RTDs are manufactured from metals whose resistance increases with temperature. Within a limited temperature range, the resistivity increases linearly with temperature. Each metals specific, and unique resistivity, can be determined experimentally. This resistance is directly proportional to a metal wire’s length, and inversely proportional to the cross-sectional area.
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 = I2 R) 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.