1. The error in temperature readings from measuring devices such as temperature controllers, recorders, or millivoltmeters is influenced by the inherent tolerances of these instruments. For example, a partial pressure voltmeter may have an error of up to 0.001% plus 0.01 mV. A digital temperature controller typically has an error of at least 0.25% of the full scale plus the error from the last digit. These variations are important to consider when high precision is required.
2. Thermocouple wires can introduce errors due to differences in material standards across countries. For instance, in the U.S., Type K thermocouples have an ordinary grade error of ±2.2°C or 0.75%, and a precision grade of ±1.1°C or 0.4%. In Japan, the JIS Type K thermocouples have similar ranges: ±2.5°C or 0.75% for ordinary grades and ±1.5°C or 0.4% for precision grades. Choosing the right standard is crucial for accurate measurements.
3. The reference junction (cold junction) can also cause errors during calibration, especially when using an ice water bath. This can result in an error range of 0.05°C to 1°C. In field applications, the thermoelectric effect between the compensation wire and the thermocouple wire must be considered. If both ends are at the same temperature, the electromotive force remains unaffected. However, improper handling can still lead to inaccuracies.
4. Compensation wires are generally less reliable than thermocouple wires, with errors approximately twice as large. This is because they are made from different materials and categorized into different levels. While the error rates are similar, proper temperature control is essential when using extension cables, as improper conditions can significantly affect accuracy.
5. Heat conduction, commonly referred to as a thermal short circuit, occurs when the thermocouple is not inserted deeply enough. This allows heat to transfer through the protective tube and the thermocouple wire, leading to measurement errors. The extent of this error depends on the thermal conductivity of the protective tube and is typically 10 to 20 times its diameter.
6. Poor insulation, often called an electrical short circuit, can occur at high temperatures. The reduced insulation resistance causes a short between the two wires, resulting in errors that can range from 1% to 10% of the measured temperature. The direction of the error (positive or negative) depends on where the short occurs.
7. Magnetic effects can also impact thermocouples. When exposed to magnetic fields, the movement of electrons in the metal changes, which affects the electromotive force. Inductive magnetic fields from transformers or motors can be shielded using copper mesh, while current-generated magnetic fields require shielding with copper and platinum.
8. Friction between the thermocouple and high-speed fluids (such as gas or liquid) can generate additional thermal energy, leading to measurement errors. This is particularly relevant in dynamic environments where fluid flow is significant.
9. Thermal radiation can cause errors if the thermocouple is too close to a heat source. The radiated heat can make the measured temperature higher than the actual value, affecting the accuracy of the reading.
10. Self-heating due to current passing through the sensor (like a thermocouple or platinum resistor) can create additional thermal errors. This effect is more pronounced in resistive temperature sensors. To minimize this, it’s recommended to use low currents when measuring to improve accuracy and reliability.
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