JJF 1280-2011Calibration Specification for Capacitive Digital Scale Units (English PDF)

6.1.2 Gauge block measurement method

The indication error of the non-enclosed capacitive digital scale unit, which has an upper limit of no more than 1000 mm, can also be measured by using 5-division gauge blocks in conjunction with a measuring fixture (see Figure 4 or Figure 5). The anvil of the measuring fixture is fixed on the main scale; the measuring plate is fixed on the running frame mounting plate. The difference between the above two measuring fixtures is that the ball probe of the measuring plate is in different positions; either one of them can be selected. When measuring, first move the running frame to make the ball probe of the measuring plate contact the measuring anvil's measuring surface; the display screen reads "zero". Then move the running frame; place a gauge block of the corresponding size between the ball probe and the measuring anvil's measuring surface for contact measurement; read the indication value from the display screen. The difference between the indication value of the capacitive digital scale unit and the actual size of the gauge block at each measuring point is the indication error of the measuring point. The difference between the maximum and minimum indication errors of each measuring point, within the measuring range, is taken as the indication error measurement result of the capacitive digital scale unit. The selection of measuring points refers to the provisions of 6.1.1.

Note: The flatness and surface roughness of the measuring surface of the measuring anvil of the measuring fixture shall meet the requirements of the 5-division gauge block; when the measuring anvil is installed, the perpendicularity between the measuring surface of the measuring anvil and the guide surface of the capacitive digital scale is not greater than 10'.

When the upper limit of measurement is not greater than 400 mm, the capacitive digital scale unit generally adopts horizontal measurement. When the upper limit of measurement is greater than 400 mm and less than or equal to 1000 mm, vertical measurement can be adopted.

Appendix A

Uncertainty evaluation of the measurement result of the indication error of the capacitive digital scale unit (Special measuring device measurement method)

A.1 Measurement method

According to this calibration Specification, the indication error of the capacitive digital scale unit, which has a resolution of 0.005 mm and a measurement upper limit of 300 mm, is measured using a special measuring device (in this example, a laser interferometer is used as a standard), to evaluate the uncertainty of the indication error measurement result.

A.2 Mathematical model

The mathematical model of the indication error of each measuring point is:

Where:

e - Indication error of the capacitive digital scale unit;

l - Indication of the capacitive digital scale unit (under 20 °C);

ls - Indication of the laser interferometer (under 20 °C).

A.3 Variance and sensitivity coefficient

Variance:

Sensitivity coefficient:

A.4 Standard uncertainties

A.4.1 Uncertainty u(deltal) introduced by the variability (deltal) of the indication of the

capacitive digital scale unit

The indication variability (deltal) obeys a uniform distribution and affects the calibration process twice. The standard uncertainty introduced is:

Appendix B

Uncertainty evaluation of the indication error measurement result of the capacitive digital scale unit (Gauge block measurement method)

B.1 Measurement method

According to this calibration Specification, 5-division blocks are used to measure the indication error of the capacitive digital scale unit, which has a resolution of 0.005 mm and a measurement upper limit of 300 mm; the uncertainty of the indication error measurement result is evaluated.

B.2 Mathematical model

The mathematical model of the indication error of each measuring point is formula (1).

Considering the influence of the measurement uncertainty factor during measurement, formula (1) is transformed into:

Where:

e - Indication error of the capacitive digital scale unit;

l - Indication of the capacitive digital scale unit (under 20 °C);

ls - Actual value of the size of 5-division gauge blocks (under 20 °C);

L - Nominal size of the calibration point;

- Average linear expansion coefficient of the capacitive digital scale unit and the gauge block;

deltat - Temperature difference between the capacitive digital scale unit and the gauge block;

deltat - Difference between the average temperature of the capacitive digital scale unit and the gauge block to 20 °C;

deltaalpha - Difference in linear expansion coefficient between the capacitive digital scale unit and the gauge block.

B.3 Variance and sensitivity coefficient

Variance:

......