In this article we will discuss how temperature is measured with RTD sensors today, with enough detail so that you will:

  • See what RTDs are and how they work
  • Learn the basic types of RTDs available and how they are used
  • Understand how RTDs can be interfaced with your DAQ system

Are you ready to get started? Let’s go!

Introduction

Did you know that temperature is the MOST often recorded physical measurement? Knowing the temperature is critical for the correct operation of everything from the human body to an automobile engine, and everything in between.

Temperature is measured with appropriate temperature sensors. There are several temperature sensors used for this, and we support them all:

In this article, we look specifically at the RTD sensor, one of the most popular temperature sensors in the world.

What is an RTD Sensor?

Compared to the thermocouple, the RTD (Resistance Temperature Detector) is generally more linear and drift-free within its measuring range. However, due to their platinum content and more complex construction, they are more expensive than thermocouples. RTDs are sometimes also referred to as “resistance thermometers.”

Typical RTD sensorTypical RTD sensor

RTDs are heavily used in scientific and laboratory applications, industrial applications, and pharmaceutical manufacturing applications. The common denominator among these applications is that accuracy and drift-free performance are more important than cost, putting the RTD ahead of the thermocouple in the selection of sensor type.

But they are also heavily used in consumer products. There are probably dozens of them in your house right now, inside your dishwasher, clothes dryer, toaster or toaster oven, the thermostat on the wall, your air conditioner, refrigerator, freezer - even your cell phone! Of course, these are very small and very inexpensive versions of the RTD.

There are three major RTD construction types:

  • Wire-Wound RTDs
  • Thin-Film RTDs
  • Coiled Element RTDs

Each has advantages and disadvantages. Let’s take a look at them.

Wire-Wound RTDs

The wire-wound RTD sensor is made of thin wire (the resistance wire that will do the sensing) wrapped around a non-conducting core, which is often made of ceramic. Platinum, nickel, and copper are most commonly used for the resistance wire because each of them has a very linear and accurate temperature/resistance relationship. We will show more about each metal in the next paragraphs.

Typical wire-wound RTD sensorTypical wire-wound RTD sensor

The wire is trimmed precisely until the specified resistance at 0°C is achieved. This is known as the “R0” resistance. It is critical that the resistance wire is constrained from bending or twisting as the temperature increases because this mechanical stress will cause a change in resistance, which will distort the measurement.

This construction is relatively fragile, which is why RTDs are usually protected with a glass coating and even installed inside a metal sheath. This is the next step of fabrication, along with an attachment of the lead wires.

Thin-Film RTDs

Thin-film RTD sensors are made up of a ceramic substrate upon which a very thin film of platinum is deposited. To protect the assembly and the film, glass or epoxy are normally applied as a coating. Most thin-film RTDs are useful only up to 300°C, although certain special configurations allow higher ranges.

Typical thin-film RTD sensorTypical thin-film RTD sensor

Thin-film RTDs cost less than the coiled and wire-wound varieties, and are therefore very popular in industrial applications. But they are not as accurate as of the other types, because the R0 cannot be adjusted as precisely. They are also more subject to the self-heating phenomenon due to their smaller size.

Coiled Element RTDs

In this configuration, the resistance wire is fashioned in very small coils which are free to expand and contract with changes in temperature. This prevents temperature-induced mechanical stress from changing the resistance and thus affecting the measurement. These coils are installed inside a ceramic body and then surrounded by non-conductive powder.

Typical coiled-element RTD sensorTypical coiled-element RTD sensor

Coiled-element RTDs are very popular in industrial applications. They are often installed inside a metal sheath to protect them.

How RTDs Measure Temperature

Unlike thermocouples which are passive sensors, RTDs operate on the principle of electrical resistance. It is necessary to pass a small current through them in order to measure a change in resistance. Therefore the DAQ system’s signal conditioner needs to supply this sensor power or “excitation” in order to read the output. 

RTDs operate on the principle that certain metals have an exceptionally stable and accurate relationship between resistance and temperature. Copper has the best resistance v. temperature ratio, however, it does not do as well at high temperatures as platinum RTDs.

R v. T

In other words, the amount of resistance change observed per change in temperature is highly linear and repeatable. This is also referred to as the temperature coefficient of resistance.

By measuring this resistance change, we can measure the temperature change. Various metals provide different accuracies, as well as different temperature operating ranges.

RTD Type Max Measuring Range Stability Corrosion Resistance R vs T Linearity
Platinum -200 to 850°C Excellent Excellent Good
Nickel -80 to 260°C Fair Good Fair
Copper -200 to 260°C Good Fair Excellent

Top-level comparison of three major RTD types

What Is an RTD’s Resistance Ratio?

The slope of the change in resistance versus the change in temperature is known as the resistance ratio. Specifically, it is the average slope across the change from 0°C (R0) to 100°C (R100). This is written as:

(R100 - R0) / R0

The specific metal that we choose - platinum, nickel, or copper - has the biggest influence on the resistance ratio. But also the purity of the metal itself is a factor. For example, RTDs used in scientific applications use the purest platinum standard, whereas RTDs used in industrial applications that do not require quite as much accuracy, can use slightly less pure platinum.

RTD Type Resistance @ 0°C (typ.) Resistance @ 100°C (typ.) Resistance Ratio Alpha (α)
Platinum 100 Ω 138.5 Ω 0.385 0.00385
Nickel 120 Ω 200.64 Ω 0.672 0.00672
Copper 9.035 Ω 12.897 Ω 0.427 0.00427

Comparison of Resistances and Alpha

The platinum resistance ratio is particularly wide, which is why they have a broader measuring range than either nickel or copper.

The constant “Alpha” (α) is simply the resistance ratio divided by 100.

RTD Models

Platinum is the most popular choice in RTDs, so you will see reference to RTD models such as PT100 sensors (or also pt100 probes). The “PT” stands for “platinum”, and the 100 Ω is the resistance at 0°C (this is the “R0”). The PT100 Platinum RTD is the most popular RTD on the market today.

PT            100

Platinum   The R0 value

There is also the PT1000 model available, which has 10 times the nominal resistance at 100°C. The output from the sensor is also higher. And while PT100 models are available in several types (wire-wound and thin-film), PT1000 models are usually only available in thin-film types.

Typical platinum Pt100 RTD sensorTypical platinum Pt100 RTD sensor

Being the de facto standard, PT100s are compatible with a very wide range of instruments and applications. However, there are advantages that PT1000s offer. For example, they are better when lead wires are very long, due to their higher resistance. Higher resistance also means less self-heating and less power consumption, which is ideal for battery-powered applications. 

It should be noted that there is a PT500 model, where 500 Ω is the resistance at 0°C. But it is relatively rare compared with the PT100 or PT1000. There is also a PT2000 model with an R0 of 2 kΩ at 0°C.

RTD Wiring Methods

Although at its core an RTD is a 2-wire sensor, the addition of one or even two more wires (3 and 4-wire hookup) provides better compensation against self-heating and lead wire resistance and is thus highly recommended. So, although a 2-wire hook-up is possible, Dewesoft signal conditioners best support 3- and 4-wire hook-ups in order to provide the best possible accuracy.

3-wire RTD connection scheme
3-wire RTD hookup

In a typical 3-wire hook-up, the RTD’s outputs are connected to pins 1 (In+)  and 2 (In-) of the signal conditioner. The positive excitation (Exc+) is applied from pin 4, which runs all the way through the sensor cable to the RTD. The negative excitation (Exc -) is jumpered between pins 2 and 3 at the DAQ system. The drain from the cable shield is connected only at the signal conditioner to avoid a ground loop.

This sensor cable therefore must have three wires in it, and a braided or foil shield.

4-wire RTD connection scheme
4-wire RTD hookup

The 4-wire hook-up is similar to except that both excitations positive and negative lines run all the way through the cable to the RTD. This method provides the best possible performance in terms of rejecting lead wire resistance and self-heating errors.

This sensor cable therefore must have four wires in it, and a braided or foil shield.

The DIN RTD Accuracy Standard

The prevalent accuracy accepted for RTDs is the DIN standard curve. It is based on the R v. T ratio of a PT100 sensor. The standard specifies a base resistance of 100 Ω at 0°C with an Alpha (α) of 0.00385.

The temperature in degrees C Resistance
0 100.00
10 103.90
20 107.79
30 111.67
40 115.54
50 119.40
60 123.24
70 127.07
80 130.89
90 134.70
100 138.50

DIN specified RTDs have three standard tolerance classes:

DIN Class Tolerance
Class A ±(0.15 + 0.002 |T|°C)
Class B ±(0.3 + 0.005 |T|°C)
Class C ±(1.2 + 0.005 |T|°C)

When To Choose Platinum, Nickel or Copper?

How are the three main metal types of RTDs chosen, and why? This table shows the main applications for platinum, nickel, and copper RTDs.

RTD Type Main Applications
Platinum Scientific and Pharmaceutical applications (laboratory-grade platinum is used)
Industrial applications (IEC 60751 standard is used)
Applications that require the widest temperature ranges
Nickel Price-sensitive applications such as HVAC, consumer products (hair dryers, clothes dryers, dishwashers, etc).
(Applications below 300° C, where nickel becomes increasingly non-linear)
Copper Mostly industrial applications, especially electromechanical applications such as electric motors, windings, power generators, because copper has the same characteristics as these test objects..
Applications that require the best inherent linearity across a narrow temperature range

Applications by RTD metal

The Wide World of RTD Applications

There are literally thousands of applications where RTDs are used around the world, but here are just a few:

  • Automotive: engine block temperature, air intake sensor, coolant temperature, outside air temperature, climate control systems, oil temperature
  • Food processing: temperature monitoring during manufacturing and storage, shipping perishables monitoring 
  • Pharmaceuticals: temperature monitoring during production and storage
  • HVAC: air temperature monitoring, fire detection, automatic climate control
  • Aerospace: Engine temperature, coolant temperature, compressor temperature, fuel tank monitoring, fire control equipment
  • Industrial: Machine temperature, electric motors, windings, power generators, thermocouple compensation (CJC), ovens, microwave power measurement
  • Medical: Infant incubators, respiration equipment, dialysis machines
  • Communications: Amplifier monitoring, transmitter temperature compensation
  • Computers: In-rush current limiting
  • Scientific and Laboratory: Research across all fields of study
  • Consumer products: coffee makers, cellphones, home weather stations, battery packs, toasters, thermostats, water heaters, refrigerators and freezers, dishwashers, clothes washers and dryers, air conditioners, electric blankets

RTD Measuring Challenges and Solutions

As mentioned, RTD hook-up is more complex than a thermocouple, however, Dewesoft DAQ systems make it easy and convenient to connect your sensors to the DAQ system. Dewesoft offers signal conditioners specifically for RTD sensors in certain product series, such as KRYPTON or IOLITE. But RTDs can be connected to all Dewesoft DAQ systems using DSI-RTD adapters.

Noise is always a consideration for any sensor with small output, but our high isolation inputs are the best prevention imaginable. Add to that our support for 2 and 3 wire hook-ups to reduce self-heating errors, and Dewesoft offers a very accurate DAQ system for RTD and virtually all sensors.

Another way to reduce noise is to place the measuring system as close to the sensor as possible. Avoiding long signal lines is a proven strategy for maximizing signal fidelity and reducing costs. Look at our SIRIUS and KRYPTON modular instruments for best-in-breed solutions here. KRYPTON models in particular can be placed directly at the measuring point, where the signal is immediately digitized and sent back time-stamped over EtherCAT to the DAQ system, avoiding the issue of long lead wires completely.

Advantages of RTD Sensors

  • Most stable
  • Most accurate
  • More linear than thermocouples

Disadvantages of RTDs

  • More expensive than thermocouples or thermistors
  • Sensor power required
  • Narrower measuring range
  • Low absolute resistance
  • Self-heating must be compensated

Basic Comparison: Thermocouples, RTDs and Thermistors

Sensor Thermistor Thermocouple RTD (Pt100)
Temperature Range1 Narrowest
(-40°C to 300°C)
Widest
Type J is -210 to 1200°C
Type K is 95 to 1260°C
Other types can range as low as -330°C OR as high as 3100°C
Narrow
-200- to 600°C
Up to 850°C is possible
Response Fast Medium to Fast
Depends on sensor size and construction
Slow
Depends on sensor size and construction
Long Term Stability Poor Very Good Best
(±0.5°C to ±0.1°C / year)
Accuracy Fair Good Better
0.2%, 0.1% and 0.05%
Linearity Exponential Non-linear
This is usually done in software
Fairly Good
But linearization is recommended
Construction Fragile Adequate
Sheaths and tubes improve fragility but increase response time
Fragile
Sheaths and tubes improve fragility but increase response time
Size Very small Small Larger
Wiring Very simple Simple Complex 
Excitation/Power Required None None Required
External Requirements None CJC (cold junction compensation) and signal linearization RTD signal conditioner
Cost Lowest 
Low-accuracy types are very inexpensive, but there are some which are more accurate and more expensive.
Low
R and S types that use platinum are more expensive
Highest

Specifications are typical

Dewesoft Measuring Instruments for RTD Sensors

Dewesoft provides several DAQ systems that can effectively measure, store, and display temperature. And they can do so by connecting the most often-used temperature sensor in the world for scientific, medical, and pharmaceutical applications: the RTD. Dewesoft DAQ systems can measure, store, analyze, and visualize temperature from one to hundreds of channels in real-time.

KRYPTON DAQ System RTD Measurement

KRYPTON is the most ruggedized range of data acquisition products available from Dewesoft. Built to withstand extreme temperature and shock and vibration conditions, KRYPTON is rated to IP67, protecting them against water, dust, and more. 

They connect to any Windows computer (including Dewesoft’s own ruggedized IP67 KRYPTON-CPU model) via EtherCAT and can be separated by up to 100  meters (328 feet), allowing you to locate them near the signal source. Like SIRIUS, they run the most powerful DAQ software on the market, Dewesoft X

RTD sensors can be connected directly to the KRYPTON RTD signal conditioning module. It is available in an 8-channel configuration (KRYPTONi-8xRTD).

KRYPTONi-8xRTD DAQ module with universal RTD signal conditioningKRYPTONi-8xRTD DAQ module with universal RTD signal conditioning

KRYPTON can also measure RTD sensors with the KRYPTON STG module and the use of the DSI-RTD Dewesoft Sensor Interface (DSI) adapter.

A multi-channel KRYPTONi-6xSTG module with various DSI adapters connected

Here is a cross-reference of KRYPTON modules and their compatibility with RTDs and thermocouples, as well as DSI adapters made for temperature measurement. KRYPTON offers an 8 channel RTD module that natively handles RTD sensors, including types PT100, PT200, PT500, PT1000, and PT2000.

  KRYPTON multi-channel modules
  KRYPTONi-8xRTD KRYPTON-3xSTG
KRYPTON-6xSTG
KRYPTONi-1xSTG
RTD Sensors
Native universal RTD input
Via DSI-RTD Via DSI-RTD
Channels 8 3 or 6
Isolation No

 

IOLITE and IOLITEd DAQ System RTD Measurement

IOLITE and IOLITEd are unique products that combine the essential capabilities of a real-time industrial control system with a powerful DAQ system. With IOLITE, hundreds of analog and digital channels can be recorded at full speed while simultaneously sending real-time data to any third party EtherCAT master controller.

IOLITEr and IOLITEs industrial data acquisition systemsLeft: IOLITEr rack-mounting system with 12 input module slots
Right: IOLITEs bench-top system with 8 input module slots

They represent great DAQ system performance plus real-time control via EtherCAT, combined with the most powerful DAQ software on the market, Dewesoft X DAQ software

IOLITEd industrial data acquisition modulesIOLITEd industrial DAQ modules

Here is a cross-reference of IOLITE input modules and their compatibility with RTDs and thermocouples, as well as DSI adapters made for temperature measurement:

  IOLITE modules
  IOLITEi-8xRTD IOLITEi-6xSTG IOLITEd-1xSTG
RTD Sensors Native RTD inputs  Via DSI-RTD Via DSI-RTD
Channels 8 6 1

The IOLITEi-8xRTD amplifier module offers both channel-to-ground isolations as well as channel-to-channel isolation up to 1000V. Data are acquired simultaneously from all 8 channels with sample rates up to 100S/s using 24-bit delta-sigma ADC. This module can also be used for resistance measurements up to 10 kΩ, and voltage measurements up to 1 V.

Universal RTD module - IOLITEi-8xRTD module for RTD sensorsUniversal RTD module - IOLITEi-8xRTD module for RTD sensors

The same sample rate and isolation specifications are true of the 6xSTG module, except that it has six channels instead of eight. The 6xSTG is a very versatile module, capable of making strain gage, resistive, and low voltage measurements, in addition to its compatibility with DSI series adapters.

SIRIUS DAQ System RTD Measurement

SIRIUS is the flagship of the Dewesoft product range. They represent the highest DAQ system performance, combined with the most powerful DAQ software on the market, Dewesoft X. 

To connect RTDs to SIRIUS data acquisition systems, we use our popular Dewesoft Sensor Interface (DSI) adapters to interface with several popular SIRIUS input modules. 

SIRIUS systems are available in a wide array of physical configurations, from modular “slices” that connect to your computer via USB or EtherCAT, R3 rack-mounting systems, and R1, R2, and R8 stand-alone systems that include a built-in computer.

SIRIUS data acquisition systemsThe SIRIUS product line-up

The DSI-RTD adapter is compatible with several popular types of RTD sensors, including Pt100, Pt200, Pt500, Pt1000, and Pt2000.

Dewesoft DSI RTD adapter for Pt100, Pt1000, Pt200The DSI-RTD adapter from Dewesoft

DSI adapters use a built-in TEDS sensor interface to automatically configure themselves in Dewesoft X DAQ software. Simply plug-in DSI-RTD adapter into the selected SIRIUS module’s DB9 input, verify your settings on the hardware setup screen, and you’re ready to start making measurements.

Here is a cross-reference of SIRIUS modules and their compatibility with various DSI adapters made for temperature measurement:

  SIRIUS DualCoreADC modules SIRIUS HD (High Density) modules SIRIUS HS (High Speed) modules
  STG, STGM, LV, MULTI HD-STGS, HD-LV HS-STG, HS-LV
DSI-RTD

1) Note that the SIRIUS-STG signal conditioner can natively handle RTDs and does not require a DSI-RTD.

DEWE-43A and MINITAURs DAQ System RTD Measurement

The DEWE-43A is an extremely portable, handheld DAQ system. Connecting to your computer via a locking USB connector, it features eight universal analog inputs. It’s “big brother” is called the MINITAURs - this is essentially the DEWE-43A combined with a computer and a few other features, in a single highly portable enclosure. The universal inputs of both systems are compatible with Dewesoft’s DSI adapters, allowing you to connect an RTD sensor to any or all of their eight input channels. 

DEWE-43A and MINITAURs DAQ systemsLeft: DEWE-43A handheld DAQ system
Right: MINITAURs model, including built-in computer

The DSI-RTD adapter is compatible with several popular types of RTD sensors, including Pt100, Pt200, Pt500, Pt1000, and Pt2000.

DSI adapters use TEDS sensor chip to automatically configure themselves in Dewesoft X DAQ software. Simply plug the DSI-RTD adapter into the selected input’s DB9 input, verify your settings on the hardware setup screen in Dewesoft X software, and you’re ready to start making measurements.