Resistance thermometers also known as RTDs are sensors used to measure temperature by associating the resistance of the sensor with a known temperature. There are a number of different types, configurations and classifications and all will have an impact on the accuracy when manufacturing a resistance thermometer sensor.
European Resistance Thermometers are usually available as 100Ω at 0°C and 138.5Ω at 100°C according to BS EN 60751 with a linear resistance change within and beyond these temperatures. Resistance Thermometers can be set to 500 or 1000 Ω
Common Resistance Thermometer Element Types
The type of element chosen during manufacturing can have a great effect on the accuracy of the product. Resistance thermometers such as the Pt100 have a sensing element which is available in a number of forms:
- Thin Film elements are the volume leader inexpensive device. They often have restricted temperature ranges. They are only available in the BS EN 60751 tolerance bands. The resistance versus temperature slope is not quite aligned with international standards because of differential thermal expansions, although it is treated as linear. The temperature range of the tolerance limits is narrower than wire wound elements.
- Wire wound elements are significantly more expensive. They tend to have a greater range, they are available in higher tolerance bands and the resistance v temperature curve is as international standards. Wire wound elements are used in the most precise assemblies. Wire wound elements can be bought in even tighter bands than the standard defines. For instance 1/10th DIN is often cited as a requirement and refers to a tolerance band of 1/10th the class B values
Accuracy of Your Resistance Thermometer
In order to ensure resistance thermometer accuracy, elements are bought and calibrated within tolerance bands as defined in BS EN 60751. The current standard separates the accuracy definition for elements and assemblies.
|Old Tolerance Class||Wire Wound New Tolerance Class||Thin Film New Tolerance Class||RTD Assembly Class||Tolerance Value in °C|
|1/3||W 0.1||F 0.1||AA||±(0.1+0.0017|t|)|
|A||W 0.15||F 0.15||A||±(0.15+0.002|t|)|
|B||W 0.3||F 0.3||B||±(0.3+0.005|t|)|
|t| = modulus of temperature in °C without regard to sign.
It should also be noted that the standard also defines limited temperature ranges for this tolerance to apply. For instance resistance thermometer assembly tolerance class AA with a thin film resistor, is only valid within the range 0 to 150°C, while an assembly may be rated to perform at a much wider span than this.
Peak Sensors Supply Position
We acknowledge the standard for producing top quality sensors with high resistance thermometer accuracy. When customers ask for a class A assembly, we will build an assembly using W0.15 or F0.15 element or better.