Types Of Temperature Transducers PDF

Types of Temperature Transducers: 1. Thermistors 2. RTDs 3. Thermocouples THERMISTOR A thermistor is also known as Thermal Resistor. Thermistor is a temperature-dependent resistor since this kind of resistor is very sensitive to a small change of temperature which is why it is used as...

Types of Temperature Transducers: 1. Thermistors 2. RTDs 3. Thermocouples THERMISTOR A thermistor is also known as Thermal Resistor. Thermistor is a temperature-dependent resistor since this kind of resistor is very sensitive to a small change of temperature which is why it is used as a temperature sensor. In a short explanation, a thermistor is a resistor that shows a change in resistance when temperatures fluctuate. (PPT) - A thermistor is often utilized as a temperature sensor due to its sensitivity to changes in temperature. The term "thermistor" is a blend of "thermal" and "resistor." While the majority of resistors try to reduce their temperature coefficient, thermistors are specifically engineered to have a high temperature coefficient. The word thermistor can be termed as Thermal Resistor. So as the name indicates it is a device whose resistance changes with the change of the temperature. Due to there high sensitivity they are widely used for the measurements of the temperature.They are usually called the ideal temperature transducer. Thermistors are generally composed of mixture of metallic oxides. (EXPLANATION) Types of Thermistors: Negative Temperature Coefficient (NTC) thermistors, which have decreasing resistance as temperature rises (The NTC type is used when a change in resistance over a wide temperature range is required. They are often used as temperature sensors in the range of -55 °C to 200 °C, although they can be produced to measure much lower or higher temperatures. Their popularity can be accounted to their quick response, reliability, robustness, and low price.) , and Positive Temperature Coefficient (PTC) thermistors, which have increasing resistance as temperature rises. (They exhibit a sudden increase in resistance above a defined temperature, called the switch, transition, or Curie temperature. The most common switching temperatures are in the range of 60 °C to 120 °C. They are often used for self-regulating heating elements and self-resetting over-current protection.) Thermistor Symbols Properties of Thermistors: They have Negative Thermal Coefficient i.e. resistance of the thermistor decreases with increase in temperature They are made up of the semiconductor materials They are made sensitive than RTD (Resistance Thermometres) and Thermocouples There resistance lies between 0.5Ω to 0.75 MΩ They are generally used in applications where measurement range of temperature -60oC to 15oC. - Temperature transducers, specifically NTC (Negative Temperature Coefficient) thermistors, exhibit the following properties due to their design and material characteristics: - Negative Thermal Coefficient (NTC): Thermistors with a negative temperature coefficient decrease in resistance as temperature increases. This behavior occurs because they are made of semiconductor materials, which have more free charge carriers (electrons and holes) at higher temperatures. As temperature rises, more electrons become available for conduction, leading to a drop in resistance. - Made of Semiconductor Materials: Semiconductors, unlike metals, have a strong temperature dependence. The electrical resistance of semiconductors decreases with increasing temperature due to the increased number of charge carriers. This makes them suitable for thermistors, as they provide significant resistance changes with small temperature variations. - More Sensitive than RTDs and Thermocouples: Thermistors are more sensitive than RTDs (Resistance Temperature Detectors) and thermocouples because they exhibit a larger change in resistance per degree of temperature change. This allows for more precise measurements in a narrow temperature range, making thermistors ideal for applications where high sensitivity is required.  - Resistance Range (0.5Ω to 0.75 MΩ): Thermistors are designed with a wide range of resistance values, allowing them to be used in various applications. Their resistance can vary significantly with temperature, enabling precise control and measurement across different environments. - Used in Temperature Range of -60°C to 15°C: Thermistors are often utilized in applications within this temperature range because they perform well in low-temperature conditions. They are sensitive enough to detect small temperature variations, making them suitable for environmental monitoring, refrigeration systems, and other low-temperature applications where accurate measurements are essential. - Overall, these properties make NTC thermistors highly effective for precise temperature sensing in a variety of applications, especially those requiring high sensitivity and performance in low temperature ranges. Thermal Packages Several package types and sizes are available. The radial leaded type is the most common and is primarily constructed from epoxy. For applications in harsh environments, glass encapsulated packages are more suitable. Integrated packages are also available, such as threaded housings, lugs, or probes for easy mounting. The following figure shows some examples of available package types. (explanation) RESISTANCE TEMPERATURE DETECTOR (RTDs) Resistance Temperature Detector or also known as RTD’s, are also considered as temperature sensor but made from high-purity conducting metals like platinum, copper, or nickel wound into a coil and which the temperature changes, electrical resistance changes as well, just like the thermistor. - An RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its temperature changes. The resistance increases as the temperature of the sensor increases. The resistance vs temperature relationship is well known and is repeatable over time. An RTD is a passive device. It does not produce an output on its own. External electronic devices are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage. Typically 1 mA or less measuring current, 5 mA maximum without the risk of self-heating. (Explanation) There resistance changes with following relation as, R = Resistance of element at given temperature α = Thermal coefficient of element Ro = Resistance of element at Main Feature of RTDs: They are highly sensitive and very cheap as compared to thermistors and thermocouples. (RTDs are more sensitive than thermocouples due to their linear, stable resistance-temperature relationship, allowing for more precise measurements. They are also cheaper when precision is needed within a specific range, as their simpler construction makes them more cost-effective compared to the more durable but complex thermocouples.) They can measure the temperature from -182.96oC to 630.74oC (RTDs, particularly platinum- based ones like the PT100, can measure temperatures from -200°C to 850°C due to platinum’s stable resistance changes. The range of -182.96°C to 630.74°C fits within RTD capabilities, making them ideal for both cryogenic and high-temperature industrial applications. This versatility is why RTDs are preferred for precise temperature monitoring.) In summary, RTDs are highly sensitive, more cost-effective for precise applications, and capable of measuring a wide range of temperatures due to their stable material properties (typically platinum) and predictable resistance-temperature relationship. THERMOCOUPLE Thermocouple is a sensor used to measure temperature. It consists of two different metals wires that joined together, such as copper and constantan. One of the metals’ junctions is at a constant reference temperature, while the other is at the measuring junction. - A thermocouple is simply a sensor that is used for measuring temperature. This design of sensor consists of two dissimilar metal wires which are joined together at one end, connected to an instrument that is capable of accepting a thermocouple input and measure the reading. A voltage develops between the junctions when they are at different temperatures, which is used to measure the temperature. (explanation) Principle of Thermocouple A potential difference is created between two metals, such as copper and constantan, when their junctions are connected. - When junctions are joined together at one end, connected to an instrument that is capable of accepting a thermocouple input and measure the reading. A voltage develops between the junctions when they are at different temperatures, which is used to measure the temperature, this is also called Seebeck effect. (explanation). Main Features of Thermocouple: Using thermocouples, one can measure extreme temperatures ranging from -200°C to over +2000°C, which is an advantage over RTD and thermosistors.( Thermocouples can measure extreme temperatures from -200°C to over +2000°C, offering a broader range than RTDs and thermistors, which is beneficial for applications involving very high or very low temperatures.) Unlike RTDs and thermoistors, these are active transducers, meaning they can measure temperature without the need for an external source. (Unlike RTDs and thermistors, thermocouples are active transducers. They generate a voltage directly in response to temperature differences without needing an external power source, making them simpler in certain applications.) They are less expensive than thermoistors and RTDs. (Thermocouples are less expensive than RTDs and thermistors, making them a more economical option for general temperature measurement.) Because of their lower accuracy when compared to RTDs and thermoistors, these are typically not utilized for highly precise tasks. (Thermocouples are typically less accurate than RTDs and thermistors. Due to this limitation, they are generally not used for tasks requiring high precision but are suitable for applications where extreme temperature ranges or lower accuracy is acceptable.)