This allows the measuring end of the sensor to be placed directly into the measurement area. Termination wires connect it to the measurement recording device. When placing the sensor in the line, it is important that the cables on the connecting head are straight and not twisted while screwing in the sensor.
Leaving the wires disconnected when inserting the sensor avoids problems with the cables. To ensure a good calibration, all of the connecting wires should be the same size and length. The insertion depth should be ten times the diameter of the stem. Frequency with which an RTD is calibrated depends on the temperature cycle, vibrations, and shock.
In most cases, the frequency of calibration is determined by the user. The calibration of the sensor is achieved by comparing its resistance to a working standard. Damaged sensors should be replaced. The different types of RTD sensors are categorized by the construction of the temperature sensing element.
The type of RTD sensor to be used is determined by the environment it will be used in and the application. The use of resistance temperature sensors began in the middle of the first industrial revolution, and they were assembled using copper wire and a galvanometer.
Copper wire was replaced by platinum when it was discovered that platinum could measure a wider range of temperatures. Thin film RTD elements have a thin layer of metal placed on the substrate of a ceramic material. The film of metal is etched into an electrical circuit pattern that offers the necessary amount of resistance.
The image below is an example of a common form of resistance pattern. Lead wires are attached, and a protective coating is applied to the substrate and element. Thin film RTD sensors are rugged, reliable, and resistant to shock and vibration damage. The wire wound version of an RTD has a wire wound around the outside of a ceramic or glass housing, referred to as a bobbin in the diagram below.
Glass core RTD sensors can be immersed in liquids. RTD sensors with ceramic cores can accurately measure extreme temperatures. Wire wound RTD sensors require skilled technical engineering and highly advanced manufacturing processes; this means they are more expensive than thin film sensors.
Coiled RTD sensors have thin wound wire enclosed in a ceramic or glass housing filled with a non-conductive powder. The resistance wire can expand and contract with the changes in temperature, minimizing errors that may be caused by mechanical strain.
The tightly packed powder around the wire increases heat transfer, improving the response time of the sensor. The ceramic or glass housing is normally inserted into a protective metal sheath. The is its resistance factor. The most common is the "". The pt is the second most used resistance sensor. A pt has greater resistance. The distortion in the lead wires is less significant and is only a small percentage of the total resistance.
Pts have a higher resistance value and require less current. They are appropriate for configurations that use less power. Since the power consumption is low, they produce less heat and have fewer errors caused by self heating.
The two wire type of RTD is the simplest circuit design. A single lead wire connects to each end of the element. The resistance in the circuit is calculated by measuring the resistance in the lead wires and connectors. This results in some degree error or readout that is higher than the actual measured temperature.
This can be eliminated with calibration. The three wire configuration is the most used in industrial applications. Two wires are connected to one end of the sensor, A and B, and to the monitoring device. The third wire, C, is connected to the element. The three wires are of equal length, so their resistance is equal. The three wire configuration also has errors that have to be adjusted by calibration.
The four wire configuration is the most complex, time consuming, and expensive to install but produces the most accurate and precise readings. DC current is provided through two leads, A and C. The voltage drop is measured by the other two leads, B and D. The voltage drop and current are known, making the resistance easy to read as well as the temperature across the system.
A variation of the four wire design has two red wires connected to the element with a white configuration that is looped. An RTD consists of a resistance element and insulated copper wires. The most common number of wires is 2; however some RTDs have 3 or 4 wires. The resistive element is the temperature sensing element of the RTD. It is usually platinum because as a material it is highly stable over time, it has a wide temperature range, it offers an almost linear relationship between temperature and resistance and it has a chemical inertness.
Table 1. Outlines usable temperature ranges for each material. Table 2A. Table 2B. Typical choices for wire insulation include nickel, nickel alloys, tined copper, silver plated copper or nickel plated copper. For RTD construction, there are two methods most used. Any RTD comprises five major components as part of its structure. Here are the details:. When a low amount of current is passed through the element, voltage, which is proportional to the resistance, is measured and converted to temperature calibration units.
The temperature is directly proportional to the resistance of RTD. This means when the temperature rises, the resistance of RTD also rises and vice versa. This shift in the temperature is sensed by the detector and accordingly a message is issued to the system. This is the working principle of RTDs- the flow of electricity is resisted with the rise in temperature which is sensed by the resistive element.
This resistance is due the metal, and is measured in Ohms. Whether you looking for any temperature measurement devices or specifically RTD sensors, ensure you source them from reliable suppliers. The Transmitter Shop TTS has a huge inventory of ready-to-ship temperature measurement devices and related accessories made by recognized manufacturers. The company provides calibration services using SI-traceable units. Related Posts.
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