JJF 1908-2021 in English

Graded requirements for indication error

According to the accuracy grade of bimetallic thermometer, the maximum allowable error is calculated as a percentage of the range:

• Grade 1.0: ±1.0%FS (such as ±3.0℃ for a 300℃ range)
• Grade 2.5: ±2.5%FS (common industrial grade)
• Grade 4.0: ±4.0%FS (suitable for low-precision scenarios)

Requirements for special types of thermometers

Adjustable angle bimetallic thermometerneeds to additionally meet the following requirements:

1. Change in indication when adjusting axially→radially≤1.0%FS
2. Calibration must be completed in the axial position

Calibration process optimization

It is recommended to adopt a three-stage temperature control method:

1. Warm-up stage: Heat up to the calibration point ±5℃ at a rate of 2℃/min
2. Equilibrium stage: Keep the temperature stable for 10 minutes
3. Measurement stage: Read when the fluctuation is controlled to ≤0.1℃

Key points for uncertainty control

• Thermostatic bath has the highest contribution to uniformity (about 28%)
• The traceability period of the standard instrument is recommended to be ≤1 year
• The switching rate of the electric contact thermometer needs to be controlled to ≤0.6℃/min