Temperature measurement, a vital part of most industrial operations, is typically accomplished by a temperature sensor--a thermocouple or a resistance temperature detector (RTD)--in contact with a solid surface or immersed in a fluid. Although these sensors have overlapping temperature ranges, each has certain application-dependent advantages.
Several factors must be considered when selecting the type of sensor to be used in a specific application: temperature range, accuracy, response time, stability, linearity, and sensitivity. These are summarized in Table 1. An RTD is the sensor of choice when sensitivity and application flexibility are the most important criteria. When it comes to component cost, though, an RTD is more expensive than a thermocouple. Choosing the perfect sensor for a particular application therefore requires an understanding of the basics of RTDs and thermocouples.
RTDs operate by exhibiting an increase in resistivity with an increase in temperature. RTDs are most commonly made from platinum, nickel, or copper. Copper and nickel versions operate at lower temperature ranges and are less expensive than platinum. Platinum is the most versatile material because of its wide temperature range (–200°C to 850°C), excellent repeatability, stability, and resistance to chemicals and corrosion. Platinum RTDs are available in 100 Ω, 200 Ω, 500 Ω, and 1000 Ω nominal resistance values at 0°C, of which the 100 Ω is the most popular.
The basic construction of an RTD is quite simple. A sensing element is connected to lead wires and supported by an insulator such as glass, mica, or a ceramic placed inside a protective sheath (see Figure 1). The detectors are available in 2-, 3-, and 4-wire configurations.
The 2-wire version is well suited to applications where the sensor is directly connected to the receiver to prevent lead length resistance errors, a problem that led to the development of more accurate 3- and 4-wire configurations.
When there is a significant distance between the sensor and the receiving instrument, 3-wire units are used. Their accuracy, although less than that of a 4-wire detector, is sufficient for many industrial applications.
In the 4-wire configuration, one pair of leads supplies the excitation current to the RTD and the other pair measures voltage across it. This technique significantly minimizes lead voltage drop and provides high accuracy.
A thermocouple is made up of two dissimilar metals, joined together at one end, that produce a voltage (expressed in millivolts) with a change in temperature. The junction of the two metals, called the sensing junction, is connected to extension wires (see Figure 2). Any two dissimilar metals may be used to make a thermocouple. Of the infinite number of candidate combinations, the ISA recognizes 12. Most of these thermocouple types are known by a single-letter designation; the most common are J, K, T, and E. The compositions of thermocouples are international standards, but the color codes of their wires are different. For example, in the U.S. the negative lead is always red, while the rest of the world uses red to designate the positive lead.
Measurement errors can be easily introduced with thermocouples. Since the voltage created by the thermocouple is due to the bonding of two dissimilar metals, the introduction of other junctions to the circuit results in voltage changes that are referred to as cold junction errors. If the temperature at the connections is determined, these errors can be corrected by a process called cold junction compensation. This is carried out at the receiving device, which is usually the signal conditioner.
Thermocouples are also available in three junction types: grounded, ungrounded, and exposed. The grounded thermocouple has its sensing junction directly attached to the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction (see Figure 3A).
The ungrounded thermocouple has its junction point detached from the probe wall. This type has a response time that is slower than the grounded style (see Figure 3B). When response time is the determining factor in selecting a thermocouple probe type, the exposed thermocouple is preferable (see Figure 3C). In this type of probe, the sensing junction protrudes out of the tip of the sheath and is exposed to the surrounding environment. Ungrounded thermocouples offer the best response time, but cannot be used in corrosive or pressurized applications.
Temperature measurement decisions can make or break the expected results of the process. Choosing the correct sensor for the application might be a difficult task, but processing that measured signal is also very critical.
The Role of Signal Conditioners
Once the temperature sensor has been selected it must be integrated into the control system, which is usually based on a DCS or a PLC. One method of integration is to directly connect the RTD or thermocouple lead wires to the controller. This technique requires a dedicated temperature conversion card. Multichannel temperature cards are available for different controllers but are expensive and do not offer system flexibility.
Another method, temperature signal conditioners, is becoming very widespread in the industry because of the benefits it offers: accuracy, noise immunity/isolation, system flexibility/diagnostics, and cost savings.
Accuracy. Temperature signal conditioners were developed to maintain the integrity of the sensor's output. Their accuracy specifications surpass those of a PLC or DCS card. Conditioning the sensor signal near the measuring point prevents degradation of the signal from errors introduced by thermal gradients with thermocouples and resistance imbalance with RTD wires.
Noise Immunity/Isolation. Thermocouple and RTD signals are very low level, making them extremely susceptible to noise. A signal conditioner can convert this low-level signal into a 4–20 mA output that is more immune to noise and which can be run longer distances. Conditioners also provide low-pass filtering that prevents high-frequency noise from passing through to the controller. Signal conditioners also provide isolation, which prevents inaccuracies due to ground loop problems that are very common with temperature measurement.
System Flexibility/Diagnostics. Signal conditioners provide the control system with complete flexibility. A 4–20 mA measurement signal can be sent directly to a recorder or the analog card. Some sophisticated temperature conditioners provide both analog and digital outputs for alarming or emergency shutdown. The modules also provide local and remote indications in case of wire break.
Cost Savings. Thermocouple or RTD wires connected directly to temperature cards are expensive, especially when labor, maintenance, and troubleshooting are added to the system cost. The use of temperature signal conditioners in conjunction with standard analog and/or digital input cards can reduce that cost considerably.
Temperature measurement and control begin with selecting the suitable sensor. The sensor signal is made useful and highly accurate by integrating temperature signal conditioners upstream of the main controller. This method of temperature control is becoming very popular due to its flexibility and reliability.