Theory of Operation | RTD Element Parameters | RTD Lead Configuration | RTD Permissible Deviation | |||||||||||||||||||||||||||||||||||||||||||||
An RTD’s basis of operation lies in a common property of any metal, the relationship between its temperature and the resistance to the flow of electrical currents through it. Once the characteristics of this relationship were established, a few metals found their way into common use in RTD’s. These are mainly, platinum, nickel and copper. Platinum’s resistance changes relative to temperature, reflected in an RTD’s TCR (temperature coefficient of resistance), is about 0.0039Ω/Ω/°C sufficient for high sensitivity to temperature at the usual 100Ω nominal resistance. Linearity of the resistance/ temperature relationship is also quite good and well defined. And finally, accuracy is excellent due to platinum’s stability and resistance to contamination, which also results in a wide recommended operating temperature range of -200 to 550°C. The most common RTD, a platinum RTD as described above but modified for industrial applications, has a platinum alloy element (usually platinum – 5% rhodium) with a TCR of 0.00385Ω/Ω/°C and shares most of the characteristics of the platinum RTD with added durability and a recommended temperature range extended to 650°C. Nickel and copper are also occasionally used as resistive elements in RTD applications. Nickel has the largest TCR at 0.00672Ω/Ω/°C for very high sensitivity but its linearity is very poor. Copper is the most linear, but has a limited temperature range. An RTD’s sensing element has several parameters from which to select depending on the application:
These are the most common values, additional TCR’s, nominal resistances, and accuracy’s are widely available. An additional consideration in the designation of RTD parameters is the lead configuration. Four possibilities exist:
Approximate error induced by lead wire at room temperature for platinum RTDs
Tolerance classifications and data for RTD elements are available for platinum RTD’s only. The two most frequently specified classes defined by the industry standard IEC-751 (for platinum RTD’s with a temperature coefficient of resistance of 0.00385Ω/Ω/°C) are “Class A” and “Class B”. The tolerances in °C are calculated as follows:
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Select from products below to configure RTDs
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RTD with SS Sheath & Leadwires
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RTD with SS Sheath & Transition Leadwires
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RTD with SS Sheath, Leadwires & Welded Hex Fitting
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RTD with SS Sheath, Connection Head & Welded Hex Fitting
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RTD with SS Sheath, Connection Head & Spring-Loaded Hex Fitting
RTD with SS Sheath, Connection Head & Spring-Loaded Hex Fitting |
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RTD with SS Sheath, Connection Head & Spring-Loaded 1/2″ NPT Nipple
RTD with SS Sheath, Connection Head & Spring-Loaded 1/2″ NPT Nipple |
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RTD with SS Sheath, Connection Head & Spring-Loaded 1/2″ NPT Nipple-Union-Nipple
RTD with SS Sheath, Connection Head & Spring-Loaded 1/2″ NPT Nipple-Union-Nipple |
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RTD with SS Sheath, Sheath Spring & 6″ Pigtails
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RTD with SS Sheath & Connector
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Cuttable RTD
The “Series 10T” Cuttable RTD is a 1/4″ OD Stainless Steel Tube. THis RTD can be cut onsite to your desired length by use of a tube cutter. |
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RTD with SS Sheath & Connection Head
RTD with SS Sheath & Connection Head |
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RTD with SS Sheath, Leadwires & Spring-loaded Hex Fitting
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