RTD Types
To decide the type of RTD element, you must first consider which instrument you are going to be reading the sensor on. Choose a type of element that is compatible with the input of the sensor reading instrument. The most common RTDs are 100 Platinum Ohm with 0.00385 temperature coefficient.
| Material Types | Resistance (Ohms) | TCR (Ohm/Ohms/C) | Resistance Tolerance +/- | TCR Tolerance % |
|---|---|---|---|---|
| COPPER | 10 Ohms at 25°C | 0,00427 | 0,2% | 1% |
| COPPER | 10 Ohms at 25°C | 0,00427 | 0,5% | 1% |
| NIQUEL | 120 Ohms at 0°C | 0,00672 | 0,5% | 1% |
| PLATINUM | 100 ohms at 0°C | 0,00385 | 0,06% | 0,12% |
| PLATINUM | 100 ohms at 0°C | 0,00385 | 0,12% | 0,35% |
| PLATINUM | 100 ohms at 0°C | 0,00385 | 0,5% | 1% |
| PLATINUM | 100 ohms at 0°C | 0,00391 | 0,12% | 0,35% |
| PLATINUM | 100 ohms at 0°C | 0,00391 | 0,5% | 1% |
| PLATINUM | 100 ohms at 0°C | 0,00375 | 0,12% | 0,35% |
| PLATINUM | 100 ohms at 0°C | 0,00392 | 0,5% | 0,1% |
Precision
Second, how accurate is your measurement? Accuracy is a combination of resistance tolerance and TCR tolerance. Any temperature above or below this temperature will have a wider tolerance band or less precision (see chart below). The most common calibration temperature is 0 ° C.
Effects of Connection Cables
Unbalanced Wheatstone bridge is the most widely used method to measure resistance. When measuring the resistance of a sensor element, all external factors must be minimized or compensated in order to obtain an accurate reading.
One of the main causes of error is the resistance of the connecting cables, especially when connecting to two wires.
The resistance is in series with the sensor element, so that the reading is the sum of the resistances of the sensor element and the connecting cables. Two-wire connection is possible when the sensor element has a high resistance and the connecting cables have a low resistance.
When the resistance of the connecting cables is comparatively high, it must be compensated. Compensation is achieved with a 3-wire configuration. As shown in the diagram below, one side of the power supply is taken to one side of the RTD via L3. This places L1 and L2 on opposite arms of the bridge so that they cancel each other out and have no effect on the output voltage of the bridge.
3-wire connections are recommended for RTDs especially when the resistance of the sensor element is low and although the resistance of the connection cables is small it can have a great effect on the reading accuracy.