Platinum Properties
The function of the Pt temperature sensor or heating element is based on the temperature dependence of the electrical resistance of the platinum metal. The relationship can be described by the following characteristic polynomial:
RT = Ro (1+αt+βt2)
RT = Measured resistance
Ro = Nominal resistance at 0°C
The constants are defined in the international standards for Pt temperature sensors.
α = 3.9083 x 10-3 °C-1
β = -5.775 x 10-7 °C2
β is so small that for most applications a linear relationship between RT and the temperature can be assumed.
Pt temperature sensors with high nominal resistances have a higher sensitivity than those with lower nominal resistances, because the slope of the characteristic is directly proportional to Ro.
Depending on the substances and processes used in the manufacture of the Pt temperature sensors, slight specific deviations from the ideal constants and the optimum characteristic can occur. These deviations define the working temperature range and the accuracy tolerance class for each sensor type.
The temperature coefficient (TC or α) is positive and defined as:
TC = (R100-Ro)/(100 * Ro).
This represents the slope of the linear approximation of the characteristic polynomial between 0°C and 100°C. The standard DIN EN 60751 for Pt temperature sensors specifies a TC of 0.003850/°C.This TC value applies to all the products described here.
In addition, customised sensors with temperature coefficients of 0.003750/°C and 0.003500/°C as well as other intermediate TC values are available, e.g. according to the JIS standard.
Accuracy tolerance classification
Kamet supplies platinum thin-film sensors in accordance with DIN EN 60751 in the accuracy tolerance classifications B and in addition
A and 1/3 DIN (see the following table).
Proportionally limited tolerances are calculated as
Δt = ±1/a (0.3°C + 0.005 Itl)
with a = 1, 2 oder 3
| Limit variations for 100Ω platinum sensors | ||||
| Limit variations | ||||
| Temperature | Class A | Class B | ||
| °C | °C | Ohm | °C | Ohm |
| -200 | ±0.55 | ±0.24 | ±1.3 | ±0.56 |
| -100 | ±0.35 | ±0.14 | ±0.8 | ±0.32 |
| 0 | ±0.15 | ±0.06 | ±0.3 | ±0.12 |
| 100 | ±0.35 | ±0.13 | ±0.8 | ±0.30 |
| 200 | ±0.55 | ±0.20 | ±1.3 | ±0.48 |
| 300 | ±0.75 | ±0.27 | ±1.8 | ±0.64 |
| 400 | ±0.95 | ±0.33 | ±2.3 | ±0.79 |
| 500 | ±1.15 | ±0.38 | ±2.8 | ±0.93 |
| 600 | ±1.35 | ±0.43 | ±3.3 | ±1.06 |
| 650 | ±1.45 | ±0.46 | ±3.6 | ±1.13 |
| 700 | ±3.8 | ±1.17 | ||
| 800 | ±4.3 | ±1.28 | ||
| 850 | ±4.6 | ±1.34 | ||
Tolerances of basic values for platinum temperature sensors are specified in DIN EN 60751. The following applies:
Class B: Δt=±(0.3°C + 0.005 Itl)
Class A: Δt=±(0.15°C + 0.002 Itl)
and according to Heraeus Sensor definition:
Class 1/3 DIN: Δt=±1/3 (0.3°C + 0.005 Itl),
Class 2B: Δt=±2(0.3°C + 0.005 Itl)
Connection technology
The best results are achieved with welding (resistance welding, laser welding etc.) or soldering (soft, hard solder). When using hard solder, it should be ensured that the platinum thin-film sensor body is not heated above its maximum rated temperature. In general, the soldering times for hard solder should be less than three seconds.
Crimping and ultrasonic welding is also possible.
