100 connection

A direct Pt100 connection can work well with short cable runs, suitable input cards and a low-interference environment. However, as soon as long cable runs, EMC interference, unclear PLC connection or diagnostic requirements are involved, a temperature transmitter is often the better solution.

The transmitter converts the sensitive sensor signal into a robust standard signal such as 4–20 mA, 0–10 V or a digital signal. This reduces cable-related errors, simplifies PLC connection and enables diagnostic functions such as sensor break or short-circuit monitoring.

For typical applications, WIKA type T15, WIKA type T32.xS, WIKA types T91.10 / T91.20, IPAQ R530, IPAQ R460, IPAQ R461 and IPAQ-4L are relevant, among others. You can find an overview of suitable devices in the category
accessories and transmitters.

FAQ: frequently asked questions about temperature transmitters

What is a temperature transmitter?

A temperature transmitter converts the signal of a temperature probe, for example Pt100 or thermocouple, into a standard signal such as 4–20 mA, 0–10 V, HART, Modbus or IO-Link.

How can a Pt100 be converted to 4–20 mA?

A Pt100 is connected to a suitable temperature transmitter. The transmitter is scaled to the required temperature range and outputs a 4–20 mA signal.

When is a transmitter useful?

A transmitter is particularly useful for long cable runs, EMC interference, missing Pt100 input card, need for 4–20 mA, diagnostic functions or digital communication.

Is a direct Pt100 connection worse?

Not in general. With short cable runs and suitable input cards, direct connection can work well. With longer cable runs or harsh industrial environments, a transmitter is often more stable.

What is a head-mounted transmitter?

A head-mounted transmitter is installed directly in the connection head of the temperature probe. It converts the sensor signal near the measuring point into a standard signal.

What is a DIN rail transmitter?

A DIN rail transmitter is installed in the control cabinet. It is particularly suitable for central signal conditioning and good accessibility for maintenance or parameterisation.

Why is 4–20 mA robust?

4–20 mA is a current signal and is therefore less sensitive to voltage drops on cables. In addition, the 4 mA live zero enables better fault detection.

When is 0–10 V useful?

0–10 V can be useful if the controller has corresponding voltage inputs and the cable run is short and low in interference. For long cable runs, 4–20 mA is often more robust.

What is HART useful for in temperature transmitters?

HART enables digital communication over the 4–20 mA signal. Parameters, diagnostic information and device status can be read out or changed.

When are Modbus or IO-Link useful?

Modbus is interesting when several measuring points are to be digitally integrated. IO-Link is particularly useful in machine building when parameterisation, diagnostics and sensor information are to be transmitted in a standardised way.

Can a transmitter detect sensor break?

Many temperature transmitters detect sensor break or short circuit and output a defined error value. This allows the PLC to clearly detect the fault.

Which products are suitable as temperature transmitters?

Depending on the application, WIKA type T15, WIKA type T32.xS, WIKA types T91.10 / T91.20, IPAQ R530, IPAQ R460, IPAQ R461 and IPAQ-4L are suitable, among others. You can find an overview in the category
accessories and transmitters.