In many production processes, the density of the media is measured to determine the normal course of the production process or to check the quality of the product. In some cases, the density is an important parameter for operational control; in other cases, if the mixed medium is two-dimensional, the density of the medium is measured, that is, the concentration of the medium can be determined indirectly.
Concentration is one of the main physical properties of the slurry. The concentration of the slurry affects the grinding efficiency, classification efficiency and beneficiation efficiency. The methods of expressing the concentration are:
(1) Weight concentration C G g
C=---------- ×100% .
G g +W g
(2) Volume concentration λ G v
λ=--------- × 100%.
G v +W v
(3) Pulp density Ï s Ï s =λ(Ïg+Ï)+Ï
Where G g , G v , Ï s - the weight, volume and density of the solid;
W g , W v , Ï - the weight, volume and density of the aqueous medium.
First, floating buoy density meter
The basic structure of this density meter is shown in Figure 1. The outer casing 2 is a cylinder, and the liquid to be measured enters the cylinder from the lower inlet pipe of the cylinder, and is discharged through the outlet pipe through the overflow port 4. The function of the overflow is to maintain a constant liquid level. The pontoon 3 is a cylinder with uneven thickness, and the thick portion of the lower portion is entirely immersed in the liquid to be tested, and the thin portion of the upper portion protrudes partially from the liquid surface, and the height of the protrusion is determined by the density of the liquid. As the liquid density increases, the buoy rises as the buoyancy increases. The upper end of the pontoon has a small rod 1 which extends out of the casing and is connected to the indicating mechanism. In order to maintain the stability of the pontoon position, the pontoon should have a good geometric symmetry to the central axis. This axis should be in a vertical position during installation. The pontoon design is calculated as follows:
Figure 1 Â Floating buoy density meter structure
1. small rod; 2. outer casing; 3. buoy; 4. overflow
Let the lower limit of the density of the measured medium be Ï and the upper limit be Ï 2 , then the average density
- - Let the pontoon itself have a weight of P. When the liquid density is Ï, the immersion volume is V. When the liquidity is Ï+ΔÏ, the immersion volume is V-ΔV. According to the Archimedes principle, there are:
Compare the above two formulas:
V. â–³Ï
â–³V=----------
Ï+â–³Ï
(1)[next]
The lower part of the pontoon has a diameter D and a height H; the upper part has a diameter d and the immersed part has a height h. When the density changes by ΔÏ, the immersion height change Δh is shown in Fig. 2. At this time there are:
1
△V=---- лd 2 . △h
4
(2)
Immersion volume:
1
V--- л(D 2 H+d 2 H)
4
(3)
The float rises:
D 2 â–³Ï
â–³h=(h+ ---H)=--------
d 2 Ï+â–³Ï
(4)
Visible from equation (4), increase The value increases the measurement sensitivity. in , The float rises with the amount of density change quantity Δh Î”Ï linear relationship.
The value increases the measurement sensitivity. in , The float rises with the amount of density change quantity Δh Î”Ï linear relationship. In the actual measurement, the factors that deviate from equation (4) are:
Figure 2 Â Floating height of the float
(1) Influence of liquid flow rate: When the flow rate is large, the impact force on the pontoon is also large. The direction of this force is from bottom to top, and the liquid flow rate should be kept constant during measurement.
(2) The small rod at the upper end of the pontoon is subjected to the reaction force given by the indicating mechanism, and its direction is from top to bottom.
(3) The density of the liquid varies with temperature. When the requirements are accurate, a temperature compensation device should be added.
The signal of the floating pontoon densitometer can be converted into an electrical signal. As shown in Fig. 3, the lower end of the pontoon is connected to a magnetic core, which is lifted and lowered together with the pontoon, and the magnetic core housing is sleeved with the primary and secondary coils of the differential transformer. [next]
Figure 3 Â Floating buoy density meter with electrical signal output
1. buoy; 2. differential transformer coil; 3. magnetic core; 4. power transformer; 5. differential transformer
Second, immersed buoy density meter
Immersed in a pontoon density meter, the entire volume of the pontoon is immersed in the liquid to be measured. When the density of the liquid changes, the buoyancy of the pontoon changes accordingly. This change is transmitted to the indicator or converted to other signals through a certain mechanical structure. . The sinking buoy density meter has two basic types: single cylinder and double cylinder.
Figure 4 shows the structure of a single immersion float type densitometer. The change in buoyancy received by the pontoon 8 is transmitted to the baffle 3, and the distance between the baffle and the nozzle 4 is changed due to the presence of the fulcrum 1. The feedback member 2 is a bellows. It is assumed that at a certain time, the density of the liquid in the measuring tank increases. At this time, the increased buoyancy felt by the pontoon reduces the distance between the nozzle and the baffle, and thus the distance between the nozzles increases, forming a negative feedback effect. The signal value is obtained by the change in air pressure at the nozzle back pressure.
Figure 4 Â Single immersion buoy density meter
1. fulcrum; 2. feedback component; 3. baffle; 4. nozzle; 5. constant air resistance;
6. Pneumatic amplifier; 7. Indicator; 8. Float; 9. Measuring cell
Figure 5 shows the double immersion float density meter structure. The standard liquid 4 has the same temperature expansion coefficient as the liquid to be tested. The inner cylinder 6 should have good thermal conductivity. The liquid to be tested is selected through the outer cylinder 3, and then enters the measuring outer cylinder 7, so that the standard liquid can be consistent with the temperature of the liquid to be measured to compensate for the influence of the temperature change on the measured value. The weight 1 is used to balance the unbalanced moment due to buoyancy changes. The following is a brief analysis of the system's balance relationship as shown in Figure 6.
Let the distance between the balance weight and the fulcrum be d, the weight is G, the distance from the suspension point of the two pontoons to the intermediate fulcrum is c, and the two pontoons have the same weight P and the same volume V, when the measured liquid and the standard When the liquid has the same density Ï 1 , the system balances at a symmetrical position (solid line position). When the measured liquid density is Ï x , the entire system is rotated through an angle θ to reach a new equilibrium state (dashed line position). The moment balance equation is:
[(P-Ï 1 V)-P-Ï xv ]lcosθ=Gdsinθ
From this:
(Ï x -Ï 1 )Ïl
Tgθ=-----------
Gd
(5)
It can be seen that reducing G and d, increasing l and V can improve the sensitivity of the instrument. The instrument indication is non-linear. [next]
Figure 5 Â Double immersion float density meter
1. balance weight; 2. pointer; 3. outer cylinder; 4. standard liquid;
5. reference buoy; 6. inner cylinder; 7. outer cylinder; 8. measuring buoy
Figure 6 Â System balance of double immersion float density meter
Third, the blown density meter
The blown gas density meter is widely used in the chemical production process, and has the advantages of simple structure and convenient operation. The physical principle is based on: different depths in the liquid, the difference in static pressure size is only determined by the depth difference and the density value of the liquid. The actual instrument used is to measure the static pressure difference of two places with a fixed depth difference. The principle and structure of this density meter are shown in Fig. 7 and Fig. 8.
After the compressed air is passed through the filter and the pressure regulating valve, it is divided into two paths, and the regulating valve is used to make the two flows equal. The reference gas path flows through the standard liquid and then vents, while the measurement gas path flows through the measured liquid. In addition, the gas pressures in the two gas paths are respectively approximated to the static pressure values ​​of the standard liquid and the corresponding depth of the measuring liquid (the insertion depth of the blowing pipe in the liquid).
Let the density of the standard solution be Ï 1 , the density of the measured annotation is Ï x , and the insertion depth of both blowing pipes is H, then the pressure difference between the two channels is:
ΔP=H(Ï x -Ï 1 )
Î”Ï has a linear relationship with Ï x . The air pressure difference ΔP is measured by a differential pressure gauge. In order to compensate for the influence of ambient temperature, the standard liquid should have the same temperature expansion coefficient as the measured liquid. If necessary, the head tube of the standard solution can be immersed in the liquid to be tested so that the temperatures of the two are the same; head pipe to be made with good thermal conductivity metal.
Figure 7 Â Blowing density meter schematic
1. Needle valve; 2. Filter; 3. Regulator valve; 4. Pressure gauge; 5. Flowmeter; 6. Standard liquid
7. measured liquid; 8. differential pressure gauge; 9. measuring gas path; 10. reference gas path
Figure 8 Â Blowing densitometer structure
1. differential pressure converter; 2. flow meter with adjusting screw; 3. constant difference device; 4. gas supply;
5. Injecting clean water; 6. Polyvinyl chloride dip tube
This type of densitometer measures high accuracy and low cost over a wide temperature range, but the medium (or slurry) needs to be well mixed during the measurement. [next]
Fourth, gravity density meter
The weight of a volume of liquid is determined by the density of the liquid. Therefore, if the weight is measured, the density value of the liquid is known. The structure of the spring gravity densitometer is shown in Figure 9. The inlet and outlet pipes of the liquid to be tested communicate with the coil spring 4, and the coil spring communicates with the measuring container 2. The liquid enters the measuring container 2 from the lower inlet through the coil spring, and is finally discharged from the upper coil spring and the outlet tube. The clarinet is also a sensitive component of the measurement that takes up the entire weight of the measuring container and liquid. When the weight changes due to a change in the density of the liquid, the secretor 2 moves up and down under the elasticity of the coil spring. The amount of movement is transmitted to the indicating recording mechanism via the ejector pin 3. In order to reduce the load on the reed pipe, water can be contained in the outer cylinder 1 (or other liquid having a density close to that of the liquid to be measured). At this time, the reduced weight is equal to the weight of the same volume of water as the container 2. In doing so, a thinner reed can be used to increase sensitivity.
Figure 9 Â Spring gravity densitometer
1. outer cylinder; 2. measuring container; 3. ejector; 4. coil spring; 5. pillar
The weight of the coil spring is:
P=V(Ï-Ï 1 )+G
(6)
Where V, G - the volume and weight of the container 2;
Ï 1 - the density of water;
Ï - the density of the liquid being measured.
Here we ignore the arm thickness of the container 2 and consider that the inside and outside volumes are equal. The measurement hysteresis of such instruments is large, and when there is a precipitate in the liquid, the reading error is increased.
Figure 10 Balance Weight Gravity Density Meter
1. outlet pipe; 2. hose; 3. inlet pipe; 4. shaft seat; 5. measuring tube;
7. balance bar; 8. tie rod; 9. support; 10. balance hammer; 11. baffle;
13. Pneumatic amplifier; 14. Pressure gauge; 15. Constant air resistance
Counterweight Gravity Density Meter The structure is shown in Figure 10. The liquid to be measured continuously passes through the U-shaped measuring tube 5. This measuring tube can be slightly rotated about the axis on the shaft seat 4. The inlet pipe is connected to the measuring pipe through the hose 2. When the density of the measured liquid changes, the measuring tube rotates around the shaft by a small angle under the action of gravity. The distance between the nozzle and the baffle is changed by the tie rod 8 and the scale rod 7, so that the pneumatic measuring system obtains a signal and is amplified for indication or recording. The counterweight 10 can be moved left and right to balance the zero position. To reduce the effects of ambient temperature, the entire measurement system is installed in a double-layered enclosure with a heating system inside to maintain a constant temperature. The structure of this instrument is cumbersome, but the measurement lag is smaller than that shown in Figure 10. It is commonly used to measure the density of lime milk. The balance weight gauge should be installed in a place where there is no vibration.
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