1 . Factors Affecting Temperature Error
Under measurement conditions, factors such as temperature, humidity, vibration, dust and corrosive gases may directly or indirectly affect measurement accuracy.
Among these factors, temperature changes have a significant effect on accuracy.
To ensure accurate results, the principle of maintaining equal temperatures between the measured object and the standard instrument is used, taking into account that different objects have varying expansion coefficients.
To ensure the accuracy of test data, all metrology experts must adhere to the temperature conditions specified by the verification regulations.
Currently, the standard temperature for measurement rooms is 20°C.
According to the manual, the relationship between the linear expansion coefficient (a), temperature change and size change can be expressed by formula (1):
Where:
- a -expansion coefficient;
- ΔL – size change;
- L – size of the object;
- ΔT – temperature variation.
The change in the size of a part resulting from a temperature deviation of 20°C can be expressed by formula (2):
In this equation, “t” represents the temperature of the object.
When the temperature of the workpiece and measuring tool differs from the standard temperature, the measurement error caused by temperature is the difference between the changes in size. This can be calculated using formula (3):
Where:
- ΔL is the size change;
- L – size of the object;
- a1, a2 – linear expansion coefficient of the part material and measuring tool;
- t1 t2 -the temperature of the part and the measuring tool.
The coefficient of linear expansion of an object is defined as the change in size of a unit length (1 mm) in response to a temperature change of 1℃ and is specific to the material of the object.
For example, consider a copper shaft with a diameter of 100 mm and a temperature of 40°C, and a steel micrometer with an outer diameter of 15°C. If the coefficients of linear expansion for the copper shaft and the steel micrometer are 17.5 x10-6 and 11.5 x10-6 respectively, then the change in dimension (△L) can be calculated as follows:
It is clear that temperature has a significant impact on part size and can result in numerous errors.
Therefore, when using measuring instruments, it is advisable to balance the temperature of the workpiece and the measuring tool before measuring to reduce temperature-related errors.
The temperature equilibration time required for each measuring tool during verification is specified in the corresponding verification regulations and calibration specifications.
Note: The process of bringing the temperature of the screw-down measuring tool and the standard measuring tool to consistency under specified conditions (such as an ambient temperature of (20 ± 6)℃) is referred to as temperature equilibrium.
two . Method to reduce temperature error
(1) Measure the temperature when it is close to the standard temperature.
In our work, we strictly comply with relevant regulations to ensure accuracy. Before checking or calibration, we ask customers to provide measuring instruments and measuring room temperature.
Customers often require immediate measurement data and it is important that temperature is taken into account when obtaining this data.
(2) When performing field measurements, do not remove the standard for measurement. Instead, place the pattern and workpiece together on a large, flat surface and measure after they reach thermal equilibrium.
(3) To avoid the influence of manual temperature on the workpiece and measuring tools, take necessary precautions. For example, when using a micrometer, hold it with a heat-insulating pad. If you suspect that heat from your hand has transferred to the micrometer, let it sit for a period of time before using or checking it.
(4) When measuring large objects outdoors, it is necessary to compensate for the temperature of the material being measured to minimize temperature errors. Use a measuring instrument specifically designed for this purpose.
