Understanding RTD Sensors: Resistance Temperature Detectors and Siemens S7-1200 Connection

In this article, we’ll explore the basics of Resistance Temperature Detectors (RTDs). We'll explain their working principles, applications in industrial temperature measurement, and cover essential aspects such as maintenance, calibration, and troubleshooting of RTD circuits.

At its core, an RTD is a passive resistive device made from metals, typically platinum, that change resistance as the temperature changes. Simply put, when the temperature increases, the resistance of the RTD also increases. For example, a Pt100 RTD, made from platinum, has a nominal resistance of 100 ohms at 0°C. This characteristic makes RTDs reliable and precise in measuring temperatures.

In industrial settings, Resistance Temperature Detectors are typically powered by a small electrical current to measure temperature precisely. Other temperature measurement devices like thermocouples and thermistors are also used, but Resistance Temperature Detectors have an advantage due to their linear resistance change with temperature..

For instance, a Pt100 RTD is nearly linear from 0°C to 400°C, and its temperature-resistance relationship is defined by a constant called the temperature coefficient (or Alpha), which for a Pt100 is 0.00385 ohms per ohm per °C. This can be simplified to a resistance change of about 0.385 ohms per degree Celsius.

Methods of RTD Resistance Detection

In the past, one common way to detect RTD resistance was through a Wheatstone Bridge, which allowed for voltage measurements proportional to the RTD’s resistance. However, the Wheatstone Bridge has some limitations, such as interference from lead wire resistance and supply voltage drifts, so it's not as widely used today.

More modern methods involve constant current sources, which are less affected by these issues. With a constant current source, the resulting voltage across the RTD gives a direct indication of temperature without being skewed by external factors.

RTD Configurations: 2, 3, and 4-Wire measurement

RTDs come in several configurations. The basic 2-wire RTD is prone to inaccuracies due to lead wire resistance, so it's rarely used in industrial applications. The 3-wire and 4-wire configurations are much more common. A 3-wire RTD connected to a Wheatstone Bridge can compensate for lead wire resistance, while a 4-wire RTD with a constant current source is often considered the most accurate setup, especially in environments where precision is critical.

How to process RTD signals for the S7-1200 PLC

To process RTD signals with a Siemens S7-1200 PLC, connect the RTD sensor (e.g., PT100) to an analog input module designed for RTD signals, such as the SM 1231 RTD module or the SB 1231 RTD signal board. In TIA Portal, configure the module by selecting the RTD type, temperature range, and filtering options. The PLC will convert the RTD's resistance to temperature data, which can be scaled and used in the control program. A typical connection involves wiring the RTD directly to the module's input terminals according to the module's wiring diagram.

Protecting RTDs and Troubleshooting

Since Resistance Temperature Detectors are sensitive, they need protection from harsh environmental conditions. This is done by housing the RTD in a thermowell - a protective sleeve that ensures Resistance Temperature Detectors maintain accuracy without being exposed directly to extreme conditions

When it comes to maintenance and troubleshooting, common issues include corrosion, moisture, or poor connections, all of which can cause erratic temperature readings. To check if an RTD is working properly, you can use an ohmmeter to measure its resistance. If the readings are off, the RTD might be faulty, and further diagnostics can be done with a process calibrator. This tool simulates the RTD in the measurement circuit to determine whether the problem lies with the RTD or elsewhere in the system.

What Does RTD Stand For?

RTD stands for "resistance temperature detector". It is also called a resistance thermometer: a temperature sensor whose resistance changes with temperature in a known and repeatable way. Platinum has a positive temperature coefficient, which means the resistance increases as the temperature rises. By passing a small, known current through the RTD and measuring the voltage, the transmitter or PLC module can measure the resistance of the sensor and convert it to a temperature reading.

The most common type of RTD in industry is the platinum Pt100 (100 ohms at 0°C). The Pt1000 (1000 ohms at 0°C) is popular in HVAC and in battery-powered devices, because its larger change in resistance per degree makes the lead resistance less important. Nickel and copper RTDs also exist, but they are rarely used in new installations.

Thin-film vs. wire-wound RTD elements

The RTD element is the actual sensing part inside the probe. A thin-film element is a platinum layer deposited on a ceramic substrate. It is small, robust against vibration and inexpensive. A wire-wound element uses a fine platinum wire coiled inside a ceramic or glass body. It offers excellent stability and is often chosen for laboratory and reference applications and for a wide temperature range.

RTD Resistance vs Temperature Characteristics

The relationship between temperature and resistance for industrial platinum RTDs is defined in IEC 60751. For temperatures from 0°C up, it is described by the Callendar-Van Dusen equation:

R(t) = R0 x (1 + A x t + B x t²), where R0 = 100 ohms for a Pt100, A = 3.9083 x 10^-3 and B = -5.775 x 10^-7.

The B term is small, so the characteristic is almost linear. The table shows typical resistance values of a Pt100 RTD:

RTD accuracy and resistance tolerance classes

How accurate is a resistance temperature detector? IEC 60751 defines tolerance classes for the RTD element. The tolerance grows with temperature, so a class gives the best accuracy close to 0°C:

Remember that the total accuracy of the measurement also depends on the wiring method, the transmitter or PLC module, and the installation (immersion depth, thermowell, response time).

Lead Resistance and Self-Heating: Two Common Sources of Error

In a 2-wire connection, the resistance of the cables is added to the resistance value of the sensor. For a Pt100, the change in resistance is only about 0.385 ohms per °C, so a total lead resistance of 1 ohm causes an error of about 2.6°C. That is why 3-wire and 4-wire connections are used in industry, or a Pt1000 sensor when only two wires are available.

The second source of error is self-heating. The measuring current through the RTD produces heat in the element. Measuring modules therefore use a small excitation current, typically around 1 mA or less, to keep the sensor temperature as close as possible to the process temperature.

Reading the RTD Temperature in the S7-1200 Program

When the RTD module is configured for a Pt100 in the standard range, the input word contains the temperature as an integer in tenths of a degree, for example 235 means 23.5°C. In the program you only need to convert it to a REAL value. A simple example in SCL:

// "RTD_Raw" is the INT input of the RTD channel (Pt100, standard range)
// 1 digit = 0.1 °C
"Tank_Temp" := INT_TO_REAL("RTD_Raw") / 10.0;

// simple high temperature alarm with hysteresis
IF "Tank_Temp" > 85.0 THEN
    "Temp_Alarm" := TRUE;
ELSIF "Tank_Temp" < 80.0 THEN
    "Temp_Alarm" := FALSE;
END_IF;

You can then compare the value with alarm limits, use it in a PID_Compact controller or display it on an HMI. If you want to practice this kind of analog processing in TIA Portal, see our Siemens TIA Portal LAD/FBD course and the Full Siemens SCL course. For a comparison of RTDs with thermocouples and thermistors, read our overview of temperature sensor types.

Resistance Temperature Detector (RTD) - FAQ

What is a resistance temperature detector (RTD)?

An RTD is a temperature measurement device that uses the change in electrical resistance of a metal, usually platinum, to measure temperature. The most common version is the Pt100 with 100 ohms at 0°C.

How do you measure resistance on an RTD?

Disconnect the sensor from the transmitter or PLC module and measure the resistance between its wires with an ohmmeter. Compare the result with the Pt100 table for the current temperature. For a 3-wire sensor, the resistance between the two wires on the same side should be close to zero (lead resistance only).

RTD or thermocouple: which is better?

An RTD is more accurate and stable in the range up to several hundred degrees. A thermocouple is better for higher temperatures, very fast response and harsh mechanical conditions.

To recap:

• An RTD is a resistive device that changes resistance as temperature changes.
• RTDs are known for their almost linear resistance-temperature relationship.
• Wheatstone Bridges were once commonly used for RTD detection but have been replaced by more accurate constant current methods.
• RTDs typically come in 3-wire or 4-wire configurations, with the 4-wire being the most precise for industrial use.
• RTDs are protected by thermowells and require regular maintenance and troubleshooting to ensure accurate temperature readings.
• Understanding RTDs can give you a solid foundation in temperature measurement, especially if you’re looking to apply this knowledge in an industrial context.