Of all the process variables measured by process manufacturers, temperature is the one most widely measured. Inaccurate or unreliable temperature measurement can impact process efficiency, energy consumption, product quality, and potentially process safety.
Emerson’s David Anderson authored a Control Engineering magazine article, Keys to achieving high accuracy and reliability in temperature measurement. He stresses that your temperature measurement results will depend on the interaction of all the devices.
David describes the concepts of measurement accuracy, repeatability, and stability. Accuracy:
…is the degree of closeness of the measurement of a temperature to that temperature’s actual (true) value.
…of a measurement system, also called precision, is the degree to which repeated measurements under unchanged conditions show the same results.
…refers to the transmitter’s ability to avoid drift in order to maintain accuracy over time. It is related to the sensor’s measurement signal, which can be influenced by humidity and prolonged exposure to elevated temperatures.
David illustrates the difference between accuracy and repeatability with a nice visual display. Think about an archery target where all the arrows land in the lower left portion of the target. The archer in this case is repeatable but not accurate. If the arrows all land close to the center of the target, the archer is both repeatable and accurate.
The actual measurements are made by the temperature sensor:
…usually a TC (thermocouple) or an RTD (resistance temperature detector), and a signal conditioning circuit (either a transmitter or a channel of an input card to a DCS or PLC) to amplify the sensor’s low level (ohm or mV) signal to a more robust 4-20mA current signal or a digital signal such as when using fieldbus.
He stresses the importance of thinking of temperature measurement as a system including the sensor, signal conditioning circuit, and combined with a field connection head and thermowell.
The requirements for accuracy and repeatability should be determined by the application. Some applications only require a general trend to give the direction and rate of temperature change. Other applications:
…may have huge financial impact due to temperature measurement errors. Examples include off-spec production that may require reprocessing, a nuisance process shut down requiring an expensive process restart, reduced production rates, and frequent recalibration costs.
To assess the performance and physical requirements of the temperature measurements, David advises:
- Review process P&IDs
- Compare diagrams to the actual installation
- Evaluate possible mounting locations
- Consult with process, mechanical, and environmental engineers during the component selection, and
- Plan scope and schedule with project managers.
It’s important to consider the overall temperature measurement system, not just the sensor. You can use:
…a total probable error (TPE) calculation. This analyzes the probable error of the transmitter and sensor system based on anticipated installation conditions. The components of this calculation include the root sum square of the transmitter and sensor accuracy effects…
You’ll want to read the article to see ways to maximize accuracy and reliability through intelligent filtering, transmitter sensor matching, hot device switchover, and sensor drift detection. This article is the fourth in the series of temperature measurement articles. Others you may want to check out include:
- Make the right choice: RTD vs. TC
- Taking the mystery out of thermowell selection
- The long and short of connecting sensors to control rooms
All of these concepts and much more are available to you by requesting a free copy of The Engineer’s Guide to Industrial Temperature Measurement. I hope you find this guide to be of value in proper temperature measurement system design, selection, installation, and ongoing maintenance through the lifecycle of your production process.
You can also connect and interact with David and other temperature measurement experts by joining the Temperature track of the Emerson Exchange 365 community.