This post was written by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). This insight was adapted from Greg’s book, Advanced Temperature Measurement and Control.

Communication from the temperature sensor to the control room should be designed to provide the greatest accessibility, flexibility, reliability, and accuracy. What was traditionally thought to be the least expensive solution has severe ramifications and hidden costs. Here we look at how the advent of smart transmitters and wireless communication has opened the door to better communication and a lower installed and life cycle cost and process performance improvements.

The most common process measurement rivaling or exceeding flow in the number of sensors in a process plant is temperature. Distillation columns may have four or more temperature sensors to find the best tray for temperature control and fluidized bed reactors may select the highest of 10 or more temperature sensors to deal with hot spots. In the past the most inexpensive hardware was used for the communication of the temperature to the control room not realizing the impact on measurement accuracy, noise, and reliability.

In fact early distributed control systems (DCS) were partly justified on the basis of eliminating the cost of temperature transmitters by wiring thermocouples (TCs) directly to a DCS TC input card that could handle 16 or 32 inputs. The results were disastrous in terms of introducing steps (analog to digital convertor noise) of 0.35 ^oF due to a 12 bit wide range input card and offset of 10 ^oF or more due to differences in sensors and lead wire errors. Noise from electromagnetic interference was prevalent. Derivative action could no longer be used and the PID gain had to be decreased. Column and reactor temperature errors and consequently composition errors caused poor process performance and off-spec product. In many cases, temperature setpoints were adjusted to compensate for the errors and a signal filter was added to reduce noise that increased the total loop dead time.

Communication

To be useful for control, safety, or monitoring applications, a temperature measurement signal must be communicated from the point of measurement to the control system of the process. The two most common ways are:

• Transmitter – the sensor is wired a short distance to the transmitter or connected directly to the transmitter, where its signal is converted to a digital, 4-20 mA, or wireless signal. The converted signal output is then communicated to the control system through transmitter wire or a wireless network.
•  Wired direct – the sensor’s lead wires are wired the entire distance to the control system. No signal conversion takes place along the route.

The benefits of using a temperature transmitter over wiring directly to thermocouple and RTD input cards of control system are:

• A more robust signal is delivered – the 4–20 mA or digital signal output from the transmitter is much more robust than a sensor signal that is wired direct. Noise interference has less impact on 4–20 mA or digital signals.
•  Measurement accuracy is optimal – Transmitters offer improved measurement accuracy over wiring direct. For example, sensors can be matched to transmitters (transmitter sensor matching), which improves the accuracy of the temperature measurement. The temperature span can be narrowed to match the process operating range (significant for older DCS with 12-bit input cards).
•  Lower Installation and Maintenance Cost - Transmitter installation is often less expensive than wiring a sensor direct because of savings from cabling costs and installation (sensor wire, especially TC wire, is relatively expensive). Also, a robust signal and accurate measurements produce time and money savings through increased functionality and diagnostic capabilities of the transmitter.
•  Better Process Performance – The greater accuracy and reliability and lower noise reduces process variability and increases on-stream time.

Insight: The use of transmitters instead of TC or RTD input cards is highly recommended to greatly improve accuracy and maintainability, by matching the calibration and nonlinearity compensation to sensor, narrowing the span, reducing noise, and offering diagnostics.

The use of digital communications allows the additional flexibility of using a single transmitter to make more than one temperature measurement and communicate these back to the control system. There are temperature devices designed to specifically take advantage of this capability, providing the ability to measure four, or eight, or potentially more individual temperatures. The most common communication techniques are the HART® (including WirelessHART™), FOUNDATION™ Fieldbus and Profibus PA standard protocols.

The reliability, security, and ease of setup of WirelessHART (Highway Addressable Remote Transducer) networks combined with increased battery life from new communication rules and PID enhancements have made wireless communication an excellent option. The temperature changes in most processes are quite slow, the refresh time can be set longer than for other types of loops, extending battery life. Also, the noise amplitude and period in temperature loops is usually quite small compared to other loops unless there are two phases (e.g., liquid and gas) or poor mixing (e.g., poor uniformity—increased variability due to insufficient agitation), decreasing the number of exception updates triggered by noise, which also extends battery life.

Technically advanced temperature communication can decrease installation costs and reduce errors and noise. The advent of the smart transmitter makes mounting the transmitter directly on the thermowell more attractive. The combination of integral mounting with the wireless option creates an incredible opportunity for process performance improvements. This post provides some general guidance. For more details including the equations to predict eight sources of measurement error see Greg McMillan's ISA book Advanced Temperature Measurement and Control, Second Edition.

Field-Mount Transmitters

Field-mount transmitters are the most rugged of all transmitter styles. Their robust housings protect against corrosion and humidity. Some field mount transmitters house the electronics in dual-compartment housings, which completely isolates them from the effects of humidity. Dual-compartment transmitters are the best design for use in harsh environments. Field-mount transmitters can be integrally or remotely mounted.

In integral mounting, the transmitter is installed directly on the thermowell by a threaded pipe nipple and a pipe union fitting to allow easy removal of the sensor. Since today’s smart transmitters are extremely reliable and have extremely low drift rates reducing calibration intervals to more than 5 years, diagnostics can be viewed remotely, and calibration can be done remotely , the need for a transmitter to be at ground level has greatly diminished. The integral mounting of transmitters reduces installation costs and eliminates errors and noise introduced by lead wires and additional terminations.

• Integral mount – the transmitter is threaded onto the sensor directly (mounts directly to U.S. style [1/2-inch NPT] sensor preferably with a pipe union fitting).
• Remote mount – the transmitter is mounted on a pipe stand or other support near the sensor. Remote mount is preferred when the measurement point is inaccessible or when the process environment is too harsh for the transmitter to be installed directly on top of the sensor.

Insight: The integral mounting of smart transmitters where permitted by accessibility and temperature, improves measurement accuracy and reliability.

The use of integral mounting and wireless transmitters provides flexibility and portability for monitoring unit operation efficiency and finding the most representative and sensitive measurement location with the least process dead time. Where ever there is a pipe connection or equipment nozzle and a line of sight to the Gateway device’s access point or nearby wireless transmitter for device hopping, the sensor with the wireless transmitter can be installed on a test basis and the benefits explored and quantified. Improvements in data analysis, equipment monitoring, and unit operation control can be prototyped and the “before” and “after” cases documented.

Insight: The integral mounting of a wireless transmitter enables flexibility and portability for online process and equipment performance metrics and optimization of measurement location.

Head-Mount Transmitters

Head-mount transmitters are small, puck-shaped transmitters. They are typically housed in a protective enclosure – a connection head for direct mounting or a junction box for remote mounting.

Rail-Mount Transmitters

Rail-mount transmitters are designed to be attached to a DIN-rail (G-rail or top-hat rail) or directly screwed onto a wall. Rail-mount transmitters are also designed for compact mounting, which allows for a number of transmitters to be mounted very closely together.  

Wiring Direct

As mentioned, wiring direct refers to wiring the sensor’s lead wires back to the control system. Because the sensor’s lead wire (and original signal) is traveling the entire distance from the point of measurement to the control system, care must be taken to avoid two key problems:

• Noise – TCs are especially sensitive to noise interference and extension wires must be routed around such sources as generators and motors.
• Heat sources – a large change in the ambient temperature can affect the sensor’s signal as it travels to the control system.
• Lead wire resistance – the total resistance of RTD lead wire and the difference in resistance of RTD lead wires will affect the RTD accuracy. A 3-wire transmitter input can compensate for total lead wire resistance. A 4-wire transmitter input can compensate for differences in lead wire resistances.

Use field transmitters for all temperature measurements important for process analysis and control. If not prohibited by accessibility or temperature, the transmitter should be integral mounted on the thermowell preferably with a pipe union to improve removability of the sensor. Consider the possibility of finding the best measurement location and exploring and prototyping the potential benefits for additional temperature measurements for process performance improvement by the use of portable wireless integral mounted transmitters. 

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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