Key points
Process Temperature is one of the most common points of measurement in process automation and is often the most critical in terms of safety, product quality and efficiency across a wide range of industries.
In today’s marketplace there are many device styles to choose from, ranging from basic sensors and bimetallic gauges through to complex control systems operating on bus networks. Selecting the correct instrument for an application can be daunting with so many alternatives available.
The best products will provide a stable, accurate, fast and reliable measurement, combined with a robust and high-quality build as well as an easy-to- use, intuitive operator interface.
Many low-cost systems using long-standing technologies are effective in basic applications where few process stresses are present and low performance is accepted.
However, with recent high levels of advancement and development in temperature measurement technologies, many traditional style components are being increasingly outperformed by their modern counterparts (especially in terms of response speed and ease of calibration, two of the biggest problems faced by operators).
The consumer benefits that were motivators to select an instrument yesterday are expected as standard today, increasing demand for higher-performance products with fresh developments.
These newer, groundbreaking designs are quickly absorbed and utilised by the marketplace to help provide a more efficient automation process and help maintain optimum performance in process automation.
Elements for Process Temperature Measurement
For process control purposes, the traditional methods of an RTD (Resistance Temperature Detector) or thermocouple element are still a best-fit solution.
RTDs rely on the linear relationship between the electrical resistance of a metal and its temperature (known as thermal resistivity) to produce a signal that can be interpreted as a temperature reading.
As the temperature of a metal heats or cools, the electrical resistance will raise or lower simultaneously and measurement of this resistance, cross-referenced through recognised standards, can be interpreted as a temperature.
There are two main styles of RTD element commonly used in process measurement. The first takes the form of a wire wound platinum element (where a length of resistive wire is formed in a helical shape around a ceramic body) and the second comprises a thin film element (where a thin resistive platinum coating is applied to a flat ceramic substrate).
Development of the latter design has led to higher performance in response times and vibration resistance, making it the preferred style to date. In both designs, the element legs are then extended away from the point of process for measuring purposes.
Conversely, thermocouple sensors simply use two different types of metals in cable form connected together. This point of connection (the hot junction) is placed at the process measurement point while the extended leads are terminated away from the process (the cold junction).
When the temperature at these two points differs, a small voltage is generated (known as the Seebeck effect) and as the temperature differential changes so does the voltage in a near linear manner. In this way the measured voltage can be interpreted as a temperature.
Either the extended RTD cables or the thermocouple cold junction end can be terminated for direct measurement or conversion to a process signal via a signal transmitter.
Transmitters for Process Temperature Measurement
While many temperature instruments are read simply as RTD or thermocouple outputs, the majority are connected to transmitter electronics in order to produce a processed signal.
Available as compact units that fit into the temperature instrument head, remote panel- mounted items (door or DIN rail) or larger wall- or pipe-mounted items, these intelligent transmitters can be scaled and programmed to tailor their output signal for the required application.
Many have the option for a connected display and common signal outputs are in the form of milliamp (with or without a HART signal), FOUNDATION Fieldbus or PROFIBUS.
As with all measurement devices, the quality and performance of the transmitter can differ from one manufacturer to another. Premium manufacturers will generally produce transmitters with greater stability, accuracy and repeatability, allowing for an overall superior performance.
Where possible, all components in the measurement loop should be purchased from the same manufacturer. It is critically important for the transmitter used to be of the highest possible quality as this section of the pathway in the measured signal will have a significant impact on the performance of the sensor in what could be a critical measuring point in the application or process.
Thermowells for Process Temperature Measurement
Both RTD and thermocouple elements are relatively fragile sensors and to be used successfully in general applications they can often require protection from the very process they are measuring.
This protection is offered in the form of a thermowell, a pocket that houses the element and safeguards it from potential damage caused by corrosive chemicals, high pressures or solids in the measured medium. Furthermore, the thermowell acts as an integral component of the process, allowing the temperature element to be removed without breaking into the pipe or tank in which it is fitted.
Connected to the process by a thread, flange or direct weld, a thermowell can be supplied in any style. Where necessary, a bespoke design will offer perfect compatibility with the process requirements.
Manufactured from either machined barstock or fabricated components, these protective pockets can be straight, tapered or stepped, produced from a variety of materials or even coated to guarantee performance and compliance with the measured medium.
It is also important to ensure that the fitted temperature element connects snugly with the inside tip of the thermowell in order to gain positive thermal transfer. This can be achieved with a spring loaded probe or with thermoconductive paste to help bridge any gap.
Future developments or process temperature measurement
As transmitters are further developed with increased accuracy, resolution and repeatability, we can see the way the technology will advance.
Similarly, the evolution of measuring elements, especially thin film sensors, has shown that quick response and resilience to process effects is a trend that will continue in future years.
Flexibility of application and ease of consumer involvement are the new, fast-growing fields of development for temperature sensors. With intelligent application of the technology, temperature measurement is being used for such diverse purposes as conveyer belt alignment detection and bearing monitoring alongside more traditional measurement purposes.
Consumer influence is also helping to shape the future of temperature instruments. Quick release mechanisms for faster removal of probes from their process has been a welcome development for those with regular calibration needs in hygienic applications, exponentially reducing the downtime required for calibration.
In the same vein, the development and future launch of a self-calibrating temperature probe from Endress+Hauser will almost eradicate the need for operator involvement to produce accredited calibration certificates on a regular basis, increasing savings both in cost and time.
This type of development is set to revolutionise the position that temperature probes hold within the process industry and align them with the capabilities and features seen in larger instruments such as flowmeters and level sensors.
However these products develop in future years it can be seen that major manufacturers of temperature probes are pushing the limits of technological development to create instruments that are fit for purpose now and in the future.
