Instrumentation engineers spend a lot of time considering how to evaluate all sorts of sensor technologies, but they often take the mechanisms for sending the information to a host system for granted. Flow meters and level sensors make this easy because they typically use standard protocols such as WirelessHART or FOUNDATION Fieldbus. Working with temperature sensors can get more complicated.
In the follow-up installment to their article published in March, Neil Larson and Todd Olin start moving up the chain from a temperature sensor to the next stage, where the signal from the sensor gets carried to the automation host system and is presented to control room operators. The April issue of Process Heating carries their article, Improving Temperature Measurement Performance: Sensor Mounting and Signal Processing.
As they point out, users may try to send the raw signal from a thermocouple or RTD directly to the host system, but this carries some downsides.
Think about the nature of the analog signal: a thermocouple sends a millivolt (mV) signal and an RTD provides an ohm (Ω) reading, but neither is robust. For example, a type K thermocouple responding to a temperature of 302°F (150°C) with a reference junction of 32°F (0°C) produces 6.138 mV. An increase to 6.540 mV corresponds to 320°F (160°C). So, a small fluctuation up or down in voltage can represent a significant temperature change. Similarly, for a Pt100 RTD, a temperature of 302°F is indicated by a resistance of 157.33 Ω. An increase of only 3.72 Ω anywhere in the circuit moves the temperature to 320°F.
Those signals can easily be distorted or shifted up or down by a variety of factors, and the longer distance, the more opportunities for problems. The solution calls for signal processing to improve overall strength and provide superior performance. The most common and practical choice is a transmitter, such as a Rosemount 644, to provide this important function.
In part one of this two-part series, it was noted that a transmitter can be used to make temperature measurements more precise. But, there is more to the picture. First and foremost, a transmitter takes the raw signal from the sensor and converts it to a more robust, standardized format. This could include a 4 to 20 mA analog signal, or digital communication protocols such as HART, Profibus PA, Foundation Fieldbus and WirelessHART.
The article goes into more detail of the mechanics of using transmitters, but it also digs into the questions of how to mount a sensor to deliver the reading. The technique by which the sensor is mounted and its ability to reach into the process to the point where the measurement needs to be is a major consideration. Sometimes it’s possible to get a useful reading from the surface of a vessel or pipe, but reaching into a process usually requires a thermowell, yet another potential area of complexity.
A thermowell is an extension through the wall to support the sensor and permit insertion without releasing the contents. Thermowells can be problematic because they become part of the larger process containment and are subject to the temperature, pressure, and corrosiveness of the process fluid. Thermowells also are subject to stresses caused by the process fluid movement. If the fluid is moving quickly, vortices can form downstream and induce serious vibrations, causing fatigue-related failures. If these are not mitigated and if the thermowell is not strong enough to withstand them, it can fracture.
Fortunately, Neil and Todd discuss ways to mitigate the problem, including the use of the Rosemount Twisted Square Thermowell. Drawing on a point made in the first article, getting a useful and accurate temperature reading is all part of a “chain of components between the sensor and final input point.” Any element can be the weak link, including something as simple as the care taken when attaching a wire to a terminal. Emerson can help minimize the potential for disruption.
You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Temperature Group and other specialty areas for suggestions and answers.