(a) Temperature Measurement through Resistance:
Think of a PT100 probe as a miniature conductor whose electrical resistance changes precisely with temperature. This principle, known as resistive temperature detection (RTD), forms the core of its operation. Here’s how it works:
- Platinum Magic: The sensing element inside the probe is made of pure platinum, chosen for its remarkably linear and predictable relationship between temperature and resistance. As the temperature increases, the platinum atoms vibrate more, disrupting the orderly flow of electrons, thereby increasing the resistance.
- Measuring the Change: An external device connected to the probe measures the resistance of the platinum element. This measured resistance directly corresponds to the temperature within the probe through the platinum’s inherent temperature-resistance characteristic.
- Conversion to Temperature: Finally, the measuring device utilizes this resistance value and converts it into a temperature reading using pre-programmed calibration equations specific to Pt100 probes.
This simple yet precise method allows PT100 probes to measure temperatures accurately and reliably across a wide range.
(b) Construction of the Sensing Element: The “PT” in PT100:
The heart of the PT100 probe lies in its sensing element, often a thin coil of pure platinum wire wrapped around a ceramic core. This construction offers several advantages:
- High Purity: Pure platinum ensures consistent and predictable temperature-resistance behavior, crucial for accurate measurements.
- Compactness: The coiled wire design creates a small and sensitive element, allowing for quick thermal response and versatility in placement.
- Durability: The ceramic core provides mechanical support and protects the delicate platinum wire from physical damage.
Now, the “PT” in PT100 stands for “Platinum,” and the “100” signifies the nominal resistance of the element at 0°C, which in this case is 100 ohms. This standard naming convention allows for easy identification and interchangeability of these probes.
(c) The Mystery of the Wires: Three or Four?
While two wires might seem sufficient to carry the current and measure the resistance, PT100 probes typically have three or four wires. Here’s why:
- Three-Wire Configuration: This setup uses three wires, one each for supplying current, measuring voltage across the element, and compensating for lead resistance. The lead resistance can introduce errors, and the third wire effectively cancels it out for accurate temperature readings.
- Four-Wire Configuration: This offers enhanced precision, especially for long lead lengths. Four wires are used, two for providing current and two for measuring voltage, completely eliminating the influence of lead resistance on the measurement.
The choice between three and four wires depends on the required accuracy, lead length, and cost