Thermocouples to meet AMS 2750F
Thomas Seebeck discovered in 1821 that when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. (Seebeck effect). The junctions can be exposed, grounded or ungrounded. Thermocouple are classified with letters and color coded according to IEC, DIN or ASTM E 230. Base metal thermocouples (J, E, K, N, T) experience more drift compared to noble metal (R, B, S) thermocouples.
Industrial heating equipment normally has to comply with pyromotric specification such as AMS 2750F which defines thermocouple requirements used for SAT, TUS, furnace and load thermocouples. Per AMS 2750F, only thermocouples with special error limits of ±2°F or ±0.4% can be used and it differentiates between expandable and non-expandable thermocouples. Digital temperature recorders must be digital and have a readability of 0.1°F after June 2022.
Thermocouples must be selected to meet application conditions. Selection considerations are:
- Response time (larger diameter reduce response time)
- Service life (larger diameter increases service life)