Measuring Temperatures Precisely: Possible Sensor Technologies and Requirements for Data Acquisition Systems

The most frequently measured quantity in the world of measurement technology is temperature. Moreover, there are various sensors and methods available for performing this task. One of the most popular ways to measure temperature is by using a thermocouple. In this article, we provide you with an overview of this subject.

All over the world, temperature is the most frequently measured quantity. The precise recording of temperature changes plays a vital role in both structural measurements and the dynamic operations of all kinds of systems.

Numerous factors trigger thermal changes. These include environmental influences (heat, cold), heating up of sensors, combustion or explosion processes, flow, and mechanically moving system parts that leads to friction or electrical current.

Temperature changes also mostly have an impact on other physical quantities, such as the amount of electrical current, strain, flow, or pressure. Ideally, these thermal dependencies are taken into consideration and compensated for during runtime. In this case, precise temperature acquisition is absolutely crucial to reduce measurement-related uncertainty due to temperature variations.

The temperature profiles obtained during testing or in a model test setup can be used to analyze and optimize a system. This could be beneficial for cooling and heating circuits, the design of materials, or determination of wire sizes for conducting electricity.

Possible methods of digital temperature measurement

Assuming that we wish to compare temperatures digitally at the same time as many other quantities, the following temperature measurement options are at our disposal:

Sensors that change their resistance, such as NTC thermistors – these lower their resistance as the temperature rises; and PTC thermistors – these increase their resistance as the temperature rises (e.g., platinum or silicone resistance thermometers or ceramic PTC thermistors).
Sensors that deliver an electrical signal, such as IC temperature sensors, alter the current or voltage as the temperature changes.
Temperature sensors that use a quartz crystal oscillator – here, the resonance frequency varies as a function of the temperature.
Pyrometers and thermal imaging cameras are non-contact tools, and they measure thermal radiation.
Fiber optic temperature sensors measure the temperature profile along a glass fiber. The principle here is the Raman effect; in other words, the temperature-dependent change in the refractive index in fiber Bragg gratings (FBGs).
And finally, one of the most common methods of measuring temperature, now more than ever, is using thermocouples – these convert temperature differences into an electric voltage as a result of the Seebeck effect. Thermocouples comprise two different metals that are soldered or welded together at one end. When this connection point becomes hot, a non-linear voltage is generated as a function of the temperature.

Type-K insulated thermocouple for the QuantumX MX809B module.

Fiber optic temperature sensors, such as the ones from the FS63 series by HBM.

Thomas Johann Seebeck (1770-1831) — Thomas Johann Seebeck (1770–1831), German physicist. Image source: Wikipedia

Seebeck effect for determining temperature

Thermocouples measure the temperature on the basis of the Seebeck effect. The relationship between heat and electricity – thermoelectricity – was discovered by physicist Thomas Johann Seebeck. If the point of connection of two different metals is heated, a temperature-dependent voltage is generated.

The tips of thermocouples consist of soldered wires that are connected to measurement electronics. If the temperature differs at different points of these wires, a charge displacement occurs. The wire’s material and conductivity determine the amount of activity of the electrons and, therefore, the extent of this charge displacement. If the soldered connection point of the two metals heats up, the electrons travel from one material to the other. The resulting difference in temperature between the connected and unconnected ends of the wires effect a thermoelectric voltage. This can then be measured from the free ends of the wires. Once we note the temperature at the free end of the wire and have measured the voltage, we can calculate the temperature at the connection point of the two wires. The measured voltage is very low in this case – only a few dozen microvolts for each degree Celsius of temperature difference. However, this is absolutely adequate for determining the temperature with precision.

Types of thermocouples

There are different types of thermocouples. They differ according to the manufacturing materials and their properties, and they cover various temperature ranges. The various types, their voltage ranges, electromotive forces, and the admissible limit of deviation temperatures are governed by DIN EN 60584.

The Type-K thermocouple is extensively used. It is an all-purpose thermocouple that is made with a connection of the alloys nickel-chromium and nickel-Alumel. It has a sensitivity of around 41 μV/°C (microvolts per degree Celsius), is low in price, and has a very wide temperature range that typically extends from -200 to +1100°C.

Other popular types are the E, T, J, N (this one is the successor to the K-Type), C, and S.

Connector for Type-K thermocouple (green)

Connector for Type-K thermocouple (yellow)

Connector for Type-T thermocouple (blue)

Connector for Type-J thermocouple (black)

Using thermocouples

Thermocouples can be used in a wide variety of applications in the fields of research and development. They can facilitate verifying and improving complex climate models of vehicle interiors, for example, to bring energy storage devices up to the thermally optimum condition and to prevent the demagnetization of electric motors.

Advantages of thermocouples

They come with a rugged design.
They can be made wafer-thin, enabling a rapid response time (up to 0.1 second/10 Hz), with an almost universal integration.
The price is low.
They can be applied for a wide temperature range.

Disadvantages of thermocouples

Compensating lines of the same material (thermoelectric wires) are required.
Contact points result in additional thermocouples, e.g. nickel-chromium (NiCr) in the Type-K thermocouple to copper in the measuring instrument, which generates thermoelectric voltages that have to be compensated (cold junction compensation).
Accuracy class
They have a non-linear characteristic.
There are many uncertainty factors.
Temperatures must frequently be measured in environments with high electromagnetic interference.

The systems being tested, such as combustion engines, compressors, and electric drives, generate their own electrical noise. Noisy environments are produced by objects that discharge high voltages, such as spark plugs, and through high currents and voltages close to the measuring point.

The optimal data acquisition system for measuring temperatures with thermocouples

The optimal data acquisition system captures the smallest signals, overcomes all challenges in varying ambient conditions, and further suppresses disturbance variables such as signal noise.

With the QuantumX, HBM offers a data acquisition system that is suitable for all common physically measurable quantities, and that provides simple data analysis with the catman software.

The following models, specially designed for the connection of thermocouples, are currently available:

MX1609KB: 16-channel data acquisition module, Type-K
MX1609TB: 16-channel data acquisition module, Type-T
MX1609KB-R: Rugged amplifier with 16 individually configurable channels for Type-K thermocouples (SomatXR series)
MX809B: 8-channel data acquisition module, all types (copper receptacle)
MX840B/MX440B: 8/4-channel universal data acquisition module, all types

Measuring temperature and numerous other signals with the QuantumX

HBM offers the QuantumX data acquisition system (DAQ) for the precise measurement of temperatures utilizing resistance thermometers or thermocouples. The QuantumX family is modular and scalable, which is in line with your requirements. It enables the connection of any signals, sensors, and transducers and synchronously digitizes physical quantities, such as temperature, strain, pressure, force, torque, velocity, acceleration, position, flow, voltage, and current.

QuantumX – the DAQ system for precise measurement results

The modular QuantumX data acquisition system processes measurable quantities, such as temperature, with exceptional precision:

It has an extremely broad range of application in R&D in fields, such as mechanical engineering and the automotive, medical engineering, and aerospace industries.
It measures temperatures with extreme precision, down to 0.1 K.
It is insusceptible to interference effects.
Reliable measurements can be performed at a high voltage potential (VDE certified).
There is automated channel parameterization using wireless TEDs (RFID).
It is ideal for structural and highly dynamic measurements, with the freedom to choose sampling rates from 0.1 to 40.000 S/s.
Optimum data transfer is facilitated via Ethernet, and it can be integrated with the software of your choice.

Ultra-rugged SomatXR MX1609KBR Thermocouple Module

MX809B Data Acquisition Module for Measurement of Temperatures and Voltages

The QuantumX offers optimum technology for almost every application:

It comes with universal or special inputs – all types, or specific types.
There is a highly accurate cold junction compensation of individual channels near the junction to copper (Pt1000).
Channels are electrically isolated from one another and from the power supply and network, effectively suppressing disturbance variables.
The measurement chains are safe owing to double insulated cable, connector with protection against contact, high electrical insulation (see measurement categories), and verification by the VDE.
It integrates polynomial linearization of curves in line with the IEC.
There is a second application-specific linearization through thermal calibration of several points (e.g., ice water and 200°C) to correct the polynomial.
It has a rugged and compact design, with an extended temperature range.
It comes with automated channel parameterization, display of measuring point, and individual linearization using TEDs (RFID technology).
It offers individual sampling rates per channel + low-pass filter.
It has an expandable, freely scalable system design.
Synchronous measurement can be performed.
It can be easily integrated with the software of your choice. With catman from HBM, it provides a simple and comprehensive solution for data acquisition and analysis.
It can be integrated in real time (EtherCAT, CAN bus).