R = k L / A                           (1) where R = resistance (ohm, Ω) k = constant of proportionality or resistivity of the material (ohm, Ω) L = length of conductor (m) A = cross sectional area of conductor (m2) Resistivity and temperature can be expressed as

kt = ko [1 + α (t – to)]                            (2) where kt = resistivity at temperature t (ohm, Ω) ko = resistivity at standard temperature to (ohm, Ω) α = temperature coefficient of resistance (1/oC, Ω/ΩoC) t = temperature (oC) to = standard temperature (oC)
Combining (1) and (2):
R / Ro = α t + 1                           (3)
Resistance Temperature Coefficients
Material Temperature Coefficient of Resistance – α – (1/oC, Ω/ΩoC)
Nickel 0.0067
Iron 0.002 to 0.006
Tungsten 0.0048
Aluminum 0.0045
Copper 0.0043
Lead 0.0042
Silver 0.0041
Gold 0.004
Platinum 0.00392
Mercury 0.0009
Manganin +- 0.00002
Carbon -0.0007
Electrolytes -0.02 to -0.09
Thermistor -0.068 to 0.14
The chemical stability, availability in pure form, and highly reproducible electrical properties, has made Platinum the metal of choice for RTD’s which are made of either IEC/DIN-grade platinum or reference-grade platinum. The difference lies in the purity of the platinum. The IEC/DIN standard is pure platinum that is intentionally contaminated with other platinum The RTD sensors can be made small enough to have response times of a fraction of a second. To measure the resistance of an RTD a small electric current (about 1 mA) must flow through the sensor to create the necessary voltage drop. The current causes the platinum element in the RTD to heat up above the temperature of the RTD’s environment (Also called Joule heating). The heating is proportional to the electric power (P = IR) in the RTD and the heat transfer between the RTD sensing element and the RTD environment. If the RTD is in a poor heat transfer medium (e.g., air), it will heat up more than if it is in a fluid, such as water. The electrical current will heat the sensor and may influence the measurement. Tolerances for RTD’s should meet the standards of ASTM E1137 Grade A or B and IEC 751 Class A or B.

ASTM Standards Related to Resistance Temperature Detectors

  • E 644-98 Standard Test Methods for Testing Industrial Resistance Thermometers
  • E 1137-97 Standard Specification for Industrial Platinum Resistance Thermometers
  • E 1652-00 Standard Specification for Magnesium Oxide and Aluminum Oxide Powder and Crushable Insulators Used in the Manufacture of Metal-Sheathed Platinum Resistance Thermometers, Base Metal Thermocouples, and Noble Metal Thermocouples

ASTM Standards Related to Temperature and Calibration:

  • E 1594-99 Standard Guide for the Expression of Temperature
  • E 344-01a…Terminology Relating to Thermometry and Hygrometry
  • E 563-97 Standard Practice for Preparation and Use of Freezing Point Reference Baths
  • E 1502-98 Standard Guide for the Use of Freezing Point Cells for Reference Temperatures
  • E1750-02 Standard Guide for Use of Water Triple Point Cells

DIN – German Industrial Standards Organization

  • DIN 43760 references nickel precision
  • DIN IEC 751 reference platinum precision resistance thermometers.
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