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RTD Definition: Resistance thermometers, also called resistance temperature detectors, are sensors used to measure temperature
RTD TOLERANCES, CLASSES AND COMPARISONS
TEMPERATURE RANGES
The internal assembly construction falls into low and high temperature ranges:
Low: –200°C to +250°C
High: –200°C to +600°C
Low temp: 3mm, 4.8mm, 6mm, 8mm, 9.5mm diameter 316 SS tube alumina powder filled.
High temp: 3mm, 6mm diameter 316 SS metal-sheathed mineral insulated.
LEAD CONFIGURATION AND COLOUR CODE
Typical values of currents recommended for sensors for minimal self heating are between 1 and 5mA.
RTD FUNDAMENTALS
Pyrosales Resistance Temperature Detectors (RTDs) are specially designed to ensure precise and repeatable temperature versus resistance characteristics. The sensors are constructed in a unique strain-free manner, and use only high quality RTD elements. Ceramic, wire-wound elements, flat film technology elements, or military specified anti-vibration elements are used as per customer’s requirement to ensure the most suitable specification is supplied.
Features and Benefits
Accuracy. A special process combines strain-free construction with full winding support for dependable, accurate readings in standard RTD elements, and in flat film elements where platinum is etched onto a substrate.
High signal-to-noise output. Increases the accuracy of data transmission and permits greater distances between the sensor and the measuring equipment.
Interchangeability. Strain-free construction and precision trimming allow elements from different lots to be substituted without recalibration.
Sensitivity. Self-heating is minimised, allowing precise measurement. The temperature coefficient (alpha) is carefully controlled at the industry standard while insulation resistance values exceed IEC-751 standards.
Standardisation. Elements are available to meet or exceed the requirements of various standardisation agencies. IEC-751 standard tolerance classes A and B respectively are very well suited for industrial applications. Tolerance classes up to 1/10th DIN can also be supplied where higher accuracy is demanded
Physical and chemical stability over a wide temperature range. Pyrosales use a highly-controlled manufacturing process. Standard elements are built to resist mechanical vibration and shock, however, where there is high exposure to mechanical vibration, specially manufactured military specification thick film RTD elements can be supplied to suit the application.
Repeatability. All elements exceed IEC-751 repeatability values even after long exposure to temperatures within the operating range.
Applications:
The main consideration when installing an RTD element is that there is sufficient immersion to ensure that the RTD is not averaging the temperature of the process and equipment or structure outside the process. The RTD is not a point-measurement device like a thermocouple, so there is an active sensing area that needs to be fully immersed to ensure that the RTD is measuring the actual process temperature.
A good thermal transfer along the shaft of the sensor for 40mm is critical when installing a thermowell.
Figure 1 shows the active sensing area of an RTD sensor. This area will vary depending on the length of the RTD element used in the construction.
Sensors can be designed to give an average temperature indication.
Choice of Sensor
There are many options to consider when choosing the correct RTD element for your requirements:
An RTD consists of a sensing element, wires to connect the sensing element to the measurement instrument, and some kind of support to position the sensing element in the process. Each of these materials sets limits on the temperature that the RTD can be exposed to.
The sensing element is an electrical resistor that changes resistance with temperature. This change in resistance is well understood and is repeatable. The sensing element in an RTD usually contains either a coil of wire, or a substrate with an etched film of platinum. Extension wires are attached to the sensing element so its electrical resistance can be measured from some distance away. The sensing element is inserted into a protective sheath (normally stainless steel). Typically, platinum-sensing elements are able to be exposed to temperatures up to approximately 650°C. Other materials such as nickel or copper can also be used, however, their useful temperature ranges are lower than for platinum. Usage temperatures for all these materials are shown in Table 1.
The wire insulation used also directly influences the temperature to which the RTD can be exposed. Table 2A contains the commonly used wire and insulation materials and their maximum usage temperatures. The wires that connect the sensing element to the readout or control instrumentation are usually made of materials such as nickel, nickel alloys, tined copper, silver plated copper or nickel plated copper. See Table 2B
The most common construction used is to place the RTD element and attached wires into a closed end metal tube, pack the tube with a vibration dampening and/or heat transfer material such as alumina powder, and seal the open end of the tube with an epoxy, silicone, or ceramic cement. The metal tubes most commonly used in RTDs are made of 316 stainless steel (used to approximately 480°C) or InconelÒ(used to approximately 650°C). The vibration dampening/heat transfer materials used vary widely in temperature range. These materials are selected by the manufacturer to provide the optimum performance based on the maximum temperature expected in use.
Another common construction is to use mineral insulated metal sheathed (MIMS) cable, where the RTD element is inserted into a drilled hole and attached to nickel or copper wires insulated by magnesium Oxide (MgO). The end is then insulated with MgO and the end welded closed, and the other end has extension wires attached before sealing as above.
Epoxy sealing compounds are typically never used above 200 to 260°C. Ceramic cements can be exposed to temperatures of 1200°C or more, but require sealant to keep moisture out of the cement and the vibration dampening/heat transfer material beneath.
The material with the lowest temperature capability in a platinum RTD is usually the wire and insulation used in its construction. Pyrosales offer two constructions, low temperature and high temperature. In low temperature constructions, PTFE insulated nickel plated copper wire is used to connect to the RTD element. The stainless steel tube is alumina powder filled to provide support to the element and has an epoxy seal. This construction is usually limited to 250°C.
High temperature constructions use either nickel wire and ceramic insulators, or MIMS cable containing nickel wires. The nickel wire and ceramic construction is alumina powder filled to support the element. The sealant used will depend on the temperature rating of the transition point. Both of these types of construction are able to be used to approximately 650°C, however with careful selection of element, and use of Inconel® sheathed MIMS cable, this can be extended to approximately 850°C.
Tolerance, Accuracy and Interchangeability:
Tolerance and accuracy are the most misunderstood terms in temperature measurement. The term tolerance refers to the degree of uncertainty or possible error at a specific point. Accuracy refers to an infinite number of tolerances over a specified range.
For example, RTDs contain a sensing element that is manufactured to have a specific electrical resistance at a specific temperature. The most common example of this requirement is what is known as the DIN/IEC standard. To meet the requirements of the DIN/IEC standard, an RTD must have a resistance of 100 Ohms ±0.12% (or 0.12 Ohms) at 0°C to be considered a Grade B sensor (a Grade A sensor is 100Ohms ±0.06%). The tolerance of ±0.12Ohms applies only to the resistance at 0°C and cannot be applied to any other temperature. Below is an interchangeability table for Class A RTDs, which provide the user with a table of tolerances at specific temperatures. See Table 3.
Lead wire resistance has a major impact on the measured resistance. The ability to compensate for this additional resistance will affect the type of assembly chosen and the system accuracy. The most common type of assembly is the 3-wire RTD. Here, the lead wire resistance is compensated for in a bridge circuit. For highest accuracy, the only option is to use a 4-wire RTD where lead wire resistance error is eliminated.
Time Response
When selecting the style of assembly that best suits your process, consideration must be given as to how quickly the sensor will react to a change in temperature. If a thermowell is used, the response time will be dramatically increased, and care must be taken in the design of the thermowell/sensor system. The thermowell bore should closely match the RTD diameter so that good thermal contact is achieved, thus maximising thermal transfer.
If quick time response is a criteria, the smaller the element and probe, the quicker the time response will be. There will be a trade-off then between the achievable response time and suitability of the sensor to stand up to the process environment.
Distance from Control or Measuring Equipment
Where the RTD assembly is installed will dictate the type of RTD required. If the control/measuring point is relatively close to the installed sensor, then it is practical to cable directly to the instrument. For longer distances, examine the input specifications of the instrument to determine whether the leadwire impedance will be too large. In these instances, the use of a 4-20mA transmitter is recommended.
(4-20mA transmitters convert the resistance to current for transmission over 2-wires with minimal losses).
Troubleshooting
Problems associated with RTD assemblies tend to be straightforward and easily fixed. As RTD elements are susceptible to damage by vibration or mechanical shock, the most likely problem will be that the element is open circuit. Depending on the type of assembly, this can be determined easily with a multimeter.
Drift problems are often more subtle. As Platinum is easily contaminated, the fundamental resistance can be changed by the introduction of impurities, and the resistance to temperature response can be dramatically different to that of pure platinum. In this instance the only method of determining whether there is an error is to calibrate the RTD sensor.
Strengths and Weaknesses of RTDs
Each type of temperature sensor has particular strengths and weaknesses.
RTD Strengths:
RTDs are commonly used in applications where repeatability and accuracy are important considerations. Properly constructed Pt100 RTDs have repeatable resistance versus temperature characteristics over time. If a process will be run at a specific temperature, the resistance of the RTD at that temperature can be determined in the laboratory and it will not vary significantly over time. RTDs also allow for easier interchangeability, since their original variation is much lower than that of thermocouples. For example, a Type K thermocouple used at 200°C has a standard limit of error of ±2.2°C. A Pt100Ohm DIN, Grade B platinum RTD has an interchangeability of ±1.3°C at this same temperature. RTDs also can be used with standard instrumentation cable for connection to display or control equipment, where thermocouples must have corresponding thermocouple wire to obtain an accurate measurement.
RTD Weaknesses:
In the same configuration, you can expect to pay more for an RTD than for a base metal thermocouple. RTDs are more expensive than thermocouples because there is more construction required to make the RTD, including manufacture of the sensing element, the hooking up of extension wires and assembly of the sensor. RTDs do not perform as well as thermocouples in high vibration and mechanical shock environments due to the construction of the sensing element. Industrial RTDs are also limited in temperature to 650°C whereas thermocouples can be used as high as 1700°C.
Summary – RTD or Thermocouple
Both thermocouples and RTDs are useful sensors for determining process temperature. RTDs provide higher accuracy than thermocouples in their temperature range because platinum is a more stable material than are most thermocouple materials. RTDs also use standard instrumentation wire to connect to the measurement or control equipment, which has the ability to decrease the overall installation cost.
Thermocouples are generally less expensive than RTDs. They are more durable in high vibration or mechanical shock applications and are usable to higher temperatures. Thermocouples can be made smaller in size than most RTDs and they can be formed to fit a particular application.
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