Aggregate Temperature Thermocouple

Function

If the aggregate temperature thermocouple drifts, fails open-circuit or reads erroneously due to sensor contamination, the PLC will under-fire or over-fire the burner — respectively delivering wet, under-heated aggregate or over-burned, oxidised material that both lead to mix rejections or pavement performance defects. A thermocouple that drifts beyond ±5 °C causes the plant to operate outside its process window without operator alert. Because thermocouple accuracy degrades gradually through element oxidation, vibration-induced wire fatigue and sheath abrasion against the aggregate flow, planned annual replacement or calibration verification is standard industry practice on continuous asphalt plants.

Construction & quality

Sensing element: Type K (NiCr/NiAl) or Type N (NiCrSi/NiSi) thermocouple pair to IEC 60584, tolerance Class 1 (±1.5 °C at 0–375 °C; ±0.4 % above 375 °C). Mineral insulation: high-density compacted MgO in the sheath annulus, maintaining electrical isolation > 100 MΩ at 20 °C / 500 VDC. Sheath: seamless 316 stainless steel, 6–10 mm OD, wall ≥ 1.0 mm, sheath length matched to chute penetration depth. Termination: stainless steel connection head with ceramic terminal block and thermocouple-grade compensating lead sockets (IEC 60584 colour coded). Process connection: 1/2-inch NPT or G1/2 male thread with ferrule compression fitting, or flanged face mount as per plant arrangement. An integrated 4–20 mA transmitter head is available for plants with analogue PLC input cards.

When to replace

• PLC aggregate temperature reading differing by more than ±10 °C from an independent hand-held IR check at the same chute location.
• Erratic or noisy temperature signal on the plant SCADA trend — indicating sheath insulation breakdown or thermocouple wire fatigue.
• Open-circuit alarm on the PLC thermocouple input channel — sensor failure or compensating cable break.
• Visible physical damage to the sheath: bent, abraded through, or fouled with coked binder at the insertion point.
• Plant consistently consuming higher or lower fuel than the historical baseline at the same production rate, aggregate type and ambient temperature.