What are the errors in steel tape measurement?
The more accurate steel tape measure must be verified when it leaves the factory and after a period of use, and the temperature, tensile force and ruler length at the time of verification must be indicated. The length marked on the ruler is the nominal length, and the difference between it and the actual length is called ruler length correction Δl. When the ruler is subjected to different pulling forces, the ruler length will change. To avoid this change, it is required to use the steel tape measure in accordance with the pull force marked on the ruler. The length of the steel tape will change at different temperatures, so a function with temperature t as a variable must be used to represent the length of the ruler. This is the ruler length equation, and its general form is:
where lt is the actual length of the steel tape at temperature t;
l Nominal length of steel tape;
Δl Ruler length correction number, that is, the difference between the actual length and the nominal length of the steel tape at temperature t0;
α coefficient of thermal expansion of steel tape;
t0 The temperature of the steel tape measure during verification;
tThe temperature at which the steel tape measure is used.
In the use of the steel tape measure, the main reasons for the error are as follows:
a. Error in temperature change
Generally, the thermal expansion coefficient of the steel tape measure is α=1.25x10-5, which is only 1/80,000 of the temperature difference per meter per degree. measurement results. The error of temperature change is taken into account in the foot length equation.
b. Pull error
The magnitude of the pulling force will affect the length of the steel ruler. If the spring scale is not used to measure the pulling force during measurement, an error will occur. The elastic modulus of steel is E=2X106 KG/CM2. According to Hooke's law, a 30-meter-long ruler will produce a length error of ±1.8 mm when the tensile force error is ±5 kg.
c. The error of the steel ruler not being level
When measuring the horizontal distance, the steel tape should be kept as level as possible, otherwise there will be errors in distance increase. For a ruler length of 30 meters, when the height difference between the two ends of the ruler reaches 0.4 meters, there will be an error of about 2.6 mm, and the relative error is 1/11200.