Thermocouples play a vital role in molten metal thermal analysis to ensure proper pouring temperatures are met, and thus the proper physical properties of the alloy are achieved. For the molten metal foundry, thermocouple monitoring of temperature requires the right choice of equipment. Choosing the correct thermocouple for the job can seem like a challenging task, but once all of the variables have been considered, the correct choice will lead to accurate measurements and reduced operating expenses.
The importance of accurate temperature measurements cannot be understated. Many of the physical properties of an alloy rely on pouring at the correct temperature, which can only be assured if the temperature measurements are accurate.
What is a Thermocouple?
Thermocouples use the Seebeck effect, which is a thermoelectric property that occurs when different metals are in contact.[1] Conductors or wires of two different metals are welded together at one end, called the hot junction. At the opposite end, the cold end, a small voltage is generated which varies as a function of the temperature at the hot end. Therefore, a properly constructed and calibrated thermocouple can determine the temperature accurately and repeatably.
The two metals chosen for the thermocouple’s construction determine the functionality of the thermocouple, and standards have been established to provide predictable and reliable results specific to different operating ranges and applications. To standardize this process, thermocouples are assigned letters to indicate which metals have been used to form the junction, which determine the operating range and accuracy. The most common thermocouples used in the foundry industry are S (Pt-10% Rh and Pt), B (Pt-30% Rh and Pt-6% Rh) and K (Ni-Cr and Ni-Al), , each of which are appropriate for different uses.[2]
Single Use Thermocouples
Single-use thermocouples are expendable, being discarded after just one sample.[3] These are available in S-type thermocouples (up to 1750 ℃ or 3182 ℉), and type B (up to 1820 ℃ or 3308 ℉), with high accuracy and suitable for high temperature environments. Single-use thermocouples are used in steel, cast iron, copper alloy, zinc, and aluminum casting operations and are accurate and economical for applications where a single reading is needed. [3] They are highly reliable when used just one time, and also limit the sources of contamination. Ultimately, single use thermocouples provide repeatable, accurate measurements for spot checking the temperature of most molten metals.
Multi-use Thermocouples
Multi-use thermocouples are a compromise between single use and resilient thermocouples, providing multiple readings before a replacement is required.[4] Available in type S (up to 1750 ℃ or 3182 ℉), and type B (up to 1820 ℃ or 3308 ℉), these thermocouples are available for virtually any molten metal operation.[4]
Because the same thermocouple can be used multiple times, it can be used make multiple readings in a heat. Each multi-use thermocouple comes equipped with a non-reactive, anti-spash sleeve that protects the thermocouple, as well as the foundry technicians taking the sample, without contaminating the heat. [4]
Typically, metal will be poured from furnace to ladle at a temperature higher than the target pouring temperature. Multiple readings will be taken until the metal has cooled to the correct temperature for final pouring, and multi-use thermocouples are ideal for this task.
Resilient Thermocouples
Of all the types of thermocouples, K-type are the most resilient. A well-constructed K-type thermocouple can be used for hundreds of immersions in molten metal before replacement is necessary.
Resilient, K-type thermocouples have a low cost per reading, as the higher upfront purchase price is divided over many readings. They are a great, general purpose thermocouple, provided the maximum temperature does not exceed 1300 ℃ (2372 ℉). This makes K-type thermocouples well suited for repeated measurements in copper alloy (bronze, brass), zinc, and aluminum melting operations.
Thermocouple Selection
In order to choose the correct thermocouple, one must consider the maximum exposure temperature (and temperature range), response time, environmental constraints, reusability and thermocouple cost. Thermocouple best practices require engineers and technicians to choose a device that can withstand the maximum exposure temperature as the first consideration. After that, the other factors can be considered.
For low overall costs, K-type, resilient, reusable thermocouples are a good choice. They are limited to low melting temperature metals (1300 ℃ or 2372 ℉) and are also not quite as accurate as B or S types, but their low cost per sample makes them attractive for routine quality control and monitoring purposes in many non-ferrous melting processes. For the best thermocouple lifespan, K-type thermocouples are hard to beat.
Sometimes, highly accurate, “one-off” measurements are needed, especially when verifying a process or to spot check operations. In this case, single-use, S-type thermocouples are a good way to reliably and repeatably take temperature measurements of virtually all metals during casting operations.
In other cases, the compromise between a resilient, many use thermocouple and a single use thermocouple is required. Multi-use B or S type thermocouples straddle that difference between low cost per sample and higher accuracy. For iron and steel processing, the multi-use thermocouples are capable of withstanding the high temperatures but have a lower cost per reading as compared to the single use thermocouples.
SYSCON Advantage
The proper choice of thermocouple will lead to reduced costs and more uniformity in metallurgy. SYSCON has been developing foundry instrumentation equipment, including thermocouples, for decades. Reach out to the technical experts at SYSCON for guidance towards the correct thermocouple choice for your foundry.