The loyal readership of our blog post series “Kaye’s Time Warp” might wonder about the significance of the Kaye Ice Point Reference in the context of validating thermal processes in the GxP environment. In fact, Kaye Ice Point references are more likely to be found on aluminum furnaces, in turbine test stands, or in power plant control rooms. However, the basic knowledge that our company founder Dr. Joseph Kaye acquired in the early 1950's in this context is crucial for the development of high-precision validation systems based on thermocouples as temperature sensors.
Like any sensor element, thermocouples, regardless of type, exhibit inherent error sources. These must be taken into account when setting up a measurement circuit. To ensure accurate temperature measurement, complete compensation of these error sources is absolutely necessary. Particularly, the careful execution of cold junction compensation is of paramount importance in thermocouples.
What is cold junction compensation?
The temperature measurement of a thermocouple is based on the so-called Seebeck effect, in which a voltage is generated when two different, electrically conductive materials are thermally connected at one point and heated differently. In short: A temperature gradient along a connection of different metals generates an electrical voltage.
The challenge now is that any connection of two different metals also acts as a thermocouple and generates a thermoelectric signal corresponding to its temperature. In practice, the signal of the thermocouple is often distorted by the unwanted thermocouples of the measuring line terminals - this is referred to as the cold solder joint effect.
Cold junction compensation is therefore a critical component in thermocouple measuring circuits. Without it, the temperature signal that is measured at the end would be a function of two temperatures - the actual desired temperature and the temperature at the cold solder joint. This compensation allows the signal of the actual measuring point to be isolated and thus provides an accurate and reliable temperature measurement.
For the purpose of addressing effective cold junction compensation in thermocouple measurement technology, three technical solutions are available:
1. Utilization of the **Seebeck effect**: This effect, named after the German physicist Thomas Johann Seebeck, describes the conversion of temperature differences into electrical voltage and vice versa. In a thermocouple, temperature differences at the junctions of the two different metals generate a voltage.
2. Utilization of the **Peltier effect**: This effect, named after the French physicist Jean Charles Athanase Peltier, describes the amount of heat that is generated or absorbed in a circuit when a current flows through two different materials.
3. The direct measurement of the actual ambient temperature directly at the cold solder joint.
Method 1: Seebeck Effect
Historically, the Ice Point Reference method (Seebeck effect) was the first to be used for cold solder joint compensation. The method is relatively simple, requiring only an ice bath to maintain a constant temperature at the junction, precisely at 0°C. This method was already implemented in the early days of thermocouple measurement technology, long before measurement systems equipped with software-controlled and fast process computers were available for the efficient compensation of this measurement error.
A thoughtful combination of ice and water, packed in a robust industrial casing, equipped with rudimentary control technology and a power supply - what sounds like building instructions from the movie "Back to the Future" is ,in reality, the basis for Dr. Kaye's multichannel ice point references, applicable for a variety of industrial uses. Yes, you read that right! As surprising as it sounds, this innovative concept found widespread acceptance in various industrial applications and has not lost its relevance to this day. Although this measuring principle still forms the foundation for today's Kaye Ice Point references, the control electronics have naturally evolved - and the remarkable thing is that the Kaye Ice Point references, despite these minimal technical changes, are still among the most accurate on the market.
Method 2: Peltier Effect
The Peltier effect is usually not explicitly used for the compensation of cold solder joints in industrial measuring instruments. The Peltier effect is typically used in Peltier coolers/heating units to create a temperature difference and thus achieve cooling, like in the case of the Kaye Block Calibrator LTR-150.
Although it is theoretically possible to use a Peltier cooler at the cold solder joint to keep the temperature at the cold solder joint constant and thus eliminate the need for compensation, this approach would be impractical and costly in most applications and is therefore only used in very specific thermocouple measurement circuits.
Method 3: Direct measurement of the actual ambient temperature directly at the cold solder joint
The now most widespread method is the direct measurement of the temperature at the cold solder joint and the subsequent compensation using software. This became possible with the advent of microprocessor technology and software algorithms.
In this method, a separate temperature sensor, often a high-precision resistance thermometer or a thermistor, is used to measure the temperature at the cold solder joint. This information is then used to compensate the thermocouple signal. The compensation is done by converting the measured cold solder joint temperature into a corresponding thermocouple voltage using the known thermocouple voltage-temperature relationship (Seebeck curve). This "compensation voltage" is then subtracted from the measured thermocouple signal to obtain only the signal of the "hot" joint (the measuring point). It is important to note that the quality of the compensation strongly depends on the precision, stability, and response of the cold solder joint temperature sensor used. In addition, the cold solder joint must be well thermally insulated to minimize disturbances due to external temperature fluctuations.
This measuring principle is also used in Kaye's data loggers. As early as the first Kaye Digistrip product family, this method was used, and the modern Kaye AVS Validators use this compensation option directly at the connection point in the SIM (Sensor Input Module), where all thermocouples are connected.
Conclusion
In conclusion, it can be noted that the basic research findings of company founder Dr. Kaye have been and continue to be consistently implemented into industrial solutions by Kaye. A continuous adaptation, based on new, more precise, and faster electronic components on a microchip basis, is an integral part of the development philosophy at Kaye. Interestingly, the fundamental physical knowledge remains the same since the early days of the company's foundation, highlighting the timeless relevance and applicability of these scientific principles.
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