5. Limitations of Temperature Measurement and the Necessity of Controlled Temperature

in Introduction to Temperature Measurement

INDEX

5.1 True Temperature and Measurement Errors
- 5.1.1 Definition of True Temperature
- 5.1.2 Why Is Measuring the True Temperature Difficult?
5.2 Necessity of Controlled Temperature
- 5.2.1 Benefits of Introducing Controlled Temperature
5.3 Summary

Temperature measurement plays a crucial role in various fields, including industrial applications, medical practices, research, and food processing. However, it is important to note that the measured temperature does not always perfectly reflect the “true temperature” .
Numerous factors, such as instrument errors, environmental conditions, and measurement methods, influence temperature measurements. If these factors are not appropriately considered, they can lead to incorrect temperature management. Therefore, in practical applications, it is essential to introduce the concept of “controlled temperature”, which allows for acceptable measurement errors while ensuring proper temperature management.
This chapter provides a detailed explanation of the limitations of temperature measurement, the factors contributing to measurement errors, methods for achieving accurate measurements, and the role of controlled temperature in practical temperature management.

5.1 True Temperature and Measurement Errors

5.1.1 Definition of True Temperature

The “true temperature” refers to the ideal temperature that an object inherently possesses, excluding any influences from measurement methods or environmental factors. However, in reality, achieving a perfect measurement is extremely difficult, as various factors, including measurement instruments and environmental conditions, introduce errors.

5.1.2 Why Is Measuring the True Temperature Difficult?

The following factors can cause discrepancies between the true temperature and the measured temperature:

1. Tolerance of Temperature Sensors
Measurement instruments have inherent manufacturing errors and accuracy limitations, which are specified as tolerance.
For example, thermocouples have a tolerance range of ±1 to 2°C, depending on the temperature range, which can introduce measurement errors.

2. Influence of the Temperature Sensor on the Measurement Target
Contact-type temperature sensors (such as thermocouples, RTDs, and thermistors) can alter the temperature of the object being measured by absorbing or dissipating heat upon contact.
If the measurement target is small, the sensor may extract heat, causing the displayed temperature to be lower than the actual value.
Additionally, temperature sensors degrade over time, leading to reduced measurement accuracy.

3. Influence of Infrared Radiation
When infrared heating is used in temperature measurement, both the measurement target and the sensor itself may be heated, leading to incorrect readings, especially when using contact-type temperature sensors.

4. Influence of Emissivity
Infrared thermometers calculate temperature based on the emissivity of the object.If the emissivity setting is incorrect by even 1%, the measured temperature can significantly deviate from the actual value.
For example, setting the emissivity at 0.90 instead of 0.95 can result in a measured temperature that is several degrees lower than the actual value.

5. Influence of the Measurement Environment
Wind, humidity, and nearby heat sources can fluctuate temperature readings.
For example, airflow can alter the surface temperature of the object, while humidity can cause condensation on the sensor, affecting the measurement.

6. Response Time of Temperature Controllers
Even in real-time temperature measurement, there is an inherent time lag, causing discrepancies between the actual and measured temperature.
This effect is particularly significant in environments with rapid temperature fluctuations.

5.2 Necessity of Controlled Temperature

Since measurement errors are inevitable, practical applications require consistent and reliable temperature control rather than an absolute true temperature.For this reason, it is essential to introduce the concept of “controlled temperature”, which allows for acceptable measurement deviations while ensuring proper temperature regulation.

Example of Controlled Temperature
Consider a scenario where an object is heated to 500°C using a heater.

1. Set Temperature: 500°C (The temperature configured in the temperature controller)
2. Measured Temperature: 500°C (The temperature recorded by the thermocouple)
3. True Temperature: Due to measurement methods and environmental factors, the actual object temperature may have an error of ±several degrees.

In this case, even if the measured temperature is 500°C, it does not necessarily indicate the true temperature. However, in practical applications, the most important aspect is ensuring that the object reaches a suitable temperature to fulfill its intended function.Therefore, by defining a controlled temperature and considering measurement errors, it is possible to implement accurate and effective temperature regulation.

5.2.1 Benefits of Introducing Controlled Temperature

1. Enables Practical Temperature Management That Accounts for Errors
Instead of striving for a precise true temperature, controlled temperature allows for consistent and reliable temperature management within measurable limits.
Measurement systems can be optimized based on the characteristics of individual devices.

2. Allows Temperature Control Adapted to Equipment Characteristics
Equipment and sensors have inherent measurement errors, but by adjusting temperature settings accordingly, stable control within an acceptable range can be achieved.

3. Ensures Product Quality in Manufacturing Processes
Rather than focusing solely on minor measurement errors, controlled temperature prioritizes meeting production and quality requirements.
For instance, in food processing and metal fabrication, maintaining the temperature within an appropriate range is often sufficient to ensure product quality.

5.3 Summary

Since it is impossible to completely eliminate measurement errors, practical operations must focus on managing temperature effectively by considering these errors.
By introducing the concept of controlled temperature and implementing appropriate temperature control strategies based on equipment and production processes, it is possible to achieve efficient temperature management while allowing for inherent measurement uncertainties.