Getting to grips with temperature measurement technologies

This simple guide will help you navigate your way through the many options available for keeping track of that most ubiquitous parameter – temperature.

Temperature accounts for approximately half the measurements taken in industrial processes. And if you consider that raising the temperature of many chemical reactions by 10°C is enough to double their speed, it’s easy to see why accurate temperature measurement is important.

Temperature measuring systems essentially comprise a sensor, a protective thermowell and a transmitter to communicate the data. This can leave users facing a bewildering choice of combinations.

Make sense of sensors
There are, fundamentally, two temperature sensing technologies utilised within the process industries – thermocouples and resistance thermometers.

Thermocouples rely on the thermoelectric effect. When two dissimilar metals are joined to form a closed circuit, an electric current flows between when their two junctions are at different temperatures.

The voltage at the junction also rises with temperature, so that heating one junction and keeping the other end cold generates a potential difference that can be translated into a temperature measurement. There are a number of alloy combinations that provide differing characteristics and temperature range possibilities, which are recognised and published in various international standards.

Resistance thermometers operate using resistance temperature detection (RTD), which measures how the resistance of a conductor changes with temperature. Platinum is normally used because it delivers a stable and almost linear response to temperature change.

RTD devices cover a temperature range of around -200°C to +600°C, although special versions are available up to +850°C.

As a family, thermocouples can be applied to a wider range of temperatures from -200°C through to 2,300°C. However, different types of thermocouple are suited to different parts of this spectrum. Thermocouples are suitable for harsh mechanical conditions where they might be subject to extreme vibrations or rapid shifts in temperature.

Although generally more accurate, RTD elements mounted in a ceramic body are more prone to failure caused by mechanical shock and extreme thermal gradients. In some extreme circumstances they may even crack and cause the measuring circuit to fail.

Protecting the element

Protecting the sensor from the external process conditions is essential when operating in harsh environments. Within a typical temperature assembly, the chosen sensor element is usually housed in a protective tube called a thermowell.

The thermowell may be machined from a single piece of material for arduous duties or fabricated for general duties. A drilled version can usually withstand up to around 550°C and 600bar, while a welded version is suitable up to 400°C and 80bar, although this is generally determined by its geometry and inherent material properties.

When choosing the thermowell, it’s important to take other factors into account, such as how chemically aggressive the medium may be and how much mechanical strain the assembly must withstand. The acceptable working life of the thermowell will vary tremendously depending on the application and might be anything from a few days up to 20 years. An experienced supplier will help users select the correct unit in order to optimise performance.

Transmitting the data

The primary measurement by the thermocouple or RTD sensor is often conditioned by a transmitter into a common industry communication format, such as analogue 4-20mA or digital fieldbus signals.

Arguably the most common installation location for a transmitter is a direct head-mounting on to the sensor. If the sensors are in a relatively harsh environment, subject to extreme heat, cold or vibration, field transmitters can be located in a convenient site, in close proximity to the measuring device. Rail and rack-mounted transmitters are also available where it is desirable to site devices remotely in control panel-type installations.

Whichever option suits the application, it’s important to check with the supplier about the transmitter’s performance. For example, if transmitters are fully potted in epoxy resin it makes them less prone to drift and they can withstand tougher conditions, like vibration, contamination, humidity and changes in ambient temperature.

Critical applications may also make it worth paying for a unit that includes self-diagnostic capabilities. This is in addition to any special requirements. For example, explosion-proof or intrinsically safe transmitters will be needed for hazardous areas.

Communications standards

The standard output from transmitters has traditionally been a 4-20mA signal, but this is increasingly being replaced in many industries by fieldbus communications. Fieldbus enables companies to take a centralised approach to controlling their operations and helps reduce maintenance costs.

The tricky thing for instrument suppliers is knowing which communication standard to work with. There are three main contenders – HART®, Profibus and Foundation Fieldbus – and none of them has yet managed to achieve real dominance in every market.

At the moment, the US seems to be favouring Foundation Fieldbus, while Profibus is more popular in Europe. It’s worth bearing this in mind by adopting transmitters that will be as flexible as possible about connecting to any new control systems that may be installed in the future.

Temperature measurement applications are so common precisely because they are so vital to the smooth running of most processes. In other words, it’s worth taking time to understand the basics behind this important technology. You might still need some expert help, but at least you’ll know which questions to ask.

These are just some of the key factors that need to be considered. As a responsible and reputable supplier of temperature equipment, ABB has all the products, expertise, service and support to meet your temperature needs. For more information email moreinstrumentation@gb.abb.com ref: ‘temperature selection’ or call 0870 600 6122.

ABB has published a comprehensive handbook covering all aspects of temperature measurement in industrial applications. For your free PDF copy email moreinstrumentation@gb.abb.com ref: ‘temperature handbook’, together with your name company and address details, or call 0870 600 6122.

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