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JJF 1280-2011: Calibration Specification for Capacitive Digital Scale Units
JJF 1280-2011
JJF
METROLGICAL TECHNICAL SPECIFICATION OF THE
PEOPLE’S REPUBLIC OF CHINA
Calibration specification for capacitive digital scale units
ISSUED ON: APRIL 12, 2011
IMPLEMENTED ON: JULY 12, 2011
Issued by: General Administration of Quality Supervision, Inspection and
Quarantine of PRC.
Table of Contents
1 Scope ... 4
2 Normative references ... 4
3 Overview ... 4
4 Measurement characteristics ... 6
4.1 Indication error ... 6
4.2 Indication variability ... 6
4.3 Drift ... 6
5 Calibration conditions ... 6
5.1 Environmental conditions ... 6
5.2 Main calibration equipment ... 7
6 Calibration items and calibration methods ... 7
6.1 Indication error ... 7
6.2 Indication variability ... 10
6.3 Drift ... 10
7 Expression of calibration results ... 11
8 Recalibration time interval ... 11
Appendix A Uncertainty evaluation of the measurement result of the indication error of
the capacitive digital scale unit ... 12
Appendix B Uncertainty evaluation of the indication error measurement result of the
capacitive digital scale unit ... 14
Appendix C Contents of calibration certificate ... 17
Calibration specification for capacitive digital scale units
1 Scope
This Specification is applicable to the calibration of various specifications of capacitive
digital scale units, which have a resolution of 0.005 mm, 0.01 mm, and a measurement
range of (0 ~ 2000) mm.
2 Normative references
This Specification refers to the following documents:
JJF 1001-1998 General terms in metrology and their definitions
JJF 1059-1999 Evaluation and expression of uncertainty in measurement
JJF 1094-2002 Evaluation of the characteristics of measuring instruments
GB/T 22518-2008 Capacitive digital scale units
When using this Specification, attention shall be paid to using the current valid versions
of the above referenced documents.
3 Overview
The capacitive digital scale consists of a capacitive main scale and a running frame
equipped with a capacitive digital display unit. It is a measuring instrument used to
measure linear displacement. The schematic diagram of its appearance structure is
shown in Figures 1 and 2.
each measuring point, within the range of the capacitive digital scale unit, is taken as
the indication error measurement result of the capacitive digital scale unit.
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(Δl) introduced by the variability (Δl) of the indication of the
capacitive digital scale unit
The indication variability (Δl) 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;
δt - Temperature difference between the capacitive digital scale unit and the gauge
block;
Δt - Difference between the average temperature of the capacitive digital scale unit
and the gauge block to 20 °C;
δα - Difference in linear expansion coefficient between the capacitive digital scale
unit and the gauge block.
B.3 Variance and sensitivity coefficient
Variance: