A pneumatic liquid level depth detection method

Abstract

The present invention discloses a pneumatic liquid level depth detection method. First, a pipetting system is built. The operation of a plunger pump is controlled through the pipetting system to move the pipette tip up and down for liquid suction or discharge. The air pressure in the air pipeline is measured by a pressure sensor. By analyzing the pressures exerted on the pipette tip in air and liquid, the relationship between the depth of the pipette tip and the air pressure value in the pipeline is then deduced, so as to realize liquid level detection and liquid depth measurement during the liquid suction and discharge processes. During the detection process, the liquid level displacement amount generated by actual pipetting is calculated using this air pressure model, and the pipette tip is adjusted in a timely manner to always be at the accurate depth within the medium to be aspirated or discharged without affecting other substances with different properties. During the detection process, the liquid level displacement amount generated by actual pipetting is calculated using the air pressure model of the present invention, and the pipette tip is adjusted in a timely manner to always be at the accurate depth within the medium to be aspirated or discharged without affecting other substances with different properties.

Description

Technical Field

[0001] The present invention relates to a pneumatic liquid level depth detection method, belonging to the technical field of liquid level detection in biomedical detection. Background Art

[0002] In the field of biomedical detection, liquid level meters are often used to detect the liquid level depth. Currently, the commonly used sensors for liquid level meters usually include the following types: electrical liquid level sensors, liquid pressure sensors, ultrasonic liquid level sensors, and laser liquid level sensors. The principles, advantages, and disadvantages of liquid level meters with these sensors are as follows:

[0003] 1. Electrical liquid level sensors: including sensors such as resistors, capacitors, and inductors. The principle is to indirectly obtain the height information of the liquid level by measuring the electrical characteristics of the liquid in the container. Among them, the most widely used is the capacitance sensor, which has advantages such as small volume and simple implementation. The disadvantage is that it is easily affected by different container materials and solution properties, and the detected liquid must be a polar liquid.

[0004] 2. Liquid pressure sensors: This measurement method uses a pressure sensor installed at the bottom. By detecting the liquid pressure at the bottom and converting and calculating the liquid level height, the reference value of the liquid pressure at the bottom is the atmospheric pressure communicated with the top or a known air pressure. This measurement method requires the use of a high-precision, flush-mounted pressure sensor, and the conversion process needs to be continuously calibrated. Its advantages are that the detection is not limited by the liquid level height. The disadvantage is that the higher the height, the higher the accuracy requirement for the sensor, and repeated calibration is required during long-term use or when the liquid is changed.

[0005] 3. Ultrasonic liquid level sensors: The principle is to calculate the liquid level height by detecting the time difference between the transmission and reflection of ultrasonic waves, and it is easily affected by the energy loss of ultrasonic wave propagation. It has characteristics such as easy installation and high flexibility, and can usually be installed at a high place for non-contact measurement. The disadvantage is that when it is used in an environment containing steam, dust, etc., the detection distance will be significantly shortened. Therefore, it is not recommended to use it in an absorptive environment with foam, etc.

[0006] 4. Laser liquid level sensors: Laser sensors are based on the principle of optical detection. They detect by reflecting light from the surface of an object to a receiver. Its light spot is small and concentrated, easy to install and calibrate, and has good flexibility. It can be applied to continuous or limit alarm of bulk materials or liquid levels, etc. The disadvantage is that it is not suitable for use in transparent liquids (transparent liquids are easy to refract light, resulting in the inability of light to be reflected to the receiver), environments containing foam or steam (unable to penetrate foam or easily affected by steam interference), fluctuating liquids (easy to cause false actions), vibration environments, etc.

[0007] During the actual pipetting process, due to the large differences in density, viscosity, etc. between samples and reagents, and the problem of volume loss caused by the transfer of liquids with different viscosity coefficients on high-throughput automated molecular detection platforms, there are large errors in the actual measurement of the liquid level depth. Liquid level gauges with the above-mentioned sensors are highly restricted, have a narrow application field, and are relatively costly, so their market applications are limited. Summary of the Invention

[0008] The purpose of the present invention is to provide a pneumatic liquid level depth detection method to solve the problem of volume loss caused by the transfer of liquids with different viscosity coefficients on high-throughput automated molecular detection platforms, so that it is always at the accurate depth within the medium to be aspirated or discharged without affecting other substances with different properties.

[0009] The present invention adopts the following technical solutions: A pneumatic liquid level depth detection method, which includes the following steps: S1. Build a pipetting system: Connect the pipette of the pipette tip to a piston pump through a gas pipeline, set a pressure sensor on the gas pipeline, and set a control mechanism for driving the pipette tip to move vertically up and down; S2. Use the control mechanism to drive the pipette tip to move downward, use the piston pump to aspirate liquid in the liquid sample, and at the same time the pipetting system feeds back the air pressure value of the pressure sensor to calculate the depth of point A where the pipette tip is located. The calculation process is as follows:

[0010] In the pipetting system, the pressure balance formula at point A of the pipette tip is:

[0011] P1 + P2 = P0 + P3 (1)

[0012] Among them, P1 is the pressure of the liquid column in the pipette on point A, P2 is the gas pressure in the gas pipeline, P0 is the external atmospheric pressure, and P3 is the pressure of the liquid in the liquid sample on point A;

[0013] Because the gas pressure in the gas pipeline is the same everywhere after stabilization, the gas pressure formula in the gas pipeline can be expressed as:

[0014]

[0015] Among them, n, R, and T are the number of moles of gas, gas constant, and thermodynamic temperature in the gas pipeline respectively, V is the volume of gas in the gas pipeline, and P w is the pressure loss along the way of the gas in the gas pipeline;

[0016] Due to the viscosity of the liquid hindering the rise of the liquid, when the piston pump starts from rest to work, the liquid cannot fill the vacancy of the gas volume in the gas pipeline in time, resulting in an increase in V. Formula (2) is further deduced as follows:

[0017]

[0018] Among them, V0 is the volume of gas in the gas pipeline before the start of liquid absorption, and ΔV is the increased volume generated due to liquid viscosity during the liquid absorption process. This volume is the difference between "the increased volume V pr in the gas pipeline when the pump piston retracts" and "the volume V lr occupied by the rising liquid fluid in the gas pipeline":

[0019] ΔV = V pr - V lr = Xt - Qt (4)

[0020] X is the set operating speed of the pump, t is the system operating time, and Q is the flow rate of the liquid in the pipette per unit time. It can be simplified by Poiseuille's law as follows:

[0021]

[0022] r is the radius of the gas pipeline, ΔP is the pressure difference between the pressure in the gas pipeline after the liquid rises and the initial state before the pipetting system works, k is the liquid viscosity coefficient, h is the liquid rising height, and ρ l is the liquid density;

[0023] For the pressure loss P w along the way of the gas, from Darcy's formula:

[0024]

[0025] λ is the friction resistance coefficient between the air and the gas pipeline wall, and R s is the hydraulic radius of the pipette, v is the gas flow velocity in the gas pipeline, ρ g is the gas density in the gas pipeline, and L is the total length from the pipette tip to the pressure sensor;

[0026] From equations (3), (4), (5), and (6), the pressure change equation in the gas path during the liquid absorption process can be obtained:

[0027]

[0028] From the liquid pressure formula P = ρgh, it can be obtained:

[0029] P1 = ρ1gh1 (8)

[0030] P3 = ρ1gh2 (9)

[0031] Among them, ρ1 is the liquid density, h1 is the liquid column height in the pipette, h2 is the liquid surface height, and h1 can be obtained by the ratio of the liquid volume V lr in the pipette to the pipeline cross-sectional area S:

[0032]

[0033] From equations (8) to (10), we can obtain:

[0034]

[0035] Substituting equations (7), (9), and (11) into equation (1), we can obtain the relationship between the liquid level height and the pressure in the pipeline:

[0036]

[0037] S3. After the liquid suction process is completed, the liquid discharge starts. At the same time, the pressure sensor feeds back the air pressure value in the air pipeline during the liquid discharge process, and calculates the depth of point B where the pipette tip is located. The calculation process is as follows:

[0038] In the liquid transfer system, the pressure balance formula at point B of the pipette tip is:

[0039] P1 + P2 = P0 + P3 (13)

[0040] Among them, P1 is the pressure of the liquid column in the pipette on point B, P2 is the gas pressure in the gas pipeline, P0 is the external atmospheric pressure, and P3 is the pressure of the liquid in the liquid sample on point B;

[0041] Since the pressure is the same everywhere in the gas pipeline after the gas in the pipeline is stable, the gas pressure formula in the gas pipeline can be expressed as:

[0042]

[0043] Among them, n is the number of moles of gas in the gas pipeline, R is the gas constant, T is the thermodynamic temperature, V is the volume of gas in the gas pipeline, and P w is the pressure loss along the gas pipeline;

[0044] Due to the viscosity of the liquid that hinders the liquid from descending, when the plunger pump starts from rest to work, the liquid level cannot drop in time, causing the gas in the gas pipeline to be compressed and V to decrease. Further derivation of the formula:

[0045]

[0046] Among them, V0 is the volume of gas in the gas pipeline before the liquid suction starts, ΔV is the increased volume due to the viscosity of the liquid during the liquid suction process, and ΔV' is the difference between "the increased volume V of the gas in the gas pipeline when the pump piston retracts" and "the volume V of the gas pipeline occupied by the descending liquid fluid": pr " and "the volume V of the gas pipeline occupied by the descending liquid fluid lr ":

[0047] ΔV = V pr -V lr = Xt - Qt (16)

[0048] X is the operating speed of the plunger pump set by the program, Q is the flow rate of the liquid in the pipette per unit time, which can be simplified by Poiseuille's law. t is the operating time of the system:

[0049]

[0050] r is the radius of the gas pipeline, ΔP is the pressure difference between the state after the liquid rises and the initial state, k is the liquid viscosity coefficient, h’ is the liquid rising height, ρ l is the liquid density;

[0051] For the pressure loss P along the gas path w , from the Darcy formula:

[0052]

[0053] λ is the friction resistance coefficient between the air and the gas pipeline wall, R s is the hydraulic radius of the pipeline, v is the gas flow velocity in the gas pipeline, ρ g is the gas density in the gas pipeline, L is the total length from the tip to the pressure sensor. In summary, the pressure change equation in the gas path during the liquid discharging process can be obtained:

[0054]

[0055] From the liquid pressure formula P = ρgh, it can be obtained:

[0056] P1 = ρ1gh1' (20)

[0057] P3 = ρ1gh'2(21)

[0058] where ρ1 is the liquid density, h1' is the liquid column height in the pipette, h'2 is the liquid surface height, and h1' can be obtained by the difference between the liquid column height after suction in the tube and the height of the discharged liquid:

[0059]

[0060] Thus, it can be obtained

[0061]

[0062] In summary, the relationship between the liquid surface height and the pressure in the pipeline can be obtained:

[0063]

[0064] The length of the gas path pipeline between the pressure sensor and the pipette tip is 1 / 3 of the total length of the gas path pipeline.

[0065] The control mechanism is a linear slide, and the linear slide includes a vertically arranged guide rail and a driver for driving the pipette tip to move on the guide rail.

[0066] The beneficial effects of the present invention are as follows: The present invention is based on a self-built pipetting system. The operation of the plunger pump and the up-and-down movement of the pipette tip are controlled by a driver. The air pressure in the air pipeline is measured by an air pressure sensor. By analyzing the pressures exerted on the pipette tip in air and liquid, the relationship between the depth of the pipette tip and the air pressure value in the pipeline is deduced, thereby realizing the liquid level detection and liquid depth measurement during the liquid suction and discharge processes. During the detection process, the actual liquid level displacement generated by pipetting is calculated using this air pressure model, and the pipette tip is adjusted in a timely manner to always be at the accurate depth within the medium to be aspirated or discharged without affecting other substances with different properties.

[0067] The present invention solves the problem of volume loss caused by transferring liquids with different viscosity coefficients on a high-throughput automated molecular detection platform, enabling it to always be at the accurate depth within the medium to be aspirated or discharged without affecting other substances with different properties. The present invention can reduce the pipetting volume error and make the extraction of stratified samples more accurate, greatly improving the accuracy of liquid sample transfer, effectively reducing the error caused by liquid level displacement during liquid suction and discharge, and improving the reliability and accuracy of the detection results. The present invention has strong universality, can be applied to all research related to micro-liquid handling in the field of biomedical detection, has low cost, does not require the use of additional materials, is easy to operate, and effectively avoids the possible cross-contamination problems during the liquid handling process. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 is a schematic block diagram of the pipetting system in the air pressure-based liquid level depth detection method of the present invention;

[0069] Figure 2 is a sectional view of the air pipeline structure of the pipetting system in the air pressure-based liquid level depth detection method of the present invention;

[0070] Figure 3 is the pharyngeal swab data curve in Example 1 of the present invention;

[0071] Figure 4 is the water data curve in Example 2 of the present invention.

[0072] In the figure: 1 - liquid sample, 2 - pipette tip, 3 - air pipeline, 4 - air pressure sensor, 5 - plunger pump, 6 - pipette tube. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0073] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0074] The air pressure-based liquid level depth detection method of the present invention includes the following steps:

[0075] S1. Set up the pipetting system: Connect the pipette tube 6 of the pipette tip 2 to the plunger pump 5 through the gas pipeline 3. Install a pressure sensor 4 on the gas pipeline 3, and set up a control mechanism for driving the pipette tip 2 to move vertically up and down. The control mechanism is a linear stage, which includes a vertically arranged guide rail and a driver for driving the pipette tip to move on the guide rail. The linear stage is used to control the position of the pipette tip 1 in the vertical direction and is controlled by an independent host computer program. The length of the gas pipeline between the pressure sensor 4 and the pipette tip 2 is 1 / 3 of the total length of the gas pipeline 3, and the pressure sensor 4 measures the air pressure at this point of the gas pipeline.

[0076] The principle block diagram of the pipetting system is as Figure 1 shown. The operation of the plunger pump and the up and down movement of the pipette tip are controlled by the driver, and the air pressure in the gas pipeline is measured by the pressure sensor.

[0077] S2. Use the control mechanism to drive the pipette tip to move downward, and use the plunger pump to suck liquid in the liquid sample. At the same time, the pipetting system feeds back the air pressure value of the pressure sensor and calculates the depth of point A where the pipette tip is located. The calculation process is as follows:

[0078] In the pipetting system, the pressure balance formula at point A of the pipette tip is:

[0079] P1 + P2 = P0 + P3 (1)

[0080] Among them, P1 is the pressure of the liquid column in the pipette tube on point A, P2 is the gas pressure in the gas pipeline, P0 is the external atmospheric pressure, and P3 is the pressure of the liquid in the liquid sample on point A;

[0081] Since the gas pressure in the gas pipeline is the same everywhere after stabilization, the gas pressure formula in the gas pipeline can be expressed as:

[0082]

[0083] Among them, n, R, and T are the number of moles of gas, gas constant, and thermodynamic temperature in the gas pipeline respectively, V is the volume of gas in the gas pipeline, and P w is the pressure loss along the gas pipeline;

[0084] Due to the viscosity of the liquid, which will hinder the liquid from rising, when the plunger pump starts from rest to work, the liquid cannot fill the vacancy of the gas volume in the gas pipeline in time, resulting in an increase in V. Formula (2) is further deduced as follows:

[0085]

[0086] Among them, V0 is the volume of gas in the gas pipeline before the start of liquid absorption, and ΔV is the amplified volume generated due to liquid viscosity during the liquid absorption process. This volume is the difference between "the amplified volume V pr in the gas pipeline when the pump piston retracts" and "the volume V lr occupied by the rising liquid in the gas pipeline":

[0087] ΔV = V pr - V lr = Xt - Qt (4)

[0088] X is the operating speed of the pump set by the program, t is the system operating time, and Q is the flow rate of the liquid in the pipette per unit time. It can be simplified by Poiseuille's law as follows:

[0089]

[0090] r is the radius of the gas pipeline, ΔP is the pressure difference between the pressure in the gas pipeline after the liquid rises and the initial state before the system works, k is the liquid viscosity coefficient, h is the liquid rising height, and ρ l is the liquid density;

[0091] For the pressure loss P w along the gas path, from Darcy's formula:

[0092]

[0093] λ is the friction resistance coefficient between the air and the gas pipeline wall, R s is the hydraulic radius of the pipette, v is the gas flow velocity in the gas pipeline, and ρ g is the gas density in the gas pipeline, and L is the total length from the pipette tip to the pressure sensor;

[0094] From equations (3), (4), (5), and (6), the pressure change equation in the gas path during the liquid absorption process can be obtained:

[0095]

[0096] From the liquid pressure formula P = ρgh, it can be obtained:

[0097] P1 = ρ1gh1 (8)

[0098] P3 = ρ1gh2 (9)

[0099] Among them, ρ1 is the liquid density, h1 is the liquid column height in the pipette, h2 is the liquid surface height, and h1 can be obtained by the ratio of the liquid volume V lr in the pipette to the pipeline cross-sectional area S:

[0100]

[0101] From equations (8) to (10), we can obtain:

[0102]

[0103] Substituting equations (7), (9), and (11) into equation (1), we can obtain the relationship between the liquid level height and the pressure in the pipeline:

[0104]

[0105] S3. After the liquid suction process is completed, the liquid discharge starts. At the same time, the pressure sensor feeds back the air pressure value in the air pipeline during the liquid discharge process, and calculates the depth of point B where the pipette tip is located. The calculation process is as follows:

[0106] In the liquid transfer system, the pressure balance formula at point B of the pipette tip is:

[0107] P1 + P2 = P0 + P3 (13)

[0108] Among them, P1 is the pressure of the liquid column in the pipette on point B, P2 is the gas pressure in the gas pipeline, P0 is the external atmospheric pressure, and P3 is the pressure of the liquid in the liquid sample on point B;

[0109] Because the pressure is the same everywhere in the gas pipeline after the gas in the pipeline is stable, the gas pressure formula in the gas pipeline can be expressed as:

[0110]

[0111] Among them, n is the number of moles of gas in the gas pipeline, R is the gas constant, T is the thermodynamic temperature, V is the volume of gas in the gas pipeline, and P w is the pressure loss along the gas pipeline;

[0112] Due to the viscosity of the liquid, it will hinder the liquid from descending, so that when the plunger pump starts from rest to work, the liquid level cannot drop in time, which compresses the gas in the gas pipeline and reduces V. Further derivation of the formula:

[0113]

[0114] Among them, V0 is the volume of gas in the gas pipeline before the liquid suction starts, ΔV is the increased volume generated due to the viscosity of the liquid during the liquid suction process, and ΔV' is the difference between "the increased volume V of the gas in the gas pipeline when the pump piston retracts" and "the volume V of the gas pipeline occupied by the liquid fluid descending": pr " and "the volume V of the gas pipeline occupied by the liquid fluid descending lr ":

[0115] ΔV = V pr -V lr = Xt - Qt (16)

[0116] X is the operating speed of the plunger pump set by the program, Q is the flow rate of the liquid in the pipette per unit time, which can be simplified by Poiseuille's law. t is the system operation time:

[0117]

[0118] r is the radius of the gas pipeline, ΔP is the pressure difference between the state after the liquid rises and the initial state, k is the liquid viscosity coefficient, h' is the liquid rising height, ρ l is the liquid density;

[0119] For the pressure loss P along the gas path w , from Darcy's formula:

[0120]

[0121] λ is the friction resistance coefficient between the air and the gas pipeline wall, R s is the hydraulic radius of the pipeline, v is the gas flow velocity in the gas pipeline, ρ g is the gas density in the gas pipeline, L is the total length from the tip to the pressure sensor. In summary, the pressure change equation in the gas path during the liquid discharge process can be obtained:

[0122]

[0123] From the liquid pressure formula P = ρgh, it can be obtained:

[0124] P1 = ρ1gh1' (20)

[0125] P3 = ρ1gh'2(21)

[0126] Among them, ρ1 is the liquid density, h1' is the liquid column height in the pipette, h'2 is the liquid surface height, and h1' can be obtained by the difference between the liquid column height after suction in the pipe and the liquid discharge height:

[0127]

[0128] Thus, it can be obtained

[0129]

[0130] In summary, the relationship between the liquid surface height and the pressure in the pipeline can be obtained:

[0131]

[0132] The following two embodiments are carried out according to the above method steps.

[0133] Embodiment 1: Monitoring the liquid level depth of the throat swab sample

[0134] The pipette tips were placed 2 cm and 5 cm away from the liquid surface respectively for pipetting. After three pipetting cycles (one suction and one discharge is one cycle, and the program is set to 10 s per cycle), a curve of the data volume and time variation was plotted. As Figure 3 shown, the abscissa is the time corresponding to the AD sampling points, and the interval between each sampling point is 0.033 s; the ordinate is the analog voltage value of the pressure sensor, and the corresponding pressure value can be calculated according to the fixed parameters of the pressure sensor. Figure 3 In it, curve (1) is the data when the pipette tip is 5 cm away from the liquid surface, and curve (2) is the curve when the pipette tip is 2 cm away. After converting the data volume into an analog pressure quantity and then substituting the values in the curve into the formula for subtraction, the depth difference can be calculated to be 2.996 cm, which is 0.004 cm different from the designed situation.

[0135] Example 2: Monitoring the depth of the water sample liquid surface

[0136] The pipette tips were placed 2 cm and 5 cm away from the liquid surface respectively for pipetting. After three pipetting cycles (one suction and one discharge is one cycle, and the program is set to 10 s per cycle), a curve of the data volume and time variation was plotted. As Figure 4 shown, the abscissa is the time corresponding to the AD sampling points, and the interval between each sampling point is 0.033 s; the ordinate is the analog voltage value of the pressure sensor, and the corresponding pressure value can be calculated according to the fixed parameters of the pressure sensor. Figure 4 In it, curve (1) is the data when the pipette tip is 5 cm away from the liquid surface, and curve (2) is the curve when the pipette tip is 2 cm away. After converting the data volume into an analog pressure quantity and then substituting the values in the curve into the formula for subtraction, the depth difference can be calculated to be 2.997 cm, which is 0.003 cm different from the designed situation.

[0137] The present invention adopts a pneumatic liquid surface depth detection technology. Using this method, the pipetting error is reduced, the extraction of stratified samples is more accurate, the precision of liquid sample transfer is greatly improved, the error caused by the liquid level displacement due to suction and discharge is effectively reduced, and the reliability and accuracy of the detection results are improved. The present invention can be flexibly applied to many fields involving liquid sample transfer such as biology and medicine, and provides a fast, safe, efficient, reliable and convenient research means for relevant researchers, which can replace researchers to perform a large number of repetitive operations and complete the inspection efficiently, in large quantities and integrally. For example, this method can be applied to many biomedical detections such as nucleic acid detection.

[0138] The present invention can be flexibly applied to many fields involving liquid sample transfer such as biology and medicine, and provides a fast, safe, efficient, reliable and convenient research means for relevant researchers, which can replace researchers to perform a large number of repetitive operations and complete the inspection efficiently, in large quantities and integrally. For example, this method can be applied to many biomedical technology fields such as nucleic acid detection.

Claims

1. A pneumatic liquid level depth detection method, characterized in that, It includes the following steps: S1. Set up the liquid transfer system: Connect the pipette of the pipette tip to the plunger pump through a gas pipeline, set a pressure sensor on the gas pipeline, and set a control mechanism for driving the pipette tip to move vertically up and down; S2. Drive the pipette tip to move downward by using the control mechanism, suck liquid in the liquid sample by using the plunger pump, and at the same time the liquid transfer system feeds back the air pressure value of the pressure sensor, and calculate the depth of point A where the pipette tip is located. The calculation process is as follows: In the liquid transfer system, the pressure balance formula at point A of the pipette tip is: P1 + P2 = P0 + P3 (1) Wherein, P1 is the pressure of the liquid column in the pipette on point A, P2 is the gas pressure in the gas pipeline, P0 is the external atmospheric pressure, and P3 is the pressure of the liquid in the liquid sample on point A; Since the gas pressure in the gas pipeline is the same everywhere after stabilization, the gas pressure formula in the gas pipeline can be expressed as: Among them, n, R, and T are the number of moles of gas, the gas constant, and the thermodynamic temperature in the gas pipeline, respectively, V is the volume of gas in the gas pipeline, and P w is the pressure loss along the way of the gas in the gas pipeline; Because the viscosity of the liquid will hinder the rise of the liquid, when the plunger pump starts from rest to work, the liquid cannot fill the vacancy of the gas volume in the gas pipeline in time, resulting in an increase in V. Further derivation of formula (2): Wherein, V0 is the volume of the gas in the gas pipeline before the liquid absorption starts, and ΔV is the amplified volume generated due to the liquid viscosity during the liquid absorption process. This volume is the amplified volume V in the gas pipeline when the pump piston retracts pr minus the volume V occupied by the rising liquid fluid squeezing the gas pipeline lr difference: ΔV = V pr -V lr = Xt - Qt (4) X is the operating speed of the pump set by the program, t is the system operating time, Q is the flow rate of the liquid in the pipette per unit time, and can be simplified by the Poiseuille's law: r is the radius of the gas pipeline, ΔP is the pressure difference between the pressure of the gas pipeline after the liquid rises and the initial state before the system works, k is the liquid viscosity coefficient, h is the liquid rising height, and ρ l is the liquid density; For the pressure loss P of the gas along the way w , according to the Darcy formula: λ is the friction resistance coefficient between the air and the wall of the gas pipeline, R s is the hydraulic radius of the pipette, v is the gas flow velocity in the gas pipeline, ρ g is the gas density in the gas pipeline, and L is the total length from the pipette tip to the pressure sensor; From formulas (3), (4), (5), and (6), the pressure change equation in the gas path during the liquid suction process can be obtained: From the liquid pressure formula P = ρgh, it can be obtained: P1 = ρ1gh1 (8) P3 = ρ1gh2 (9) where ρ1 is the liquid density, h1 is the height of the liquid column in the pipette, h2 is the liquid surface height, and h1 can be obtained from the ratio S of the liquid volume V lr in the pipette to the cross-sectional area of the pipeline: From formula (8) and formula (10), it can be obtained: Substitute formulas (7), (9), and (11) into formula (1) to obtain the relationship between the liquid level height and the pressure in the pipeline: After the liquid suction process is completed, start to discharge the liquid. At the same time, the pressure sensor feeds back the air pressure value in the gas pipeline during the liquid discharge process, and calculate the depth of point B where the pipette tip is located. The calculation process is as follows: In the liquid transfer system, the pressure balance formula at point B of the pipette tip is: P1 + P2 = P0 + P3 (13) Wherein, P1 is the pressure of the liquid column in the pipette on point B, P2 is the gas pressure in the gas pipeline, P0 is the external atmospheric pressure, and P3 is the pressure of the liquid in the liquid sample on point B; Since the gas pressure in the pipeline is the same everywhere after stabilization, the gas pressure formula in the gas pipeline can be expressed as: where n is the number of moles of gas in the gas pipeline, R is the gas constant, T is the thermodynamic temperature, V is the volume of gas in the gas pipeline, and P w is the frictional pressure loss of the gas in the gas pipeline; Because the viscosity of the liquid will hinder the liquid from descending, when the plunger pump starts from rest to work, the liquid level cannot descend in time, resulting in the compression of the gas in the gas pipeline and a decrease in V. Further derivation of the formula: Wherein, V0 is the volume of the gas in the gas pipeline before the liquid absorption starts, ΔV is the amplified volume generated due to the liquid viscosity during the liquid absorption process, and ΔV' is the amplified volume V in the gas pipeline when the pump piston retracts pr minus the volume V of the liquid fluid descending and squeezing into the gas pipeline lr difference: ΔV' = V pr -V lr = Xt - Qt (16) X is the operating speed of the plunger pump set by the program, Q is the flow rate of the liquid in the pipette per unit time, and can be simplified by the Poiseuille's law, and t is the system operating time: r is the radius of the gas pipeline, ΔP is the pressure difference between the state after the liquid rises and the initial state, k is the liquid viscosity coefficient, h' is the liquid rising height, and ρ l is the liquid density; For the pressure loss P of the gas along the way w , according to the Darcy formula: λ is the friction resistance coefficient between the air and the wall of the gas pipeline, R s is the hydraulic radius of the pipeline, v is the gas flow velocity in the gas pipeline, ρ g is the gas density in the gas pipeline, and L is the total length from the gun head to the pressure sensor; in summary, the pressure change equation in the gas path during the liquid discharge process can be obtained as follows: From the liquid pressure formula P = ρgh, it can be obtained: P1 = ρ1gh1' (20) P3 = ρ1gh'2 (21) Where ρ1 is the liquid density, h1' is the height of the liquid column in the pipette, h'2 is the liquid level height, and h1' can be obtained by the difference between the liquid column height after liquid suction in the pipe and the liquid discharge height: From this, it can be obtained In summary, the relationship between the liquid level height and the pressure in the pipeline can be obtained:

2. The pneumatic liquid level depth detection method according to claim 1, characterized in that: The length of the gas pipeline between the pressure sensor and the pipette tip is 1 / 3 of the total length of the gas pipeline.

3. The pneumatic liquid level depth detection method according to claim 1, characterized in that: The control mechanism is a linear slide, and the linear slide includes a vertically arranged guide rail and a driver for driving the pipette tip to move on the guide rail.

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