3 minutes to understand: JJF 1257-2010 Calibration method for dry-type temperature calibrators

I. What is the standard used for?

Scope of application: Applicable to the calibration of dry-type temperature calibrators (referred to as "dry block calibration furnace") with a temperature range of -80℃ to 1300℃. The calibration temperature cannot exceed the range calibrated by the manufacturer.

II. What is a dry block calibration furnace?

A dry block calibration furnace is a portable temperature calibration device with a metal temperature-isolating block at its core. By heating or cooling, the thermometer (such as thermocouple, platinum resistance, etc.) inserted into it reaches the same temperature as the temperature-isolating block, thereby calibrating the accuracy of the thermometer.

Key terms:

Measurement area: The area in the dry-type oven hole used for calibration, usually located at the bottom of the hole, with a length of ≥40mm.

Matching bushing: A metal sleeve used to fit thermometers of different sizes, which must be used according to the manufacturer's requirements.

III. Why is calibration required?

The displayed temperature of the dry-type oven may deviate from the actual temperature, and calibration can ensure that it provides a stable and uniform temperature environment. Factors that affect calibration accuracy include:

Temperature fluctuation (changes over time)

Temperature difference between different temperature measuring holes

Uneven axial temperature distribution

Number and size of inserted thermometers

IV. Preparation conditions before calibration

Environmental requirements: temperature 15℃~35℃, humidity ≤85%.

Equipment requirements:

Thermometer outer diameter ≤6mm, insertion depth ≥15 times outer diameter.

The supporting electrical measuring equipment needs to be high-precision, and the thermometer needs to be traceable and calibrated.

Bushing selection: Use an adapter metal bushing as specified by the manufacturer.

V. Core steps of calibration

1. Temperature deviation calibration

Method: Compare the dry body furnace display value with a high-precision reference thermometer.

Steps: Set the calibration temperature point (at least 3), measure the temperature rise and fall once, and take the average value.

Formula: Deviation = dry body furnace display value - reference thermometer measured value.

2. Calibration of other key indicators

Temperature fluctuation: half of the maximum temperature fluctuation value within 20 minutes (for example: fluctuation ±0.5℃).

Inter-hole temperature difference: measure the maximum temperature difference between different holes.

Axial temperature field uniformity: measure the temperature difference at the bottom, middle and top of the hole.

VI. What does the calibration report contain?

Temperature deviation and uncertainty of the dry body furnace.

Temperature fluctuation, inter-hole temperature difference, axial temperature field uniformity and other indicators.

Calibration environment, equipment information, traceability certificate, etc.

VII. Usage and maintenance suggestions

Correct operation:

The thermometer is inserted deep enough to ensure good contact.

Preferentially use the center hole for calibration and place the dry body furnace vertically.

Regular inspection: It is recommended to calibrate once a year, or verify regularly with a standard thermometer.

Notes:

At high temperature (>660℃), the gap between the thermometer and the hole is ≤1mm.

Thermometers with an outer diameter of >6mm require additional evaluation of thermal conductivity error.

Summary

JJF 1257-2010 provides a standardized process for dry block calibration. The core is to ensure the accuracy of the temperature display by comparing with the reference thermometer. Calibration needs to pay attention to indicators such as temperature deviation, fluctuation, uniformity, and regular maintenance. Following the standard operation can greatly improve the reliability of thermometer calibration.