How to calculate the sensitivity of liquid surface tension force
Publish: 2021-05-07 04:07:36
1. You don't need Limin. What area do you use to measure this lile? As long as the surface of the sensor has a certain area, attached to the liquid surface, lift up, measure the force divided by the attached area is the tension coefficient. This is the simplest. I didn't think of a more economical way.
2. You don't need Limin. What area do you use to measure this lile? As long as the surface of the sensor has a certain area, attached to the liquid surface, lift up, measure the force divided by the attached area is the tension coefficient. This is the simplest.
3. Generally expressed in MV / V, the higher the value is, the higher the sensitivity is.
for example, if the power supply is 10V, the general output is 20mV.
the sensitivity is 2mV / v
for example, if the power supply is 10V, the general output is 20mV.
the sensitivity is 2mV / v
4. tolerable. When measuring, hang different weights, the mass value is x-axis, and the corresponding digital meter display value is y-axis. When drawing, the slope is the sensitivity.
5. Measuring liquid surface tension coefficient with silicon pressure resistance sensor
2. Learn the physical principle of silicon pressure resistance sensor and measure the surface tension coefficient of water and other liquids< Experimental apparatus
Fig.1 surface tension coefficient tester
wbm-1a liquid surface tension tester and vernier caliper
III. experimental principle (two pictures missing)
surface tension is a manifestation of molecular force, which occurs at the boundary of liquid and gas contact and is determined by the special situation of liquid molecules in the surface layer. The molecules in the liquid are almost close to each other. The equilibrium distance between the molecules is always maintained. A little farther away, the molecules will attract each other, and a little closer, the molecules will repel each other. This determines that the liquid molecules can not diffuse infinitely like the gas molecules, but can only vibrate and rotate near the equilibrium position. For the molecules near the liquid surface, because the attraction of the gas molecules in the upper space is less than that of the internal liquid molecules, the resultant force of the molecules is not equal to zero, and its resultant force direction is perpendicular to the interior of the liquid. This kind of contraction force is called surface tension. The repulsive force between molecules in the surface layer decreases with the increase of their distance from each other, and the gravitational interaction between molecules is dominant in this special layer. If a dividing line Mn is drawn arbitrarily on the surface of the liquid to divide the liquid surface into two parts A and B (as shown in Fig. 2), f represents the attraction of the molecules in the surface layer of part a to part B, F & # 180; It represents the attraction of molecules in the surface layer of the right part to part A. the forces of these two parts must be equal in magnitude and opposite in direction. The mutual traction force between any two parts of the surface layer promotes the shrinkage of the liquid surface layer. Due to the effect of surface tension, the liquid surface tends to shrink as much as possible, so the small droplets in the air tend to be spherical
Fig.2 schematic diagram of liquid surface tension
the direction of surface tension is tangent to the liquid level and perpendicular to the dividing line between the two parts. If the liquid level is a plane, the surface tension is on this plane. If the liquid surface is a curved surface, the surface tension is on the section of the curved surface. Surface tension is a property of matter, which is related to temperature and the nature of interface. The surface tension f is proportional to the length L of the dividing line Mn, which can be written as
F= α L (1)
coefficient α It's called the surface tension coefficient, and its unit is n / m. Numerically, the surface tension coefficient is equal to the mutual traction force per unit length between two adjacent parts of the liquid surface. The surface tension coefficient is related to the temperature and purity of the liquid, but not to the size of the liquid surface. The liquid temperature increases, α After the pure liquid is mixed with trace impurities, α It decreased obviously
Fig. 3 force analysis of pull-out process
the common methods for measuring surface tension in general physics experiments include pull-out method, capillary method and maximum bubble pressure method. Here we use the pull-out method to measure the surface tension of liquid with silicon pressure resistance sensor. The specific measurement method is to hang an aluminum alloy ring with clean surface on the pull hook of the force sensor, raise the lifting table to make the aluminum alloy ring vertically immerse in the liquid, lower the lifting table, and the liquid level drops. When the bottom of the ring is flush with the liquid level or slightly higher, the inner and outer walls of the ring will take up part of the liquid e to the effect of the liquid surface tension, as shown in Figure 3. In equilibrium, the gravity mg, the upward pull F and the liquid surface tension F of the lifting ring meet the requirements of
F = Mg + fcos φ 2)
when the lifting ring is separated from the liquid critically, φ= 0, that is cos φ= The equilibrium condition is approximately
F = f-mg= α( D1+D2) π 3)
where D1 and D2 are the inner diameter and outer diameter of the lifting ring respectively, and the liquid surface tension coefficient is
α=( F-mg)/ π( D 1 + D 2) (4)
F, Mg, d 1 and D 2 should be measured
the force sensor is used to measure the force. Firstly, the silicon pressure resistance sensor is calibrated to obtain the sensor sensitivity B (MV / N), and then the reading U1 of the voltmeter when the lifting ring is about to pull off the liquid surface (F = Mg + F) is measured, and the reading U2 of the digital voltmeter after pulling off (F = mg) is recorded α=( U1+U2)/B π( D1+D2) 5) 4. Experimental steps
1. Experimental preparation
start the machine and preheat it for 15 minutes to clean the glassware and rings; Use vernier caliper to measure the inner and outer diameter D1 and D2 of the lifting ring respectively
2. Calibration of silicon pressure resistance sensor
(1) hang the weight plate on the hook of the force sensor and select the "200 mV" gear to zero and calibrate the sensor
(2) put 1 g (1 weight) into the weight plate each time, record the reading of the digital voltmeter until 7 g is added, and record the data in Table 1 (read again after the output of the voltmeter is basically stable)
3. Measure the surface tension
put the water to be measured in the glassware and place it on the lifting table, hang the metal lifting ring on the hook of the force sensor, keep the lifting ring level, turn the lifting table clockwise slowly to make the liquid level rise, when the lower part of the lifting ring is immersed in the liquid, turn the lifting table anticlockwise slowly to make the liquid level fall, Observe the physical process and phenomenon when the ring is immersed in the liquid and pulled up from the liquid, pay special attention to the reading U1 of the digital voltmeter immediately before the ring is pulled off the liquid surface and the reading U2 of the digital voltmeter after the ring is pulled off, record these two values, repeat the above measurement process for five times, and record the corresponding U1 and U2 in Table 2
v. precautions
(1) when using the force sensor, the force should not be greater than 30 g, otherwise the sensor will be damaged and the weight should be handled with care
(2) after cleaning, the utensils and rings should not be touched by hands or liquid
(3) keep the lifting ring level, rotate the lifting table slowly to avoid water shaking, and read U1 and U2 accurately
(4) wipe and wrap the rings after the experiment< Experimental data
Table 1 Calibration of force sensor
weight mass / g
1
2
3
4
5
6
7
output voltage / MV
according to the calibration formula u = b * mg, the sensitivity of the instrument B, g = 9.80 M / S2 is determined by the least square method
Table 2 Determination of surface tension coefficient of water
times
U1 / MV
U2 / MV
Δ( U1-U2)/mV
α/(× (10-3n / M)
1
2
3
4
5
inner diameter D1 / mm
outer diameter D2 / mm
7. Thinking question
(1) what other methods can be used to process the experimental data of force sensor sensitivity B
(2) analyze the reason why the reading value of voltmeter changes from large to small at the moment when the lifting ring is about to break the liquid level
(3) analyze the systematic error and random error of the experiment, and put forward the methods and measures to rece the error and improve the experiment?
2. Learn the physical principle of silicon pressure resistance sensor and measure the surface tension coefficient of water and other liquids< Experimental apparatus
Fig.1 surface tension coefficient tester
wbm-1a liquid surface tension tester and vernier caliper
III. experimental principle (two pictures missing)
surface tension is a manifestation of molecular force, which occurs at the boundary of liquid and gas contact and is determined by the special situation of liquid molecules in the surface layer. The molecules in the liquid are almost close to each other. The equilibrium distance between the molecules is always maintained. A little farther away, the molecules will attract each other, and a little closer, the molecules will repel each other. This determines that the liquid molecules can not diffuse infinitely like the gas molecules, but can only vibrate and rotate near the equilibrium position. For the molecules near the liquid surface, because the attraction of the gas molecules in the upper space is less than that of the internal liquid molecules, the resultant force of the molecules is not equal to zero, and its resultant force direction is perpendicular to the interior of the liquid. This kind of contraction force is called surface tension. The repulsive force between molecules in the surface layer decreases with the increase of their distance from each other, and the gravitational interaction between molecules is dominant in this special layer. If a dividing line Mn is drawn arbitrarily on the surface of the liquid to divide the liquid surface into two parts A and B (as shown in Fig. 2), f represents the attraction of the molecules in the surface layer of part a to part B, F & # 180; It represents the attraction of molecules in the surface layer of the right part to part A. the forces of these two parts must be equal in magnitude and opposite in direction. The mutual traction force between any two parts of the surface layer promotes the shrinkage of the liquid surface layer. Due to the effect of surface tension, the liquid surface tends to shrink as much as possible, so the small droplets in the air tend to be spherical
Fig.2 schematic diagram of liquid surface tension
the direction of surface tension is tangent to the liquid level and perpendicular to the dividing line between the two parts. If the liquid level is a plane, the surface tension is on this plane. If the liquid surface is a curved surface, the surface tension is on the section of the curved surface. Surface tension is a property of matter, which is related to temperature and the nature of interface. The surface tension f is proportional to the length L of the dividing line Mn, which can be written as
F= α L (1)
coefficient α It's called the surface tension coefficient, and its unit is n / m. Numerically, the surface tension coefficient is equal to the mutual traction force per unit length between two adjacent parts of the liquid surface. The surface tension coefficient is related to the temperature and purity of the liquid, but not to the size of the liquid surface. The liquid temperature increases, α After the pure liquid is mixed with trace impurities, α It decreased obviously
Fig. 3 force analysis of pull-out process
the common methods for measuring surface tension in general physics experiments include pull-out method, capillary method and maximum bubble pressure method. Here we use the pull-out method to measure the surface tension of liquid with silicon pressure resistance sensor. The specific measurement method is to hang an aluminum alloy ring with clean surface on the pull hook of the force sensor, raise the lifting table to make the aluminum alloy ring vertically immerse in the liquid, lower the lifting table, and the liquid level drops. When the bottom of the ring is flush with the liquid level or slightly higher, the inner and outer walls of the ring will take up part of the liquid e to the effect of the liquid surface tension, as shown in Figure 3. In equilibrium, the gravity mg, the upward pull F and the liquid surface tension F of the lifting ring meet the requirements of
F = Mg + fcos φ 2)
when the lifting ring is separated from the liquid critically, φ= 0, that is cos φ= The equilibrium condition is approximately
F = f-mg= α( D1+D2) π 3)
where D1 and D2 are the inner diameter and outer diameter of the lifting ring respectively, and the liquid surface tension coefficient is
α=( F-mg)/ π( D 1 + D 2) (4)
F, Mg, d 1 and D 2 should be measured
the force sensor is used to measure the force. Firstly, the silicon pressure resistance sensor is calibrated to obtain the sensor sensitivity B (MV / N), and then the reading U1 of the voltmeter when the lifting ring is about to pull off the liquid surface (F = Mg + F) is measured, and the reading U2 of the digital voltmeter after pulling off (F = mg) is recorded α=( U1+U2)/B π( D1+D2) 5) 4. Experimental steps
1. Experimental preparation
start the machine and preheat it for 15 minutes to clean the glassware and rings; Use vernier caliper to measure the inner and outer diameter D1 and D2 of the lifting ring respectively
2. Calibration of silicon pressure resistance sensor
(1) hang the weight plate on the hook of the force sensor and select the "200 mV" gear to zero and calibrate the sensor
(2) put 1 g (1 weight) into the weight plate each time, record the reading of the digital voltmeter until 7 g is added, and record the data in Table 1 (read again after the output of the voltmeter is basically stable)
3. Measure the surface tension
put the water to be measured in the glassware and place it on the lifting table, hang the metal lifting ring on the hook of the force sensor, keep the lifting ring level, turn the lifting table clockwise slowly to make the liquid level rise, when the lower part of the lifting ring is immersed in the liquid, turn the lifting table anticlockwise slowly to make the liquid level fall, Observe the physical process and phenomenon when the ring is immersed in the liquid and pulled up from the liquid, pay special attention to the reading U1 of the digital voltmeter immediately before the ring is pulled off the liquid surface and the reading U2 of the digital voltmeter after the ring is pulled off, record these two values, repeat the above measurement process for five times, and record the corresponding U1 and U2 in Table 2
v. precautions
(1) when using the force sensor, the force should not be greater than 30 g, otherwise the sensor will be damaged and the weight should be handled with care
(2) after cleaning, the utensils and rings should not be touched by hands or liquid
(3) keep the lifting ring level, rotate the lifting table slowly to avoid water shaking, and read U1 and U2 accurately
(4) wipe and wrap the rings after the experiment< Experimental data
Table 1 Calibration of force sensor
weight mass / g
1
2
3
4
5
6
7
output voltage / MV
according to the calibration formula u = b * mg, the sensitivity of the instrument B, g = 9.80 M / S2 is determined by the least square method
Table 2 Determination of surface tension coefficient of water
times
U1 / MV
U2 / MV
Δ( U1-U2)/mV
α/(× (10-3n / M)
1
2
3
4
5
inner diameter D1 / mm
outer diameter D2 / mm
7. Thinking question
(1) what other methods can be used to process the experimental data of force sensor sensitivity B
(2) analyze the reason why the reading value of voltmeter changes from large to small at the moment when the lifting ring is about to break the liquid level
(3) analyze the systematic error and random error of the experiment, and put forward the methods and measures to rece the error and improve the experiment?
6. It depends on how many times you test. If it is an even number, it is directly divided into two groups: suppose there are eight data, which are divided into (1,2,3,4) and (5,6,7,8), and the corresponding values of the former group are subtracted from the latter group, such as 5-1,6-2; Then add these differences, divide them by 4 (each group has 4 data), and then divide them by a 4 (= 5-1 = 6-2 = 7-3 = 8-4). You can get what you want. If you give it meaning, it can be expressed as the average value of reading change for each weight added. If the number of experiments is odd, you just need to reuse the middle number. For example, there are 7 groups of data, 1,2,3,4,5,6,7, and the components are divided into (1,2,3,4) and (4,5,6,7). Just divide by 4 (each group has 4 data), and then divide by 3 (= 4-1 = 5-2 = 6-3 = 7-4). The application condition of the successive difference method is that the independent variable changes at equal intervals (such as reading data at equal intervals on the screw rod or screw micrometer), and the function relationship is linear. When the function relation is nonlinear, it can't be dealt with by the successive difference method. In addition, it must be remembered that when using the successive difference method, the data measured at equal intervals should be divided into two groups in half, so that the calculation results are more accurate.
7.
Calculation of sensor sensitivity:
(1) the sensitivity is numerically equal to the slope of the output input characteristic curve
if there is a linear relationship between the output and the input of the sensor, then the sensitivity s is a constant. Otherwise, it will change with the input
(2) the dimension of sensitivity is the ratio of the dimension of output and inputfor example, when the displacement of a displacement sensor changes by 1 mm and the output voltage changes by 200 mV, its sensitivity should be expressed as 200 mV / mm. When the dimensions of the output and input of the sensor are the same, the sensitivity can be understood as the magnification
When the sensitivity is improved, higher measurement accuracy can be obtained. But the higher the sensitivity, the narrower the measurement range and the worse the stability 
extended data
selection of sensor sensitivity:
generally, within the linear range of the sensor, the higher the sensitivity of the sensor, the better. Because only when the sensitivity is high, the value of the output signal corresponding to the measured change is relatively large, which is concive to signal processing. However, it should be noted that the sensitivity of the sensor is high, and the external noise irrelevant to the measured is easy to mix in, which will also be amplified by the amplification system, affecting the measurement accuracy
Therefore, the sensor itself should have a high signal-to-noise ratio to minimize the interference signal from the outside The sensitivity ofsensor is directional. When the measured signal is unidirectional and its directivity is high, the sensor with low sensitivity in other directions should be selected; If the measured vector is multi-dimensional, the smaller the cross sensitivity of the sensor, the better
reference source: network sensor
8. A small tray is hung under the sensor hook, and then zeroed (at present, the sensor is composed of a silicon diffusion resistance non-equilibrium bridge and a digital voltmeter. The sensitivity is high. Therefore, after adjusting the zero, the number may be bouncing and need to be adjusted repeatedly). Put 500 mg and 92 into the tray in turn; 1000mg\ 1500mg.......\ 3000mg small weight, respectively read out the corresponding voltage value, into the least square formula (general physical experiment Chapter 1 has introced, some high school textbooks also have, can also be directly substituted into the scientific calculator or excel) to get the K value The standard formula of the least square is y = a + BX. The mass is converted into gravity by the local acceleration of gravity as the value of X, and the voltage as the value of Y. the obtained B is the value of K). The linear correlation coefficient r can be obtained by the same method The sensitivity of this experiment is very high. Generally, the data is considered valid when R exceeds 0.99.)
9. The sensitivity of load cell is the output voltage under the nominal load, which can be used as a reference for the design of amplifier circuit
theoretically, the higher the sensitivity, the higher the resolution, that is, the more precise the graation can be
in practical application, sensitivity needs to be combined with linearity, stability and overload limit value to judge whether it is more accurate than other sensors. It is not significant to simply compare the sensitivity.
theoretically, the higher the sensitivity, the higher the resolution, that is, the more precise the graation can be
in practical application, sensitivity needs to be combined with linearity, stability and overload limit value to judge whether it is more accurate than other sensors. It is not significant to simply compare the sensitivity.
10. tolerable. When measuring, hang different weights, the mass value is x-axis, and the corresponding digital meter display value is y-axis. When drawing, the slope is the sensitivity.
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