Selecting and using appropriate devices can be the difference between a company saving or losing thousands of dollars. Equally important to selecting the sensor is how the measurement is transmitted to the control room. When planning your temperature measuring approach, understand both to ensure maximum effectiveness.
It is important to first understand the different sensor technologies and wiring measurement options. Although many industrial temperature sensor types are available, resistance temperature detectors (RTDs) and thermocouples (T/Cs) are the most common. The technology behind RTDs and thermocouples is drastically different, each giving their own benefits that drive appropriate selection.
RTDs are based on the concept of thermal resistivity. As the measuring element increases in temperature, the electrical resistance increases as well. RTD elements typically are constructed from resistive materials such as platinum, copper or nickel, each having a standard correlation between resistance measured and process temperature.
Two styles of RTDs — wire-wound and thin-film — commonly are used. Wire-wound RTDs are constructed either by winding the resistive wire around a ceramic mandrel or by winding it in a helical shape. Thin-film RTDs have a thin resistive coating that is applied atop a ceramic substrate.
By contrast, thermocouples consist of two wires of dissimilar metals that are joined at the end. A phenomena known as the Seebeck effect causes a current to be created when the temperature applied to one end, or “junction,” differs from the temperature at the other end of the wire pair. The end placed inside a sensor sheath and exposed to process temperatures is called the “hot end” while the other end, used as a reference temperature outside the process, is called the “cold junction.” The Seebeck effect states that a voltage measured at the cold junction is proportional to the difference in temperature between the two junctions.
Each sensor type has benefits and drawbacks that can help the user determine which sensor is best for a given application. For instance, wire-wound RTDs have a temperature range of -328 to 1,562°F (-200 to 850°C), making these sensors suitable for cryogenic applications as well as most general-temperature measurements. Wire-wound RTDs are less susceptible to hysteresis effects from temperature changes, keeping their accuracy over time. They also typically are capable of higher accuracies than thin-film styles.
Thin-film RTDs are designed to have a robust mechanical design and moisture seal. Their relatively low mass, size and design — the wire is encapsulated by the ceramic substrate — make these sensors a good option in high vibration applications. Thin-film RTDs typically are less expensive than wire-wound RTDs because fewer materials are needed for their construction. One drawback is that these sensors are more limited in temperature range, being effective between -58 and 752°F (-50 and 400°C).
The many different types of thermocouples available differ in that they use various metal combinations. The most common types are J, which uses iron and Constantan, and K, which uses Chromel and Alumel. Thermocouples have faster response times and higher temperature ranges than RTDs, but they are far less accurate.
Why choose an RTD over a thermocouple?
- Better accuracy and repeatability.
- A higher signal-to-noise ratio, which makes them less susceptible to noise such as radio frequencies, electromagnetic interference and voltage spikes.
- Better linearity over temperature ranges.
By contrast, thermocouples offer:
- Much higher temperature ranges, with the highest to 3,272°F (1,800°C).
- Typically, a faster response time than RTDs.
- Lower cost.
The final choice depends on your process and temperature measurement accuracy needs, among other factors.
Reading Your Sensor
There are two ways to get sensor temperature measurements back to a monitoring and control system. They are:
- Utilize sensor extension wires to carry the low level signals generated by RTDs and thermocouples.
- Install temperature transmitters at or near the measurement point.
Although direct wiring strategies originally were deemed less expensive and sometimes easier, modern transmitters are highly functional and affordable. Transmitters also can save time and minimize the cost of maintenance and installation.
Direct wiring sensors to control a system requires one to use sensor extension wires. These wires are fragile and cost much more than the shielded copper wire used for a temperature transmitter’s analog or digital signal. The further away the control system is from the point of measurement, the longer the wire run, and the greater cost associated with this type of wiring system.
Temperature transmitters allow for accurate measurements. Transmitters can be calibrated to any range within a sensor’s capability. By measuring a narrow range, a more accurate measurement is produced. Temperature transmitters contain sensor diagnostics that allows the user to track sensor operation and find and diagnose sensor failure. These diagnostics can indicate unwanted conditions, sensor burn out and wire breakage. The information obtained from a transmitter can minimize the maintenance cost of removing a sensor from a process to check if it is working properly.
To get the most from your temperature measurement, thought must be put into the type of sensor used as well as the way in which the data collected from the sensor is obtained and read. Both of these elements can affect your temperature reading and the effectiveness of your system. PH