Home > Solutions > Content

# Several basic principles of length measurement (I)

May 04, 2017

The Minimum Deformation Principle

In length measurement,the cause to the deformation of the pieces to be tested and the measuring instruments is mainly due to the thermal deformation and elastic deformation (the contact deformation and the deformation caused by gravity). The deformation makes the measured piece and the size of measuring equipment change, and affects the measurement result’s accuracy and reliability. Therefore, during the measurement process, we should try to minimize the deformation due to a variety of causes, this is the minimum deformation measurement principle.

1. Thermal Deformation

(1) Overview

Heat bilges cold shrink, this is a natural phenomenon, it is this feature that often leads to serious misalignment of the measurement result.

Formula for the linear thermal deformation is:

△L=L•a•△t

L——Object sizemm

a——Linear Thermal Expansion Coefficient10-6/℃；

t——temperature variation，℃。

For example: third line scale standard metal linear thermal expansion coefficient: a=18.5×10-6/, if the temperature changes to t=1, so 1 meter will change: L=La•△t=1000×18.5×10-6/=18.5μm

For precision measurement, the gap is very big.

As a result, to test the parts of high precision, large size parts, the influence of temperature is a factor that can not be ignored.

All precision test needs to rule the temperature conditions, especially for the length measurement, almost all the verification regulation indicating the temperature requirements. It means that under specified conditions of temperature measuring doesn’t need temperature correction, otherwise to be revised. For high precision, large size material measurement is required isothermal.

(2) The thermal deformation leads to error of measurement

The measurement error caused by hot deformation is mainly due to the difference in temperature between the pieces to be tested and the measuring tool. We should place the pieces to be test and measuring tool (meter) in the lab for isothermal before measurement (isothermal time length is related to temperature difference, mass, heat dissipation area and the surrounding medium and other factors). But despite isothermal, large surface temperature and internal parts are not equal; even in the thermostatic chamber, temperature field distribution is not necessarily homogeneous. There will be a certain error for temperature measurement. Measuring temperature environment due to human body, such as lighting heat source will be volatile. After isothermal, measuring error caused by the thermal deformation will become very small, in a certain precision of measurement, it can be neglected.

In the measurements, almost people only pay attention to the condition of constant temperature, ignored the influence caused by body's temperature conduction. For example:a inside calipers with length of 2804000mm, put in hands for 2-5mins, its length will increase by 2050μm, use your index finger and thumb (without gloves)to take a gage block for 20 mm 30 s, its size will increase by 0.5μm(it is not allowed ).

Therefore, the high precision measuring instruments, such as contact interferometer, flat and thick interferometer and the flat crystal thickness interferometer must be equipped with  prevent and reduce thermal isolation device.