3.6.4 Experiment procedures
1)Calibration of silicon piezoresistive force sensor
(1)Install the fixed rod of the force sensor on the column of the iron frame platform,and ensure that the direction of measuring force is perpendicular to the plane of the sensor spring plate.
(2)Switch on the power to make the digital voltmeter preheated until the number is stable,and then set zero.
(3)Add plate weights(0.5 g/each)in turn and read out the corresponding voltage value U to determine the sensitivity of the sensor.Fill the value into Table 3.6.1.Attention:two methods can be employed to determine the sensitivity k of the force sensor:
①linear fitting of the U-F curve gets its slope.
②using the successive difference method calculates sensitivity k=ΔU/(Δmg×10-3)(mV/N).
2)The surface tension coefficient α of distilled water and absolute ethanol(https://www.daowen.com)
(1)After the lifting ring was hung up on the force sensor,the lifting table can be rotated in the anti-clockwise direction to raise the liquid level in the dish until the ring is immersed into the target liquid.This is the starting position of a surface tension experiment.
(2)The lifting table will be rotated in the clockwise direction to move the ring out of the liquid with a constant speed.A liquid meniscus is pulled out by the ring leading to the increase of the force approaches to a maximum value.Record the maximum value U1 at the moment before the ring breaks from the liquid and the value U2 after that.
3)Tips and Attentions
(1)The instrument needs to be preheated for at least 15 minutes.
(2)The fluctuation of the liquid in the dish should be as small as possible during rotating the lifting table.
(3)Keep fingers away from the distilled water to avoid polluting the liquid.
(4)Try to avoid the influence of wind on the lifting ring,which may make the zero point fluctuate and affect the measurement results.
