To determine whether a sensor may be a thermocouple or a thermal resistance is measured using a volt ohmmeter. If the sensor has two wires, measure the resistance between the two wires. If the sensor is a thermocouple, you only need to measure the resistance of this wire.

 

Most RTDs are limited to a maximum temperature of 1000 degrees Fahrenheit. In contrast, some thermocouples can be used to measure 2700 degrees Fahrenheit.

 

The pyroelectric resistance temperature detector (RTDS) is specially designed to ensure accurate and repeatable temperature and resistance characteristics. The sensor is built in a unique strain free way and uses only high-quality RTD elements. According to the customer's requirements, use ceramic, wire wound components, flat film technology components or military special anti vibration components to ensure that the most appropriate specifications are provided. Features and benefits accuracy. A special process combines strain free construction with full winding support for reliable, accurate readings on standard RTD elements, and platinum in flat film elements that are etched on the substrate. High SNR output. It increases the accuracy of data transmission and allows a greater distance between the sensor and the measuring equipment. Interchangeability. Strain free structure and fine tuning allow elements to be replaced from different batches without recalibration. Sensitivity. Self heating is minimal and allows accurate measurement. When the insulation resistance value exceeds IEC-751 standard, the temperature coefficient (alpha) shall be strictly controlled according to the industry standard. Standardization. These elements can meet or exceed the requirements of various standardization bodies. IEC-751 standard tolerance classes a and b are very suitable for industrial applications respectively. The tolerance class up to 1/10 DIN can also provide higher accuracy requirements. Physical and chemical stability over a wide temperature range. Fireworks sales use a highly controlled production process. Standard components are built to resist mechanical vibration and shock. However, in case of high mechanical vibration exposure, specially manufactured thick film RTD components with military specifications can provide suitable applications. Repeatability. The repeatability value of all components exceeds IEC-751 even if they are exposed to the temperature within the operating range for a long time.

 

Applications • Air conditioning and refrigeration services • Food processing • Stoves and grills • Textile production • Plastic processing • Petrochemical processing • Microelectronics • Air, gas and liquid temperature measurement • Exhaust gas temperature measurement.

 

When to use RTDs? When accuracy and stability are required by customer specifications. Accuracy must be extended to a wide temperature range. • When area sensing rather than point sensing improves control. • When a high degree of standardization is required. Advantages • Linearity vs. wide operating range • Wide operating temperature range • High operating temperature range • Interchangeability vs. wide operating temperature range • Good stability • Low sensitivity • High cost • No point sensing • Affected by shock and vibration • Requires three or four wire operation

 

The main consideration for installing RTD elements is to have sufficient immersion to ensure that the RTD does not average the temperature outside the process and equipment or structure. RTD is not a point measuring device like thermocouple, so it is necessary to have a fully immersed active sensing area to ensure that RTD is measuring the actual process temperature. When installing thermowells, a good 40mm heat transfer along the sensor shaft is critical. Active sensing area of RTD sensor. This area depends on the length of the RTD element used in the build. The sensor can be designed to provide an average temperature indication. There are many options for sensor selection to consider when selecting the correct RTD elements for your requirements: temperature rated tolerance, accuracy, and interchangeability. Time response distance control or measuring equipment. The most common structure is to put RTD elements and connecting wires into closed metal tubes, wrap the tubes with vibration damping and/or heat transfer materials such as aluminum oxide powder, and seal the open ends of tubes with epoxy resin, silicone resin or ceramic cement. The most commonly used metal tubes in Rtd are made of 316 stainless steel (for about 480 ° c) or Inconel (for about 650 ° c). The damping/heat transfer materials vary greatly in the temperature range. These materials are selected by the manufacturer to provide the best performance based on the highest temperature expected for use. Another common structure is to use mineral insulated metal sheathed (MIMS) cables, in which RTD elements are inserted into a drill hole and connected to magnesium oxide (MgO) insulated nickel or copper wires. The end is insulated with magnesium oxide, the end is sealed with welding, and the other end is connected with extension wire, and then sealed as described above. Epoxy sealing compounds will not normally be used above 200 to 260 ° C. Ceramic cement may be exposed to temperatures of 1200 degrees Celsius or higher, but a sealant is required to prevent moisture from entering the cement and the underlying vibration suppression/heat transfer materials. The materials with the lowest temperature capability in platinum thermistors are usually wires and insulating materials used in buildings. Pyrosales offers two structures, low temperature and high temperature. In low-temperature structure, PTFE insulated nickel plated copper wire is used to connect RTD components. The stainless steel tube is filled with aluminum oxide powder to provide support for the element and has an epoxy seal. This structure is usually limited to 250 degrees Celsius. Nickel wire and ceramic insulator are used in high temperature buildings, or MIMS cables contain nickel wire. The nickel wire and ceramic structure are filled with aluminum oxide powder to support the element. The use of sealant will depend on the transition point of the temperature rating. These two structures can be used to a temperature of about 650 ° c. However, if the components are carefully selected and Inconel sheathed MIMS cables are used, this structure can be extended to about 850 ° c. Tolerance, precision and interchangeability: Tolerance and precision are the most easily misunderstood terms in temperature measurement. The term tolerance refers to the degree of uncertainty or possible error at a specific point. Precision refers to an unlimited number of tolerances within a specified range. For example, RTDs contain a sensing element that has a specific resistance at a specific temperature. The most common example of this requirement is the DIN/IEC standard. In order to meet the requirements of DIN/IEC standards, an RTD must have a resistance of 100 ohm ± 0.12% (or 0.12 ohm) at 0 ° c to be considered as a class b sensor (100 ohm ± 0.06% for a class sensor). The tolerance of ± 0.12 ohm is only applicable to the resistance at 0 ° c and cannot be applied to any other temperature. The following is the interchangeability table of class a RTDs, which provides the user with a tolerance table at a specific temperature. Refer to Lead wire resistance has a significant effect on the resistance being measured. The ability to compensate for these additional resistances will affect the type of assembly selected and the accuracy of the system. The most common type of assembly is the three wire RTD. Here, the lead resistance is compensated in the bridge circuit. To achieve the highest accuracy, the only option is to use a 4-wire resistor, where the wire resistance error is eliminated.

 

When selecting the assembly style most suitable for your process, you must consider the reaction speed of the sensor to temperature changes. If thermowells are used, the response time will be greatly increased, and care must be taken when designing thermowell/sensor systems. The inner diameter of the thermowell should closely match the RTD diameter in order to achieve good thermal contact, thereby maximizing heat transfer. If fast response is a standard, the smaller the element and probe, the faster the time response will be. The tradeoff is then made between the achievable response time and whether the sensor is suitable to withstand the process environment. The distance from the control or measuring equipment where the RTD components are installed will determine the type of RTD required. If the control/measurement point is relatively close to the installed sensor, it is feasible to connect the cable directly to the instrument. For long distances, check the input specifications of the instrument to determine if the lead impedance is too large. In these cases, 4-20mA transmitters are recommended. (4-20mA transmitter converts resistance into current and transmits it through two wires with minimum loss). Troubleshooting problems related to RTD assemblies is often straightforward. As RTD components are easily damaged by vibration or mechanical impact, the most likely problem is that the components are open circuit. Depending on the type of assembly, this can be easily determined with a multimeter. Drift is usually more subtle. Because platinum is easy to be polluted, the introduction of impurities can change the basic resistance, and the resistance to temperature response is very different from that of pure platinum. In this case, the only way to determine if there is an error is to calibrate the RTD sensor. Advantages and disadvantages of temperature sensors Each type of temperature sensor has its own advantages and disadvantages. RTD strength: RTD is usually used in applications where repeatability and accuracy are important considerations. Properly constructed Pt100 RTDs have repeatable temperature characteristics of resistance changing with time. If a process will operate at a specific temperature, the RTD resistance at that temperature can be measured in the laboratory and will not change significantly over time. Rtd also allows for easier interchangeability because its original variation is much lower than thermocouples. For example, the standard error limit of the k-type thermocouple used at 200 ° c is ± 2.2 ° c. At the same temperature, Pt100Ohm DIN, class b platinum resistor has ± 1.3 ° c interchangeability. The Rtd can also be used with standard instrument cables to connect display or control equipment. The thermocouple must have corresponding thermocouple wires to obtain accurate measurement. Weakness of RTD: In the same configuration, you can expect to pay more thermal resistance than base metal thermocouples. RTDs are more expensive than thermocouples because more structures are needed to make RTDs, including the manufacturing of sensing elements, wiring and assembly of sensors. Due to the structure of the sensing element, the performance of rtd in high vibration and mechanical shock environment is not as good as that of thermocouple. The temperature of industrial RTDs is also limited to 650 ℃, while thermocouples can be used up to 1700 ℃.

 

Thermocouple Fundamentals 12-15 Useful thermocouple life is a very difficult prediction, even if most details of the application are known. Unfortunately, such information is often difficult to determine. The best test for any application is to install, use, and evaluate the in use performance of a design that is considered likely to succeed. The recommendations and non recommendations listed in the thermocouple type description are a good place to start when selecting the assembly style to be installed in the process for the first time. Offset and Drift

 

Stability All thermocouples will be affected by calibration drift during use, which is only a matter of how much and how fast this happens. The performance of thermocouple depends heavily on the absolute uniformity of physical and chemical properties of the whole circuit length. When producing thermocouple materials, careful steps should be taken to ensure that this uniformity (or homogeneity) is achieved. During use, different parts of the circuit will experience different heat, chemical exposure and other conditions, so the physical structure and composition of these parts will change from the original thermocouple wire. As the thermoelectric EMF generated by a given temperature difference is very sensitive to the changes in the chemical and metallurgical properties of the wire, the total EMF generated by the used probe may be different from that generated by other identical new probes under the same conditions. During the observable time, these changes are usually small (usually small enough to be negligible). However, under unfavorable conditions, it is possible to achieve large drift speed quickly. In order to achieve long-term reliable thermocouple life, the usual strategy is to operate the equipment comfortably at the highest temperature and provide it with the cleanest possible working environment. Enclosures, such as sheaths, protective tubes, and thermowells, are common methods of controlling the environment around thermocouples themselves.

 

The protective tube, sheath and even thermowell may fail due to corrosion or mechanical damage. The processing process may exceed the temperature, exposing the thermocouple to an environment higher than the expected temperature. If the output of the sensor controlling a process is low, the process responds to its controller and may be forced to reach a temperature higher than expected. Base metal components are vulnerable to some chemicals. They can also be changed by adverse operating conditions. In the case of supply, the content of impurities in high-quality precious metal thermocouple wires is very low. Therefore, it is easy to be polluted by the factors that can affect its thermoelectric performance. Platinum is particularly sensitive to the presence of free silicon. It can combine with free silicon to form eutectic alloys, which will melt at or below the normal service temperature. The protection tubes of high-purity insulators and precious metal components, as well as the cleaning during operation, are essential to prevent this situation. Human error may also be a contributing factor. The control may be set incorrectly, the connection may be incorrect, and the operating conditions of the inappropriate action response may be wrong. The combination of instrument redundancy with training and responsibility is a common means to deal with such errors. The troubleshooting method is to evaluate the problem and check whether the system performance meets the conditions: Does the change in the control produce logical results? How about the product? Does its condition conform to the instrument description? How to detect used thermocouples First, remove the suspected thermocouples from use. It is not always practical to "detect" them in another place. Once the device is used, it means that it may no longer be homogeneous. Placing an uneven thermocouple under a set of different temperature gradients, even if it is slightly different, will result in different outputs and readings. Recalibrating a used thermocouple will certainly generate a number, but this number may not be meaningful in the place where the thermocouple is used. The best way to evaluate used thermocouples is to "detect" their position by placing a new thermocouple with a known output, and then place the suspect thermocouple during operation, and compare the readings. It is not practical to install two sensors at the same time. Remove the suspect probe and replace it with another probe with known good performance. Then, as long as the good probe and the removed probe are placed at the same location and the process has not changed during the exchange, the readings of the two probes can be compared. Note that it is not necessary to reserve and use an unlimited number of new detectors for these tests. Some suitable replacement equipment can be kept available, and one can be selected for testing. Under normal conditions, the drift or degradation of thermocouples is a gradual and very slow process. Therefore, a single replacement probe can be used to detect a process multiple times, and is considered a reliable repeat test. Moreover, when a drifting detector is found, the test detector can simply remain in place as a working sensor, while the next substitute becomes the test equipment. A useful instrument for system test thermocouple system troubleshooting is a portable temperature indicator. Many of these devices can use two or more different types of thermocouples, and some provide output functions that can generate electrical output to simulate thermocouples operating at any selected temperature. In use, the instrument is usually connected to the wire of the circuit and tested at a convenient access point, such as a connector. Care should be taken to ensure that the correct polarity is maintained. In Australia, we use ANSI color codes where the negative is always red. There, the output of an operational sensor can be monitored and evaluated. Alternatively, using the "output" function of the instrument, an analog thermocouple signal may be sent back to the permanent indicator or controller of the circuit to verify the normal operation of the remaining circuits. When driving the signal back to the instrument, it is usually necessary to disconnect one side of the circuit to avoid the low resistance of the thermocouple itself "loading" the portable tester. The extension wiring part of the thermocouple circuit can also be checked for correct connection with a portable tester. The part under test should be electrically isolated from the rest of the circuit, and one end of a pair of extension wires should be shorted together. If the tester is connected to the other end of the short circuit pair, the tester should display the approximate temperature of the short circuit pair. Note that if both ends of the extended pair happen to be at the same temperature, it may be necessary to heat the shortened end slightly and verify that the tester has correctly seen the temperature change. The possibility of incorrect reverse connections is being checked in this test.

 

Thermal resistance or thermocouple: thermocouple and thermocouple are useful sensors for measuring process temperature. In its temperature range, RTDs have higher accuracy than thermocouples because platinum is a more stable material than most thermocouple materials. The Rtd also uses standard instrument lines to connect to measurement or control equipment, which can reduce the overall installation cost. Thermocouples are generally cheaper than rtds. They are more durable for high vibration or mechanical shock applications and can be used at higher temperatures. Thermocouples can be smaller than most rtds and can be formed for specific applications.