The development of Flow Measuring Instrument
Flow refers to the quantity of fluid flowing through the effective section of the pipe in unit time. The quantity of fluid expressed in volume is called volume flow, and the unit is meter ³/ Hour, liter/hour, etc; The quantity of fluid expressed by mass is called mass flow, and the unit is ton/hour, kilogram/hour, etc.
As early as 1738, Swiss Daniel First Bernoulli used the differential pressure method to measure water flow based on the Bernoulli equation; Later, the Venturi tube was used to measure the flow in the Italian Humane Churi Research, and the research results were published in 1791; In 1886, American Herschel made a practical device for measuring water flow with a Venturi tube.
From the early to the middle of the 20th century, the original measurement principles gradually matured, and people began to explore new measurement principles. Since 1910, the United States has begun to develop a trough flowmeter to measure the water flow in open ditches. In 1922, Parshall reformed the original Churi flume into the Parshall flume.
From 1911 to 1912, Carmen, an American Hungarian, put forward the new theory of Carmen Vortex Street; In the 1930s, the method of measuring the flow rate of liquid and gas with sound waves appeared again, but no great progress was made until the Second World War. Until 1955, there was a Maxon flowmeter using the sound cycle method to measure the flow rate of aviation fuel. In 1945, Colin successfully measured the blood flow with an alternating magnetic field.
After the 1960s, measuring instruments began to develop in the direction of precision and miniaturization. For example, in order to improve the accuracy of differential pressure instruments, force balance differential pressure transmitters and capacitive differential pressure transmitters have emerged; In order to miniaturize the sensor of the electromagnetic flowmeter and improve the signal-to-noise ratio, the electromagnetic flowmeter with non-uniform magnetic field and low-frequency excitation has emerged. In addition, a practical Karman vortex flowmeter with a wide measuring range and no moving detection components was also introduced in the 1970s.
With the rapid development of integrated circuit technology, ultrasonic flowmeter with phase-locked loop technology has also been widely used. The wide application of microcomputers has further improved the capacity of flow measurement. For example, laser Doppler flowmeters can process more complex signals after being applied to microcomputers.
Flow can be measured indirectly by various physical phenomena, so there are many kinds of flow-measuring instruments. According to the measurement method, the flowmeter includes differential pressure type, variable area type, volumetric type, velocity type, and electromagnetic type.
Differential pressure flow
It is a kind of flow-measuring instrument widely used, accounting for about 70% of the total number of flow-measuring instruments. It consists of a throttling device and a differential pressure gauge. When the fluid-filled with a circular tube flows through a throttling element (such as an orifice plate), the flow beam forms a local contraction at the orifice plate. As the flow rate increases and the static pressure decreases, a differential pressure is generated in front of and behind the orifice plate. This differential pressure is proportional to the square of the flow rate.
Measuring instrument
There are strain, capacitance, and vibrating wire differential pressure transmitters, and double bellows differential pressure gauges. This kind of instrument is convenient for debugging and has been standardized. As long as the throttling device is matched with the differential pressure gauge, it can be used to measure the fluid flow. Double bellows differential pressure gauges are widely used in petroleum, chemical, metallurgy, power, and light industrial industries. When the instruments are used together with throttling devices, they can measure the flow of liquid, steam, and gas; When used together with the balancer, the liquid level can be measured; When used alone, it can measure the pressure and suction of liquid and gas in the form of indication, recording, alarm and accumulation. Welded bellows are used to ensure the consistency of effective areas of high and low-pressure bellows, eliminate static pressure error and improve measurement accuracy.
Main forms
It is a rotary (floating) flowmeter, which is composed of a tapered glass tube and a float. The float can move up and down in a vertically installed tapered glass tube. When the measured fluid flows from bottom to top through the annular gap between the pipe wall and the float, lift the float upward. At this time, the area of the annular gap between the pipe and the float increases, until the force formed by the pressure difference on both sides of the float is equal to the gravity of the float, and the float will be in a balanced position.
When the flow changes, the force formed by the pressure difference on both sides of the float will also change so that the float will be re-balanced at a new position. The floating height of the float is the reading of the flowmeter.
Turbine flow
It is composed of a sensor and display instrument, and the sensor is mainly composed of a magnetoelectric induction converter and turbine. When the fluid flows through the sensor, it first passes through the front flow guide and then drives the turbine made of ferromagnetic materials to rotate. The rotating turbine cuts the magnetic line of force of the magnetoelectric induction converter on the solid shell, and the magnetoresistance in the magnetic circuit changes periodically, thus inducing an alternating current signal.
Signal frequency
It is proportional to the volume flow of the fluid to be measured. The output signal of the sensor is amplified by the preamplifier and then transmitted to the display instrument for flow indication and integration. The turbine speed signal can also be detected by the photoelectric effect, Hall effect, and other converters.
Electromagnetic flow
It is composed of sensors, converters, and display instruments, and operates according to Faraday's electromagnetic induction law.
Karman Vortex Street
A non-streamlined cylinder (cylinder or triangular cylinder, etc.) is placed in the fluid, and a regular vortex will be generated alternately on both sides behind the cylinder within a certain Reynolds number range. According to the relationship between the distance between the vortices on both sides and the distance between the vortices on the same side, the vortex frequency can be measured to obtain the volume flow.
General flow
Volume flow is measured, and fluid density changes with temperature and pressure. Therefore, in the case of density change, the calculated volume flow is inaccurate for a specified working condition, while the mass flow is independent of temperature and pressure changes. Therefore, in some cases, it is necessary to use a mass flowmeter. Mass flow meters can be divided into the direct type and derivative type.New flow
In order to meet the special requirements of flow measurement, with the development of new technology, there are some new flow measurement instruments. For example, the Doppler laser anemometer can measure the change velocity of airflow, supersonic airflow and turbulence, and combustion flame, especially the velocity distribution; When pneumatic conveying various materials, it is necessary to measure the flow rate of gas-solid two-phase flow. Therefore, a correlation flowmeter without separate calibration is developed.
Other flow
ultrasonic flowmeter
The difference in the time between the transmitting end and the receiving end is measured by the difference in the speed of ultrasonic propagation in different flow velocities, so as to know the flow velocity of the fluid. The volume flow of the fluid can be obtained by multiplying the cross-sectional area of the pipe.
Turbine flowmeter
When the fluid flows through the turbine blade in the pipe, the turbine blade rotates. The rotating speed of the blade is proportional to the fluid flow. The fluid flow can be known by measuring the rotating speed.
Target flowmeter
The target flowmeter is a fluid resistance flowmeter. When the medium flows through the target in the pipeline, the target receives the force of the fluid. The force is proportional to the square of the fluid flow. The flow can be measured according to the force.
Measuring principle of electromagnetic flowmeter
B: Magnetic induction intensity V: average flow velocity of liquid in pipeline D: inner diameter of the pipeline
It can be seen from the above equation that the electromotive force is proportional to the volume flow rate flowing through the pipe. In practical work, because the induced electromotive force generated by the permanent magnetic field is DC, it can lead to electrode polarization or medium electrolysis, causing measurement errors, so the alternating magnetic field is often used in industrial instruments.
B=Bmax sin ω t E= Bmax sin ω t D V
The induced electromotive force obtained is also alternating. In this way, the polarization effect of the medium can be eliminated. At the same time, the measured signal is easy to be amplified, but the induction error is correspondingly increased.
In addition, in order to solve the solid flow measurement of cut tobacco, cement, and corn flour, an impulse flowmeter is developed; In order to solve the transportation of ore, paper, and coal into slurry after crushing, and the metering problem in the transportation of sewage treatment, dredging, and other sludge, there are wear-resistant linings and electromagnetic flowmeters with concentration compensation; In addition, insert a plug-in flowmeter made of small diameter turbine, vortex street, and electromagnetism into the large diameter to measure large flow, and the instrument is cheap, low-pressure loss and easy to maintain.
Measurement error
1. Definition of measurement error
The measurement error is the difference between the measurement result and the measured true value, referred to as error. Because the true value (also known as theoretical value) cannot be obtained accurately, in fact, the agreed true value is used. The agreed true value needs to be characterized by the measurement uncertainty, so the measurement error cannot be obtained accurately.
Measurement uncertainty: it indicates the dispersion of the measured value, which is related to people's understanding of the measured value. It is an interval obtained through analysis and evaluation.
Measurement error: refers to the difference value indicating that the measurement result deviates from the true value. It exists objectively but cannot be determined by people.
For example, the measurement result may be very close to the true value (that is, the error is very small), but due to insufficient understanding, the value given by people falls in a large area (that is, the measurement uncertainty is large); It is also possible that the actual measurement error is large, but the uncertainty given is small due to insufficient analysis and estimation. Therefore, various influencing factors should be fully considered when evaluating the measurement uncertainty, and the evaluation of uncertainty should be verified as necessary.