What is the approved thermometer accuracy under R 68 for measuring temperatures of aggregates, asphalt binder, and asphalt mixtures?

• A. ±2.0 °C
• B. ±0.5 °C
• C. ±1.0 °C
• D. ±2.5 °C

Answer: (D)

Measuring temperatures accurately is essential because temperature governs binder viscosity, aggregate coating, and the workability of the mixture. Under R 68, the allowed thermometer accuracy for measuring temperatures of aggregates, asphalt binder, and asphalt mixtures is ±2.5 °C. This means readings should be within 2.5 degrees of the true temperature to be considered acceptable. For example, if the target mix temperature is 150 °C, a thermometer reading between 147.5 °C and 152.5 °C would be within tolerance. This level of accuracy balances practical field and plant measurement needs with the control required to ensure proper coating, compaction, and performance of the final asphalt pavement. If a thermometer reads outside this range, it should be calibrated or replaced to maintain reliable temp control.