In the industrial field liquid level detection, there is a type of instrument that performs liquid level detection according to the buoyancy of the sensor by putting the sensor into the liquid to be tested. These instruments are collectively referred to as buoyancy liquid level detecting instruments. The buoyancy liquid level meter is further divided into constant buoyancy and variable buoyancy liquid level gauge according to whether the buoyancy of the sensor varies with the liquid level.
The buoyancy force of the constant buoyancy liquid level meter does not change with the height of the liquid during operation, that is, the sensor always floats above the liquid surface, and the height of the sensor is the height of the liquid. Such liquid level detecting instruments are more numerous, such as floating. Liquid level gauges such as discs, floats, and floats. The buoyancy force of the variable buoyancy liquid level meter changes with the height of the liquid. The sensor cannot float in the liquid. The height of the sensor cannot characterize the liquid level. Other conversion methods are needed to know the liquid. Height, such a liquid level detection instrument application is more than a float level gauge.
Construction and measurement principle of the float level gauge
The sensor of the float level gauge - the float does not float with the change of the liquid surface during the whole test. The reason for this phenomenon is that the float of the float level gauge is much more sinking than the float and the float. The buoyancy of the pontoon is not sufficient to float against its own gravity. Even if the liquid level is full, the float is completely immersed in the liquid, and the buoyancy of the float cannot overcome its own gravity, so the float level gauge is also called the sink cylinder level gauge.
Measuring principle
When the liquid to be tested has no liquid level, the buoy is not buoyant, and the connecting wire connected to the top of the buoy is in a tight state. The torsion bar is affected by the gravity of the buoy to generate a fixed torsional force. At this time, the buoy level gauge should output 4 mA. The current, ie the float level gauge, is zero.
When the liquid level of the liquid to be measured rises, the liquid enters the measuring cylinder, and as the liquid level rises, the volume of the discharged liquid increases. According to Archimedes' law, the buoyancy of an object is equal to the volumetric weight of the liquid discharged by the object. Since the float rod is a small-weight, heavy-duty metal tube, the weight of the liquid discharged from the float is much smaller than the weight of the float itself, so the connecting piece on the upper part of the float is still in a tight state, and the torsion rod is subjected to the gravity of the float minus the discharge of the float. The weight of the liquid (the buoyancy of the pontoon) produces a varying twist, at which point the float level gauge outputs a varying current greater than 4 mA.
When the liquid level of the measured liquid reaches zu high, the pontoon is completely immersed in the liquid to be tested. At this time, the volumetric weight of the liquid discharged from the pontoon is a fixed value. At this time, the torsion force of the torsion bar is the weight of the pontoon itself. The weight of the liquid discharged from the pontoon. At this time, the torsion bar is subjected to a fixed torsion force, and the current output from the pontoon level gauge is 20 mA, that is, the fullness of the pontoon level gauge.
According to Archimedes' law, the tension applied to the upper part of the pontoon during the entire level measurement
F= mg-ÏVg=G -Ï€d2/4ÏgH
In the formula, G is the weight of the float, d is the outer diameter of the float, Ï is the density of the liquid to be measured, and H is the height of the liquid to be tested.
The above formula is converted to:
H=4(GF)/Ï€d2Ï=K(GF)/Ï
Where K is a fixed constant 4/Ï€d2
It can be seen from the formula that the tensile force F of the steel wire is inversely proportional to the single value of the height H of the liquid to be measured when the liquid density Ï is constant. As long as the tensile force F of the upper wire of the pontoon is detected, the height of the liquid to be measured can be obtained.
Construction of the float level gauge
First, the classification of the float level gauge
The float level gauge is also divided into two types: the inner float and the outer float according to the position of the float. The inner and outer division refers to the place where the float is installed. In the container of the liquid to be tested, it is the inner float, and the liquid container to be tested. Outside is called the outer buoy.
The inner float level gauge itself does not have a measuring cylinder. The float is directly installed in a container containing liquid, and is mainly used in an environment lower than the ground such as a zero tank or an underground pool. The inner float level gauge is mounted vertically on top of these containers through the top flange.
The outer float level gauge itself has a measuring cylinder, and the measuring cylinder is connected to the side of the container of the liquid to be measured through the interface flange to form an equal liquid level connector. The buoy is suspended in the center of the measuring cylinder through the top torsion bar and the connecting wire. .
Second, the construction of the float level gauge
The pontoon level gauge is mainly composed of the following parts: pontoon, connecting piece (wire, fixed card), torsion bar, torque sensor transmitter and measuring cylinder (outer pontoon) and other body accessories.
The pontoon is welded to both ends of a thick metal tube, and one end of the interface ring is the upper end of the pontoon.
The connecting wire is a stainless steel strip with low toughness. One end is connected and fixed on the float ring, one end is fixed on the fixing card of the torsion bar, and the buoy and the torsion bar are connected, which is a force soft conductive component.
The torsion bar is a mechanical torsion arm that converts the tension generated by the interaction of the pontoon gravity and the liquid buoyancy on the connecting wire into an axial displacement about the axis.
The torque sensor transmitter detects and transforms the shaft displacement on the torsion bar arm, and then compares it with the internal program parameters of the transmitter to calculate the liquid level height at this time, and displays and outputs the standard current signal remote transmission. Achieve liquid level detection of liquids.
Flotation of the float level gauge
As a standard instrumentation, the float level gauge needs to be periodically calibrated to confirm the accuracy and linearity of the float level gauge; the float level gauge in the field operation is also repaired after the fault is repaired. Calibration makes it suitable for on-site use. The purpose of the two calibrations is different, and the actual use tends to focus on the latter. The calibration of the float level gauge has two methods: the standard method - the weight hanging method and the comparison method - the on-site water school method.
First, the hanging weight calibration method
According to the measurement principle of the float level gauge, the float level gauge measures the liquid level of the liquid according to the magnitude of the torque. Therefore, the calibration of the float level gauge should be based on the set torque to perform zero point and full scale calibration. This calibration method is called hanging weight calibration. Like the calibration process of the target flowmeter, the float level gauge also needs to pass the standard weight to simulate the force of the float, and adjust the zero and fullness of the float level gauge separately.
The weight calibration method has high calibration accuracy and good linearity, and can be adjusted within the full range. It is mainly used for periodic calibration of the performance test of the float level gauge itself. In addition, the internal float level gauge has its own calibration tube without calibration. Also use this method.
Weight calibrating the float level gauge, first calculate the wire pull force at the zero point and full stroke of the float level gauge. By formula: F=mg-Ï€d2/4ÏgH, the weight of the weight required for the liquid level is zero. It is the weight of the pontoon itself. The weight of the weight required at the full stroke is the value of F when H is the maximum value. At this time, the density Ï of the measured liquid is the density of the liquid under standard conditions.
When calibrating, remove the pontoon from the connecting wire, and use the lightweight plastic bag to put the weights required for zero and full stroke respectively. Observe the display of the transmitter to see the zero point, whether the full range is accurate, and carry out the corresponding Adjustment. Then calculate the weight of the weight required for the liquid level height of 25%, 50%, 75%, and then mount to check the linearity of the level gauge.
Second, the site water school law
The water calibration method is a general calibration method for the outer float level gauge, which is simulated by using the clean tap water that is the most readily available in life as the liquid to be tested. This calibration method can be quickly adjusted in the field without disassembling the level gauge. It is widely used in the field external float level gauge setting calibration, fault repair calibration and zero point full degree calibration.
To calibrate the float level timer with water, first modify the parameters in the transmitter, change the density value of the measured liquid to the density of water, and then obtain a new measurement range. Note that this measurement range cannot exceed the maximum acceptance of the transmitter. range. The water school method is a comparison calibration method. The density of clean tap water is considered as 1. The density Ï of the measured liquid is compared with the density of water, and three conditions can be obtained:
1. If the density of the liquid to be tested is greater than 1, the calibration value of the wire will not reach the value of the wire pulling force when the measuring cylinder is full of tap water even if the measuring cylinder is full of tap water. The maximum calibration value is within the real setting range. It is not fully calibrated.
2. If the density of the liquid to be tested is less than 1, when the water is filled with tap water, the tension of the wire exceeds the normal setting range. Especially when the density of the measured liquid is very small, the over-range calibration may damage the transmitter. At this point, you should check the parameter range requirements on the pontoon label, and perform calibration height conversion so that the water cannot fill the measuring cylinder and protect the transmitter from damage.
3. When the density of the liquid to be tested is very close to 1, the water is used to simulate the liquid to be tested for calibration effect.
On-site water school law operation steps
1, preparation work
First cut out the outer float level gauge, close the hand valve in front of the outer float, open the drain relief valve, drain the measured liquid in the measuring cylinder, unscrew the exhaust plug at the top of the measuring cylinder, and clean the tap water from the wire. The plug is filled into the measuring cylinder to fully clean the residual liquid and dirt in the measuring cylinder.
2, modify the measurement range
Modify the parameters in the transmitter, change the density of the measured liquid to 1, and check if the new measurement range exceeds the zu license.
3. Calibration of zero point.
After the measuring cylinder is cleaned, check if the zero point of the transmitter is 4 mA. If there is deviation, adjust the zero point.
4, full scale calibration
Close the drain valve at the bottom of the measuring cylinder, pour the tap water from the top plug of the measuring cylinder until it is full, and observe whether the display of the transmitter is full 20 mA. If there is deviation, adjust the full scale.
5, linearity view
Slightly open the drain relief valve under the measuring cylinder, let the tap water in the measuring cylinder flow slowly, check whether the transmitter shows linear decrease until 4 mA at zero point. If the zero point is not accurate, adjust again.
After all the water in the measuring cylinder has been drained, close the drain valve again, then slowly pour water from the plug to see if the transmitter's display increases linearly until it reaches 20 mA. If the fullness is not correct, adjust again.
Repeat several times to see if the change process of the float level gauge is linear, with or without death, hop count, and sudden change.
6, the end of the application
After the calibration is finished, change the density of the measured medium in the transmitter back to the original set value, screw the top of the measuring tube, close the bottom drain valve, and put it into the float level gauge. The calibration is over.
Float level gauge failure and elimination
The float level gauge is made based on the Archimedes principle. It can be obtained by the formula H=K(GF)/Ï. Only when the measured liquid level height H and the tensile force F on the wire are in a single value, can it be accurate. The resulting liquid level is obtained. In this case, G and Ï are a fixed value.
In actual use, the values ​​of G and Ï will change, resulting in the single value correspondence between H and F in the above measurement formula breaking, resulting in inaccurate measurement.
First, the impact of the change in the weight of the pontoon
G is the weight of the float rod. During use, the impurities in the liquid to be measured will gradually adsorb to the outer surface of the float rod, causing the overall weight G of the float to increase continuously, especially in the measurement of dirty medium, if the outer float level The measuring cylinder of the meter does not carry out maintenance and sewage discharge for a long time, and the appearance of the floating rod rod will adsorb more dirt, which causes the overall weight of the buoy cylinder to become larger, and the zero point of the float cylinder level gauge becomes higher.
Treatment method: Regularly draining and cleaning the medium of the measuring cylinder, zu is likely to eliminate the dirty impurities adsorbed on the outer surface of the pontoon, check the zero point for adjustment, and always ensure the torque and transmission detected by the transmitter when the liquid level is zero. The stress in the parameter settings is the same.
Second, the impact of changes in measured liquid density
Ï is the density of the liquid to be measured. Compared with the measurement zero offset caused by the adsorption of impurities on the surface of the pontoon, the measurement error caused by the density change of the measured liquid is more serious. From the formula, the liquid density Ï is a denominator value, which is carried out throughout the measurement. If the true density of the measured liquid deviates from the set density, the slope of the formula changes, causing the overall offset of the measuring point of the float level gauge. Setting the trajectory causes the pontoon level gauge to display inaccurately throughout. The change in the density Ï of the measured liquid is mainly caused by the following two points:
1, maintenance work is not in place
The external float level gauge is not maintained in time, causing the liquid in the measuring cylinder to deteriorate and precipitate impurities in a static state for a long time, causing the density of the liquid to be measured where the float rod is located to change, resulting in inaccurate measurement.
Treatment method: Discharge and replace the degraded liquid in the measuring cylinder of the external buoy, and let the fresh liquid fill the measuring cylinder.
2. The set density is inconsistent with the actual density.
The standard density set inside the transmitter is inconsistent with the density of the liquid in the actual production, which makes the measurement inaccurate. It can be divided into temporary inconsistency and intermittent inconsistency.
Temporary inconsistency mainly occurs during the driving of the device, due to the instability of production or the mutual conversion of various driving replacement materials (washing, oil transportation, material composition changes), the density of the liquid to be measured and the density of the normal operating level gauge. Inconsistent, but the float level gauge is not accurate. At this time, it can be processed according to the actual situation. If the fast liquid level requirement of the device is not strict, it can be processed without treatment. After the device is stable, the liquid in the measuring cylinder is replaced with a normal liquid and the display returns to normal. If the device is highly dependent on the float level gauge when driving, and the liquid level of the container to be tested is strict, it can be modified intermittently according to the actual material density in the container. After the device is normal, it will be changed back to the original density.
Intermittent inconsistency, if the materials running in the device will change intermittently, such as a set of lubricating oil equipment in our factory will randomly process different raw materials according to the market demand. At this time, the density of the medium set in the pontoon parameters is If the new media density is inconsistent, it needs to be converted to the density of the new material, and its liquid level display will be accurate.
Third, the transmitter failure
The probability of a transmitter failure is small. The most common fault is that the zero point of the transmitter drifts. At this time, the float level gauge should be zero-calibrated. In addition, the transmitter should be waterproof. Many transmitter faults on the site are caused by the failure of the transmitter to prevent the rain from entering the terminal of the transmitter, causing a short circuit, draining the ground or even burning the circuit board.
Fourth, antifreeze measures
The structure of the outer float level gauge determines that the liquid in the measuring cylinder is less exchanged with the medium in the process vessel, and the area exposed in the outer space is very fast. If the liquid to be tested has characteristics such as easy condensation and crystal water content, it is necessary to carry out heat tracing and heat preservation measures for the external float level gauge to prevent the liquid level in the cylinder from freezing when the temperature is low in winter, causing the liquid level meter to malfunction or even freeze and crack.
to sum up
The measuring principle of the float level gauge determines that the float can be made into a thick or even solid metal rod, so its pressure resistance is much higher than that of the float level gauge, and it is widely used in high pressure and interlock protection systems. At the same time, due to this feature, the range of tension of the connecting wire of the float level gauge is small, which results in poor anti-interference ability of the float level gauge. If the density of the measured liquid is unstable, the measurement error of the float level gauge will be large. The resistance to medium density fluctuation is not as good as the magnetic float level gauge, so it should be reasonably selected according to the actual situation on site.
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