Match Temperature Sensor Configuration to the Application for Best Results

Special construction features can better adapt a temperature
sensor to measuring process conditions.
Image courtesy Pyromation

There are more temperature controlled operations than any of us could count in a lifetime, each with a set of signature performance requirements and design challenges. Matching the means of temperature measurement, the control loop characteristics, and heat delivery method to the application are essential to achieving successful operation.

Step one is to measure the process temperature. This sounds simple until you start researching products and technologies for measuring temperature. Like the temperature controlled operations mentioned previously, they are numerous. To filter the possible candidates for temperature sensing devices, consider these aspects of your application and how well a particular sensor may fulfill your requirement.

• Response Time - How rapidly the sensor will detect a change in process temperature is a function of how the sensor is constructed and how it is installed. Most temperature sensors are enclosed or encapsulated to provide protection for the somewhat vulnerable sensing element. Greater mass surrounding the sensing element, or a shape that inhibits heat transfer from the process to the sensor, will slow sensor response. Whether the slower response time will adversely impact process operation needs to be considered. More consideration is due to the manner in which the temperature sensor assembly is installed. Not all applications involve a fluid in which the sensor assembly can be conveniently immersed, and even these applications benefit from careful sensor placement.
• Accuracy - Know what your process needs to be effective. Greater levels of accuracy will generally cost more, possibly require more care and attention to assure the accuracy is maintained. Accuracy is mostly related to the type of sensor, be it RTD, thermocouple, or another type.
• Sensitivity - Related to the construction, installation, and type of sensor, think of sensitivity as the smallest step change in process temperature that the sensor will reliably report. The needs of the process should dictate the level of sensitivity specified for the temperature sensor assembly.

Take a simple application as an illustration. Heat tracing of piping systems is a common function throughout commercial and industrial settings experiencing periods of cold weather. Electric heat trace installations benefit from having some sort of control over the energy input. This control prevents excessive heating of the piping or applying heat when none is required, a substantial energy saving effort. A temperature sensor can be installed beneath the piping's insulation layer, strapped to the pipe outer surface. A specially designed sensor assembly can improve the performance of the sensor and the entire heat trace control system by enhancing the response time of the temperature sensor. A right angled sheath permits insertion of the sensor beneath the piping insulation while orienting the connection head upright. A surface pad at the tip of the sheath increases the surface contact with the pipe to provide faster sensor response. The surface pad is a metal fixture welded to the sensing end of the temperature sensor assembly. It can be flat, for surface temperature measurements, or angled for installation on a curved surface, like a pipe. The increased surface contact achieved with the surface pad promotes the conduction of heat to the sensor element from the heated pipe in our illustration. This serves to reduce and improve the response time of the sensor. Adding some thermally conductive paste between the pad and the pipe surface can further enhance the performance. While the illustration is simple, the concepts apply across a broad range of potential applications that do not allow immersion of the temperature assembly in a fluid.

A simple modification or addition of an option to a standard sensor assembly can deliver substantially improved measurement results in many cases. Share your temperature measurement requirements and challenges with a process measurement specialist. Leverage your own process knowledge and experience with their product application expertise.

Achieving Close Control of Process Temperature

Process Temperature Controller
Courtesy Yokogawa

Temperature control is a common operation in the industrial arena. Its application can range across solids, liquids, and gases. The dynamics of a particular operation will influence the selection of instruments and equipment to meet the project requirements. In addition to general performance requirements, safety should always be a consideration in the design of a temperature control system involving enough energy to damage the system or create a hazardous condition.

Let's narrow the application range to non-flammable flowing fluids that require elevated temperatures. In the interest of clarity, this illustration is presented without any complicating factors that may be encountered in actual practice. Much of what is presented here, however, will apply universally to other scenarios.

What are the considerations for specifying the right equipment?

KNOW YOUR FLOW

First and foremost, you must have complete understanding of process fluid properties.

• Specific Heat - The amount of heat input required to increase the temperature of a mass unit of the media by one degree.
• Minimum Inlet Temperature - The lowest media temperature entering the process and requiring heating to a setpoint. Use the worst (coldest) case anticipated.
• Mass Flow Rate - An element in the calculation for total heat requirement. If the flow rate will vary, use the maximum anticipated flow.
• Maximum Required Outlet Temperature - Used with minimum inlet temperature in the calculation of the maximum heat input required.

MATCH SYSTEM COMPONENT PERFORMANCE WITH APPLICATION

• Heat Source - If temperature control with little deviation from a setpoint is your goal, electric heat will likely be your heating source of choice. It responds quickly to changes in a control signal and the output can be adjusted in very small increments to achieve a close balance between process heat requirement and actual heat input.
• Sensor - Sensor selection is critical to attaining close temperature control. There are many factors to consider, well beyond the scope of this article, but the ability of the sensor to rapidly detect small changes in media temperature is a key element of a successful project. Attention should be given to the sensor containment, or sheath, the mass of the materials surrounding the sensor that are part of the assembly, along with the accuracy of the sensor.
• Sensor Location - The location of the temperature sensor will be a key factor in control system performance. The sensing element should be placed where it will be exposed to the genuine process condition, avoiding effects of recently heated fluid that may have not completely mixed with the balance of the media. Locate too close to the heater and there may be anomalies caused by the heater. A sensor installed too distant from the heater may respond too slowly. Remember that the heating assembly, in whatever form it may take, is a source of disturbance to the process. It is important to detect the impact of the disturbance as early and accurately as possible.
• Controller - The controller should provide an output that is compatible with the heater power controller and have the capability to provide a continuously varying signal or one that can be very rapidly cycled. There are many other features that can be incorporated into the controller for alarms, display, and other useful functions. These have little bearing on the actual control of the process, but can provide useful information to the opeartor.
• Power Controller - A great advantage of electric heaters is their compatibility with very rapid cycling or other adjustments to their input power. A power controller that varies the total power to the heater in very small increments will allow for fine tuning the heat input to the process.
• Performance Monitoring - Depending upon the critical nature of the heating activity to overall process performance, it may be useful to monitor not only the media temperature, but aspects of heater or controller performance that indicate the devices are working. Knowing something is not working sooner, rather than later, is generally beneficial. Controllers usually have some sort of sensor failure notification built in. Heater operation can be monitored my measurement of the circuit current.

SAFETY CONSIDERATIONS

Any industrial heater assembly is capable of producing surface temperatures hot enough to cause trouble. Monitoring process and heater performance and operation, providing backup safety controls, is necessary to reduce the probability of damage or catastrophe.

• High Fluid Temperature - An independent sensor can monitor process fluid temperature, with instrumentation providing an alert and limit controllers taking action if unexpected limits are reached.
• Heater Temperature - Monitoring the heater sheath temperature can provide warning of a number of failure conditions, such as low fluid flow, no fluid present, or power controller failure. A proper response activity should be automatically executed when unsafe or unanticipated conditions occur.
• Media Present - There are a number of ways to directly or indirectly determine whether media is present. The media, whether gaseous or liquid, is necessary to maintain an operational connection between the heater assembly and the sensor.
• Flow Present - Whether gaseous or liquid media, flow is necessary to keep most industrial heaters from burning out. Understand the limitations and operating requirements of the heating assembly employed and make sure those conditions are maintained.
• Heater Immersion - Heaters intended for immersion in liquid may have watt density ratings that will produce excessive or damaging element temperatures if operated in air. Strategic location of a temperature sensor may be sufficient to detect whether a portion of the heater assembly is operating in air. An automatic protective response should be provided in the control scheme for this condition.

Each of the items mentioned above is due careful consideration for an industrial fluid heating application. Your particular process will present its own set of specific temperature sensing challenges with respect to performance and safety. Share your requirements with temperature measurement and control experts, combining your process knowledge with their expertise to develop safe and effective solutions.

Selecting and Using the Right Temperature Sensor

Temperature measurement is an ubiquitous primary element of almost every fluid process control operation, as well as a host of other industrial applications. Selecting and properly applying a temperature sensing technology is key to effective process measurement. Of the two common technologies, RTD and thermocouple, each has advantages under a certain set of operating conditions, and it is useful to be familiar with the basics of how they work and how they should be applied.

Pyromation, a long standing manufacturer of temperature measurement devices for industrial and scientific applications, provides us with a useful white paper detailing thermocouple and RTD operation, as well as the strengths and weaknesses of each in application.

Read the white paper. Share your process temperature measurement challenges with a product specialist. Combining your knowledge of the application with their knowledge of the products will produce the best solution.

Selecting and Using the Right Temperature Sensor from Classic Controls, Inc.