Measuring Thermocouples with the MCC 134 and Raspberry Pi

Thermocouples are a popular choice among engineers for measuring temperature due to their affordability, ease of use, and wide measurement range. This article details the intricacies of accurate thermocouple measurements and highlights how the MCC 134 DAQ HAT (230-6181) addresses these challenges and offers tips to minimize measurement errors for MCC 134 users.

Understanding the Function of Thermocouples

Thermocouples operate based on the Seebeck effect, which converts thermal gradients into electrical potential differences. They consist of two wires made of dissimilar metals joined together at one end, forming a junction. The dissimilarity between the metals generates varying electric potentials over a temperature gradient, resulting in a measurable voltage in the circuit.

Different types of thermocouples employ different metal combinations and are suitable for specific temperature ranges. For instance, J type thermocouples use iron and constantan and are ideal for measurements between -210 °C to 1200 °C, while T type thermocouples use copper and constantan for measurements ranging from -270 °C to 400 °C.

The temperature difference between two junctions—the hot junction where the measurement is taken and the cold junction at the measurement device connector block—is referred to as the thermal gradient.

Fundamentals of Thermocouple Measurement

Thermocouples generate a voltage relative to the temperature gradient between the hot and cold junctions. To determine the absolute temperature of the hot junction, it is necessary to know the absolute temperature of the cold junction.

Historically, ice baths were used to establish a known cold junction reference. However, modern thermocouple measurement devices utilize sensors to measure the terminal block (cold junction) where the thermocouples connect to the measurement device.

Identifying Sources of Thermocouple Errors

Several factors contribute to thermocouple measurement errors, including noise, linearity and offset error, the characteristics of the thermocouple itself, and the measurement of the reference or cold junction temperature. In modern high-accuracy 24-bit measurement devices, advanced ADCs are employed, and design practices are implemented to minimize noise, linearity, and offset errors.

While it is impossible to completely avoid thermocouple errors, it is feasible to minimize them. These errors primarily stem from the inherent imperfections in the alloys used, which may vary slightly from batch to batch. Certain thermocouple types exhibit less error; standard type K and J thermocouples have an error range of up to ±2.2 °C, whereas type T thermocouples have an error range of up to ±1 °C. More expensive thermocouples, known as SLE (Special Limits of Error), are manufactured using higher-grade wire and can reduce these errors by a factor of 2.

Accurate Measurement of the Cold Junction Measuring the cold junction, where the thermocouples connect to the measurement device, presents a challenge. In more expensive instruments like the DT MEASURpoint products, an isothermal metal plate is employed to maintain a consistent and accurately measurable cold junction. However, in lower-cost devices, incorporating isothermal metal blocks becomes cost-prohibitive. As a result, accurately measuring the temperature at the exact point of contact between the thermocouple and the copper connector becomes challenging. This limitation makes the measurement of the cold junction temperature susceptible to temporary errors caused by rapidly changing temperatures or power conditions near the cold junction.

Design Challenges of the MCC 134

To comprehend the design challenges associated with the MCC 134, let's compare it to MCC's popular E-TC, a high-accuracy ethernet-connected thermocouple measurement device. The cold junction temperature of the E-TC is measured using the Analog Devices ADT7310 IC temperature sensor.

The IC sensor design works effectively in the controlled and consistent environment of the MCC E-TC. The outer plastic case regulates airflow, and the electronic components and processors operate under a constant load. Consequently, the IC sensor accurately measures the cold junction temperature in this controlled environment.

However, during the initial design of the MCC 134, an IC sensor was employed to measure the cold junction temperature. However, it became apparent during the device validation process that the accuracy was insufficient. Due to the inability to position the IC sensor close enough to the connector block, large and uncontrolled temperature gradients caused by the Raspberry Pi and the external environment resulted in poor measurement repeatability.

MCC addressed this issue by redesigning the MCC 134 with an improved scheme that offers significantly enhanced accuracy and repeatability while maintaining a low cost. Instead of using an IC sensor and a single terminal block, the new design incorporates two terminal blocks and three thermistors. These thermistors are placed on either side and in between the terminal blocks. Although this redesign introduced complexity, the thermistors more accurately track temperature changes in the cold junction, even during variations in processor load and environmental temperature.

This design yields excellent results that are less susceptible to the uncontrolled environment of the Raspberry Pi. However, certain factors can still impact accuracy, and users can improve their measurement results by minimizing sudden changes in temperature gradients across the MCC 134.

Best Practices for Accurate Thermocouple Measurements with the MCC 134

The MCC 134 is designed to achieve results within the maximum thermocouple accuracy specifications when operated within the documented environmental conditions. Operating under conditions with excessive temperature transients or airflow fluctuations may affect the results. In most cases, the MCC 134 will meet the typical specifications. To achieve the most accurate thermocouple readings, MCC recommends the following practices:

• Reduce the load on the Raspberry Pi processor. Fully loading all four cores of the Raspberry Pi processor with a program can increase the processor's temperature above 70 °C. Running a program that loads only one core will operate at approximately 20 °C cooler.
• Minimize variations in environmental temperature. Place the MCC 134 away from heat sources or cooling devices that cycle on and off. Sudden environmental changes can lead to increased measurement errors.
• Provide steady airflow, such as from a fan. A consistent airflow helps dissipate heat and reduce errors.
• When configuring multiple MCC DAQ Hats in a stack, position the MCC 134 farthest from the Raspberry Pi board. Since the Raspberry Pi generates significant heat, placing the MCC 134 at a greater distance from the Pi will enhance accuracy.

Conclusion

Thermocouples offer engineers a cost-effective and versatile means of measuring temperature. However, accurately measuring thermocouples poses challenges. Through innovative design and extensive testing, MCC has overcome the difficulties of measuring thermocouples accurately in the uncontrolled environment of the Raspberry Pi. The MCC 134 DAQ HAT enables the use of standard thermocouples with this rapidly expanding, cost-efficient computing platform.