Microforce calibration
For calibration of the bending stiffness of microforce sensors and/or microforce standards, a microforce measuring device, based on a compensation balance (Sartorius SC2: measuring range 20 mN, 1 nN resolution, 2.5 nN reproducibility, 9 nN linearity deviation, 0.1µN uncertainty) nd a precision linear adjusting device (PIFOC: 80 µm traverse path, 1 nm resolution) has been developed.
Figure 2: Microforce measuring device
Figure 2: Microforce measuring device
If the standard to be calibrated has a probing tip of its own, this tip is used for probing of the load receptor. At the same time, the deflection z of the standard and the force F required for it are measured. In the case of the compensation balance used, the position of the load receptor remains constant during calibration, i. e. the displacement of the standard z corresponds to the deflection of the standard to be calibrated. The bending stiffness k is calculated as a quotient of the force F and the deflection z.
An essential disadvantage of this force measuring device is its traceability to the SI units with the aid of mass standards. The achievable uncertainties of mass standards are insufficient, especially for small masses. A typical measurement uncertainty for a mass of 1 mg is 20 µg. The gravitational force which corresponds to this mass is thus affected by an uncertainty of 200 nN.