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RTDs (resistance temperature detectors) are one of the most common temperature sensor types used in industrial applications. Thermocouples and thermistors are popular temperature sensors as well, but RTD sensors are more accurate over a wide temperature range and more stable over time, making them an excellent choice for many applications.
An RTD sensor is essentially a resistor whose resistance value increases with temperature. Due to the predictable change in resistance of certain materials as temperature changes, it is possible to acquire highly accurate and consistent temperature measurements. Most RTD sensors have a response time between 0.5 to 5 seconds or more. RTD sensors can be constructed with pure platinum, nickel or copper. RTDs made with platinum are also known as PRTs (platinum resistance thermometer) and are the most frequently used given their higher temperature capabilities, stability and repeatability.
Specifications for RTD sensors include a base resistance value and a temperature coefficient of resistance (TCR) value. Typical base resistance values can range from 10 to several thousands of Ohms (Ω) depending on material and type. The base resistance value indicates the nominal resistance of the sensor at 0°C (nickel and platinum) or 25°C (copper), with 100Ω being the most common.
The temperature coefficient of resistance does not affect a sensor’s accuracy, but is important to the measuring device that calculates changes in temperature based on the base resistance. PRTs have two standards of TCRs; the European standard (IEC 751) requires a TCR of 0.00385Ω/Ω/°C; and the American standard requires a TCR of 0.00392Ω/Ω/°C. Assuming a TCR of 0.00385Ω/Ω/°C — meaning that for every degree change in temperature, the resistance increases by 0.385Ω — a 100Ω PRT’s resistance will be 138.5Ω at 100°C. Likewise, assuming a TCR of 0.00392 Ω/Ω/°C will result in a resistance of 139.2Ω at 100°C. Thus, the measuring device used needs to be attuned to the TCR of an RTD sensor in order to accurately report changes in temperature, but the difference in TCR value has no impact on the sensor itself.
Thermocouples and thermistors are some of the other popular temperature sensors used in industrial applications. Thermocouples basically convert thermal energy into electrical energy, and use that to measure the temperature. While thermocouples measure the highest temperatures, respond quickly to temperature changes—within fractions of a second—and are easily obtainable at low cost, they are the least stable and repeatable, and suffer from poor accuracy. Thermistors are semiconductors that present a non-linear change in resistance as temperature changes; unlike an RTD, the resistance in a thermistor decreases as temperature increases. In comparison, thermistors feature high sensitivity to small temperature changes and become more stable with use, but are fragile, have a limited temperature range and currently lack standardization.
Between the three types of temperature sensors, RTD sensors are the most accurate and stable over time, and are resistant to contamination under 660°C. They also boast high repeatability, which means that RTDs can accurately measure identical temperatures even when exposed to repeated heating and cooling cycles with minimal discrepancies. This means that an RTD sensor will consistently measure 100°C after being put into an oven and subsequently a freezer multiple times. In contrast, a thermocouple is more likely to measure 100°C, then 98°C, then 103°C and so on when placed in the same situation. Since most applications do not require immediate responses (less than 0.5 to 5 seconds) to temperature changes, RTDs are an ideal solution for many industrial applications, which Network Technologies Inc (NTI) includes in its product line of ENVIROMUX® Enterprise Environment Monitoring Systems and Accessories.
NTI offers a line of platinum 100Ω RTD sensors that can be used in conjunction with one of three available transmitters to accurately monitor temperatures in many industrial applications. The temperature ranges of the available RTDs are: -67 to 240°F (-55 to 115°C), accurate to within ±0.27°F (±0.15°C); 35°F to 140°F (2°C to 60°C), accurate to within ±0.6°F (±0.33°C); and -30°F to 230°F (-34°C to 110°C), accurate to within ±0.6°F (±0.33°C). Rugged, waterproof RTD sensors are available for harsh environments. Some of the common installations for the RTD sensors include: plenum mounting, duct mounting, immersion wells, direct mounting onto sheet metal duct systems, remote temperature sensing for building automation systems and mechanical equipment room instrumentation.
Transmitters are necessary to convert the resistor values into temperature values, and can be connected to NTI’s Enterprise Environment Monitoring Systems for a variety of alert and logging functions. The ENVIROMUX-RTDT-x 100Ω Platinum RTD Transmitter is available in two ranges, -20 to 140°F (-28 to 60°C) and 30 to 240°F (-1 to 115°C), and is accurate to within ±0.8°F (±0.45°C). Both units support 2-wire connections and can be calibrated for higher accuracy. With a wider temperature range of -328 to 1562°F (-200 to 850°C), the ENVIROMUX-RTDT-1562 High-Accuracy Platinum RTD Transmitter is accurate to within ±0.2°F (±0.1°C). It supports 2, 3, or 4-wire connections and is configurable to support 100Ω platinum, 120Ω nickel or 10Ω copper RTD sensors. With RS485 signal output, the transmitter boasts precise temperature measurements.
When combining the RTD sensors and transmitters with NTI’s ENVIROMUX Enterprise Environment Monitoring Systems, companies not only can accurately monitor temperature, but they also can monitor a wide range of other environmental threats such as humidity, liquid water presence, power, intrusion and smoke, and receive alert notifications when a sensor goes out of a configurable threshold — an ideal preventive measure for many industrial applications.
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