Understanding Temperature and Transmitters
Because temperature is one of the basic metrics used in the measurement of flow, density, and other factors, the temperature transmitter is one of the most widely used field devices in industry today. Understanding one helps in selecting the other. AT A GLANCE · Temperature history · Accep · Transmitter types Temperature has a long history, grounded in personal understanding, like sticking a toe into the water before jumping in. Understanding a bit of history can help before you dive into selecting your next temperature transmitter. Temperature is defined as the measure of the average kinetic energy of the molecules of a substance, represented in a numerical scale with larger values indicating a greater average kinetic energy. However, this has not always been the case. The road to standardized temperature measurement has been a long one. Historians date the first attempt to establish a temperature scale back to 130-200 B.C. by the Greek doctor Galeano, who suggested the sensations of hot and cold be measured in a scale with four divisions, thus giving birth to the first temperature scale. It would still be centuries until well-defined temperature standards emerged. With the invention of the first thermometer by Galileo Galilei in 1592, advancements in temperature measurement began picking up pace. In the following decades, many temperature scales were conceived. All were based on one or more arbitrarily fixed points, but none were universally accepted. In 1714 Gabriel Fahrenheit, a Dutch manufacturer of precision instruments, created the first accurate and repeatable mercury thermometer. His scale, degrees Fahrenheit, became the first universally accepted temperature scale in h
ted scales
In 1742, the Swede Anders Celsius created another scale. Celsius set the freezing and boiling points of water as the definition of his scale. Celsius chose 0 degrees as the boiling point of water and 100 degrees as the freezing point of
water. Later, these points were inverted and the Centigrade scale was born. In 1948 the Ninth General Conference on Weights and Measures changed the name to the 'Celsius scale' in his honor.
The Celsius and Fahrenheit scales are both relative; their numerical values of reference are arbitrary. Because of the desire for more-grounded reference points, the additional temperature scales of Kelvin and Rankine were developed. These scales assign 0 to the thermodynamic absolute zero, the theoretical point of zero molecular kinetic energy.
With creation of universally accepted temperature scales, scientists were now free to research the effects of temperature on various substances. In 1821, Thomas Seebeck discovered that when two different metal wires are joined at two points and one of the points is heated, an electric current will circulate. It was this discovery that led to the modern development of the thermocouple, one of the most-used temperature sensors for industrial applications.
By the 20th century, the necessity of universally accepted temperature properties of various materials was clear. This would foster consistency and repeatability within the scientific community and promote scientific advancement. The most recent ratification of temperature standards was on January 1, 1990, when these scales and values were fully standardized under the assignment of the International Temperature Scale—the ITS-90. In addition, there are localized standards used in the measurem
ent of temperature: ANSI (USA), DIN (Germany), JIS (Japan), BS (UK).From the temperature scale comparison in the graphic, the following relations between scales can be deducted:
(A formula here)
Great progress in the evolution of temperature measurement has concurrently advanced the accuracy, reliability, and repeatability of temperature transmitters used in the automation and process control industry. This, in conjunction with the advancement and availability of diverse temperature sensors, h
as contributed to continuous improvement of controlled processes and final product quality.
Intelligent transmitters
An intelligent temperature transmitter is defined as a transmitter that combines the technology of the temperature sensor with additional electronics. Generally these electronics allow for remote monitoring and configuration of the transmitter parameters. Looking at the market, three distinct lines of intelligent temperature transmitters have emerged. Each has its own advantages and disadvantages based upon application and cost.
Explosion-proof and weather-proof transmitters. This type of transmitter is normally used in critical applications with high-performance requirements. The transmitter is enclosed in a sealed, explosion-proof compartment. This enclosure is generally made of stainless steel, but can be any approved explosion-proof material. The enclosure generally contains two compartments, separating the electronics and sensor. Primary advantages of this transmitter type are high accuracy, robust safety precautions, reliability, and weatherproofing. The primary disadvantage is cost. This type of transmitter generally also includes a local indicator with local adjustment. This allows the temperature to be monitored, and the transmitter adjusted in the field.
DIN rail, panel-mount transmitters. This type of transmitter can be mounted to a DIN rail and is generally used for centralized control room installations. Panel-mount transmitters are low cost, allo
w easy installation and maintenance, and can be configured for use with a diverse selection of sensor types. The primary disadvantage is the lack of explosion proofing and the tendency to have somewhat lower accuracy due to the long wiring required to mount the sensor remotely.Head-mount transmitters. This type of transmitter can be mounted directly in DIN connection heads. The primary advantage is low cost of installation, small size, and compatibility with a diverse selection of sensor types
. Because the transmitter is installed directly in the connection head, electrical connections and sensor wiring is simplified.
Communication protocols used in temperature transmitters have followed the general trend of other field devices in the process industries, with the predominant protocols being HART, Foundation Fieldbus, and Profibus. Intelligent temperature transmitters are widely available in all these protocols.
César Cassiolato is product manager, Smar Equipamentos Ind. Ltd.; Chris Murphy is Sales Engineer, Smar Research Corp.; www.smar.com 
