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RTD's

Resistance Temperature Detectors

RTD THEORY

RTD's function as a result of increasing resistance in proportion to increasing temperature: i.e., with a known current, as temperature increases, voltage increases.  As a known current is placed through the RTD, the output voltage indicates RTD resistance.  Since the relationship between the resistance and the temperature is known, the temperature can thus be determined. 

The resistivity of the metal used in an RTD is dependent upon the temperature.  Platinum has the best range and is the most common metal used.

The platinum RTD element is manufactured to have a set resistance at 0 C; however, once incorporated into an instrument loop, the lead resistance of the loop must be compensated for in order to achieve an actual temperature reading.  For this reason, while the 2-wire RTD probe may be adequate for very short lead lengths, it is inherently inaccurate and rarely used.

The most commonly used RTD is the 3-wire probe, which allows for the compensation of loop resistance by introducing a third wire, known as a reference wire, that is equal in length and size to the two wires connecting the RTD to the readout device.  This third wire bypasses the RTD at its junction to one of the other two wires, and allows the readout device to automatically subtract the lead resistance from the overall resistance of the circuit.

Offering even greater potential for accuracy is the 4-wire RTD,  which uses two reference wires.  However, the system must be designed to accommodate this design.

PLATINUM RTD

The usual nominal resistance of platinum RTDs is 100ohms at 0 C.  Unfortunately, standards are not identical worldwide, which presents a problem when a RTD built to one standard is used with an instrument designed to a different standard.  In addition, manufactured tolerances must be considered.  Not only do they vary with the manufacturer and the standard, but the tolerances are also affected by the manufacturing process itself.

GAYESCO offers platinum RTDs to all standards; the DIN and American standards the most available.  Our normal tolerance is 0.1% of the reading.  The following table delineates the most common standards.

GAYESCO RTD assemblies are TIG-welded to avoid possible contamination from brazing flux.  Our strict manufacturing procedures are designated to prevent element contamination from chemicals and moisture.

 


 

 



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