How do I choose between different types of RTD sensors?
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Choosing the right type of RTD sensor is a mater of matching the sensor to your measurement requirement. Here are some areas to take into consideration.
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The highest-accuracy of all PRTs are the Ultra Precise Platinum Resistance Thermometers (UPRTs). This accuracy is achieved at the expense of durability and cost. The UPRT elements are wound from reference-grade platinum wire. Internal lead wires are usually made from platinum, while internal supports are made from quartz or fused silica. The sheaths are usually made from quartz or sometimes Inconel, depending on temperature range. Larger-diameter platinum wire is used, which drives up the cost and results in a lower resistance for the probe (typically 25.5 Ω). UPRTs have a wide temperature range (−200 °C to 1000 °C) and are approximately accurate to ±0.001 °C over the temperature range. UPRTs are only appropriate for laboratory use.
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Another classification of laboratory PRTs is Standard Platinum Resistance Thermometers (Standard SPRTs). They are constructed like the UPRT, but the materials are more cost-effective. SPRTs commonly use reference-grade, high-purity smaller-diameter platinum wire, metal sheaths and ceramic type insulators. Internal lead wires are usually a nickel-based alloy. Standard PRTs are more limited in temperature range (−200 °C to 500 °C) and are approximately accurate to ±0.03 °C over the temperature range.
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Industrial PRTs are designed to withstand industrial environments. They can be almost as durable as a thermocouple. Depending on the application, industrial PRTs can use thin-film or coil-wound elements. The internal lead wires can range from PTFE-insulated stranded nickel-plated copper to silver wire, depending on the sensor size and application. Sheath material is typically stainless steel; higher-temperature applications may demand Inconel. Other materials are used for specialized applications.
Difference in RTD Types:
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Thermistors vs. Pt100 vs Pt1000 (RTDs)
Resistance thermometers on the basis of Pt100, Pt1000 (positive temperature coefficient PTC) and NTC (negative temperature coefficient) are used everywhere in the industrial temperature measurement where low to medium temperatures are measured. In the process industry, Pt100 and Pt1000 sensors are used almost exclusively. In machine building, however, often an NTC is used – not least for cost reasons. Since meanwhile the Pt100 and Pt1000 sensors are manufactured in thin-film technology, the platinum content could be reduced to a minimum. As a result, the price difference compared to the NTC could be reduced to such an extent that a changeover from NTC to Pt100 or Pt1000 becomes interesting for medium quantities. Particularly since platinum measuring resistors offer significant advantages over negative temperature coefficients.
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The lead resistance affects the measurement value of 2-wire temperature sensors and must be taken into account. For copper cable with a cross-section of 0.22 mm2, the following guide value applies: 0.162 Ω/m → 0.42 °C/m for Pt100. Alternatively, a version with Pt1000 sensor can be chosen, with which the influence of the supply line (at 0.04 °C/m) is smaller by a factor of 10. The influence of the lead resistance compared to the base resistance R25 for an NTC measuring element is far less noticeable. Due to the sloping characteristic curve of the NTC, the influence at higher temperatures increases disproportionately in case of higher temperatures.
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conclusion :
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The main difference between thermistors and RTDs is the temperature range. If your application involves temperatures above 130°C, the RTD is your only option.Below that temperature, thermistors are often preferred when accuracy is important. RTDs, on the other hand, are chosen when tolerance (i.e. resistance) is important. In short: thermistors are better for precision measurement and RTDs for temperature compensation.
RTD sheath Material selection
All TEMPDURA RTD sensors are made using the highest quality materials. The sensors are all ANSI special limits of error to give your measurements the best possible results. The various sheath materials are dependent on the application and the following will help you make the best selection.
304 SS Maximum temperature of 1650° (900°) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900°F to 1600°F (480 to 870°) range.
310 SS Maximum temperature of 2100°F (1150°C) and offers good mechanical and corrosion resistance similar to 304SS. Very good heat resistance. Not as ductile as 304SS.
316 SS Maximum temperature of 1650°F (900°C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C)
Teflon ® designed for applications that require contact temperature measurement in corrosive or chemical environments. The Teflon coating can be applied directly to the sheath of the RTD sensor, providing protection while minimizing effects to response. In applications that require considerable long-term protection, a welded Teflon sleeve can be used adding as much as 1/16" thick of Teflon protection to the surface of the RTD temperature sensor probe.
Hastelloy X Maximum temperature 2200°F (1205°) widely used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric conditions. Excellent high temperature strength.
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Nicrobell Maximum temperature 2340°F (1300°)Highly stable in vacuum and oxidizing atmospheres. Corrosion resistance generally superior to stainless steels. Can be used in Sulfurous atmospheres at reduced temperatures. High operating.
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Refractory Oxide recrystallized, e.g.Alumina Impervious Maximum temperature 3150°F (1750°) Good choice for rare metal thermocouples.
Good resistance to chemical attack. Mechanically strong but severe thermal shock should be avoided.
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Titanium: MELTING POINT: 3135°F or (1725°C). Light weight, excellent strength in the 300°F to 800°F (150°C to 470°C) temperature range. Exellent resistance to oxidizing acids such as nitric or chromic. Resistant to inorganic chloride solutions, chlorinated organic compounds and moist chlorine gas. Resistant to salt water spray and sea water.
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Copper: MAX OXIDIZING TEMPERATURE: 400°F or (205°C). Excellent thermal conductivity. Used in special applications for research and low temperature applications.
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Choosing between a RTD Sensor and Thermocouple
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Resistance Thermometers utilise a high precision sensing resistor, usually platinum, the resistance value of which increases with temperature. The dominant standard adopted internationally is the Pt100 which has a resistance value of 100.0 Ohms at 0°C and a change of 38.50 Ohms between 0 and 100°C (the fundamental interval).
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The platinum sensing resistor is highly stable and allows high accuracy temperature sensing. Resistance thermometer sensing resistors are 2 wire devices but the 2 wires will usually be extended in a 3 or 4 wire configuration according to the application, the associated instrumentation and accuracy requirements.
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Thermocouples comprise a thermoelement which is a junction of two specifield, dissimilar alloys and a suitable two wire extension lead. The junction is a short circuit only, the EMF is generated in the temperature gradient between the hot junction and the ‘cold’ or reference junction. This characteristic is reasonably stable and repeatable and allows for a family of alternative thermocouple types (e.g. J,K,T,N) to be used. The alternative types are defined by the nature of the alloys used in the thermoelements and each type displays a different thermal EMF characteristic. In both cases, the choice of thermocouple or RTD must be made to match the instrumentation and to suit the application.
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Thermocouples are, generally:
• Relatively inexpensive
• More rugged
• Less accurate
• More prone to drift
• More sensitive
• Tip sensing
• Available in smaller diameters
• Available with a wider temperature range
• More versatile
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RTD’s are, generally:
• More expensive
• More accurate
• Highly stable (if used carefully)
• Capable of better resolution
• Restricted in their range of temperature
• Stem, not tip sensitive
• Rarely available in small diameters (below 3mm)
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In both cases, the choice of thermocouple or RTD must be made to match the instrumentation and to suit the application