Introduction:
In the field of temperature measurement, thermocouples are widely used due to their simplicity, reliability, and wide temperature range. To ensure the accuracy of temperature measurements, the selection of materials for the thermo-well is crucial. In this case, the most appropriate material for the thermo-well is steel.

Explanation:
1. Thermal Conductivity:
The accuracy of temperature measurement depends on the ability of the thermo-well material to conduct heat effectively. Steel has a higher thermal conductivity compared to other materials like brass, copper, and aluminum. This means that steel can transfer heat more efficiently, resulting in better thermal contact between the hot gas and the thermocouple. As a result, the temperature measurement will be more accurate.

2. Mechanical Strength:
The thermo-well needs to withstand the pressure and flow conditions of the hot gas in the pipe. Steel has excellent mechanical strength, making it a suitable material for the thermo-well. It can withstand high pressures and flow velocities without deforming or failing, ensuring the integrity of the temperature measurement system.

3. Corrosion Resistance:
The thermo-well is exposed to the hot gas, which may contain corrosive elements or compounds. Steel is known for its corrosion resistance, especially when compared to materials like brass and copper. This ensures that the thermo-well remains durable and maintains its accuracy over time, even in harsh environments.

4. Compatibility:
Steel is compatible with thermocouple materials, such as Type K or Type J thermocouples. This compatibility ensures that there are no compatibility issues between the thermo-well and the thermocouple, which could affect the accuracy of temperature measurement.

Summary:
In summary, the most accurate material for a thermo-well in measuring the temperature of hot gas flowing through a pipe is steel. Steel offers high thermal conductivity, mechanical strength, corrosion resistance, and compatibility with thermocouples. These properties ensure efficient heat transfer, durability, and accurate temperature measurements in various operating conditions.