RU2457461C1 - Method and apparatus for measuring density of liquid - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method is realised by making a float with elastic walls and air inside. The float is placed in the analysed liquid in a sealed vessel, after which pressure inside the vessel is gradually raised (reduced). At a certain pressure value, the float begins to sink (or emerge, if pressure is reduced) due to change in volume of air inside the float. Pressure at which the float will sink or emerge will vary with density of the liquid. The method also enables to avoid maintenance of a certain temperature when measuring density.

EFFECT: possibility of measuring density in small volumes of liquid owing to creation of excess pressure or pressure drop in a vessel with liquid and use of floats with variable volume.

9 cl, 2 dwg

Description

The invention relates to measuring equipment and can be used to measure the density of liquids in the oil, chemical, food industry and in other fields. The technical result of the invention is the ability to measure density with small amounts of liquid, as well as eliminating the need to withstand a certain temperature when measuring density.

A known hydrometer made in the form of a glass ampoule with a load and a scale (GOST 18481-81). To measure the density, the test fluid is poured into a vessel and a hydrometer is immersed in it, on a scale of which the density is determined.

Hydrometers have the following disadvantages:

- to measure density using a hydrometer, a suitable capacitance and a relatively large amount of liquid are required;

- when measuring the density of wetting liquids, the meniscus, making it difficult to read information, serves as an additional source of measurement error.

A known method for determining the surface tension and density of a liquid (RF patent No. 2323722, G01N 13/02, G01N 9/26, 07/10/2008), which includes supplying gas to the inlet of a gas supply tube immersed at a given depth in a controlled liquid, and measuring the maximum pressure in the tube. Additionally, the number of gas bubbles entering the liquid is measured, and the surface tension and density of the controlled liquid are judged by the maximum pressure in the tube and the number of gas bubbles.

The disadvantages of the method include the need for a pressure sensor and the difficulty of calculating the density, taking into account the influence on the readings of surface tension instruments.

The closest in technical essence and the achieved result is a method of measuring density (RF patent No. 2091756, G01N 9/26, G01N 7/18, 09/27/1997), in which they are placed in a test liquid in a U-shaped cell. A rarefaction is applied to one of the knees of the cell, achieving equality in the other (measuring) knee to the atmospheric. Then increase the pressure in the first elbow, keeping the gas pressure in the measuring elbow constant. The measuring elbow is hermetically connected to the upper end of the calibrated capillary, which is the pressure gauge of the micromanometer. Changing the volume of the gas phase in a calibrated capillary, measure the movement of the liquid level in the measuring elbow. Knowing the constants of the device, calculate the density value.

The disadvantage of the prototype is the bulkiness of the design.

The objective of the invention is the ability to measure the density in small volumes of liquid by creating excessive pressure or vacuum in the tank with liquid and the use of floats of variable volume.

This problem is solved by manufacturing a float with elastic walls and with air inside. The float is placed in the test liquid in a sealed container, after which the pressure in the vessel gradually increases (or decreases). At a certain pressure value, the float begins to sink (or float in the case of a decrease in pressure) due to a change in the volume of air that is inside the float. As the density of the fluid changes, the pressure at which the float sinks or floats will change.

Figure 1 shows the sequence for determining the density of a liquid by creating a vacuum. Figure 2 shows the sequence for determining the density of the liquid by creating high pressure.

The implementation of this method may be as follows:

Option 1. In a graduated sealed container 1 with a piston 2, a tube 3 and a filter 6, a float 4 is placed, the average density of which is greater than the density of the liquid under investigation (at atmospheric pressure, the float sinks in the liquid), figa. The end of the tube 3 is lowered into the auxiliary tank 5 with the test fluid. Then using the piston 2 create a vacuum (after that the piston is fixed to prevent its return to its original position).

According to the Boyle-Mariotte law, at a constant temperature and mass of an ideal gas, the product of its pressure and volume is constant, p · V = const. When creating a vacuum, the initial volume of air (V1) increases significantly, its pressure decreases:

p ₂ = p ₁ · V ₁ / V ₂ ,

where V ₁ - the volume of air in the tank to create a vacuum;

V ₂ - the volume of air in the tank after creating a vacuum;

p ₁ - air pressure in the tank to create a vacuum;

p ₂ - air pressure in the tank after creating a vacuum.

After creating a vacuum, the test liquid begins to flow through the tube from the dishes 5 into the container 1. The air in the float tends to expand, P _n ^_ > p ₂ , V _p -> V ₂ , but its expansion is limited by the walls of the float. The wall resistance to expansion depends on the material, shape of the float, wall thickness.

Due to the expansion of air, the volume of the float increases, its average density decreases, as a result of which the float floats, figb. As the tank is filled, the pressure gradually returns to atmospheric, the volume of the float decreases, and at a certain moment the float begins to sink, figv. The pressure of the tank at this moment can be calculated by the liquid level:

p ₃ = p ₁ · V ₁ / V ₃ ,

where V ₃ - the volume of air in the tank at the time of the start of the immersion of the float;

p ₃ - air pressure in the tank at the time of the beginning of the immersion of the float.

Knowing the dependence of the volume of the float on the pressure in the tank V _p = f (p) (obtained on the basis of experiments or by calculation), we can calculate the volume of the float, by volume and mass - the density of the float at the time of the start of the dive:

ρ = m _p / V _p ,

where V _p - the volume of the float at the time of the start of the dive;

m _p - mass of the float.

The density of the float at the start of the dive is equal to the density of the liquid.

A filter can be installed on the tube to determine the degree of contamination of the liquid and to measure the density of the purified liquid.

The degree of contamination of the liquid is determined as follows.

When contaminated liquid enters the tube, contaminants (for example, soot, wear particles, dust in engine oil) are deposited on the filter, and when there is a large amount of them, the rate of liquid intake decreases. The speed of filling the tank is inversely proportional to the cross-sectional area of the channel through which the fluid enters. Knowing the time of filling a fixed volume of the tank with pure liquid, and comparing it with the time of filling the same volume with contaminated liquid, it is possible to establish the fact of liquid contamination and assess the degree of contamination. If instead of a filter, a magnet is placed outside the tube, it is possible to evaluate the degree of contamination of the liquid with metal particles.

Option 2. In a calibrated sealed container 1 with a piston 2 and a tube 3, a certain amount of the studied liquid is collected (also leave a sufficient amount of air - up to half the volume of the container) and a float 4 is placed, the average density of which is less than the density of this liquid (at atmospheric pressure, the float floats in liquid), figa. Then using the piston 2 create excess pressure (after that the piston is fixed to prevent it from returning to its original position). The test liquid will begin to flow out of the tank 1 through the tube 3. Under the influence of excess pressure, the air inside the float is compressed, the volume of the float decreases, its average density increases, as a result of which the float sinks, fig.2b. As the fluid flows from the tank, the pressure gradually returns to atmospheric, the volume of the float increases, and at a certain moment the float begins to float, figv. Similarly to option 1, by the level of the liquid at the moment the float begins to emerge, one can judge the density of the investigated liquid.

A simpler option is to determine the density not by calculation, but on the basis of a preliminary calibration of the capacitance. Several experiments are conducted with liquids of known density, the levels at which the float begins to sink (or float) are noted on the tank indicating the appropriate liquid density. If necessary, intermediate density values are obtained by interpolation.

By changing the shape of the float, the material and the thickness of its walls, it is possible to achieve the desired range of changes in the volume (and therefore density) of the float. A float in the form of a flat circle, which takes the form of a ball when thin, with a thin wall of elastic material will have a large measurement range, because there is a significant change in its volume. Using a float, the volume of which varies slightly with pressure, you can determine the density in a smaller range, but with greater accuracy (to do this, reduce the volume of air inside the float, or increase the stiffness of the float body). To increase the density of the float, metal shot or wire can be placed inside it.

With increasing temperature, the density of petroleum products decreases. A float made of an elastic material with air inside also increases volume with increasing temperature (and therefore its density decreases). According to Charles's law, when a gas is heated by 1 ° C, its volume increases by 1/273 of the volume of this gas at 0 ° C. By changing the volume of air in the float and the stiffness of the walls of the float, it is possible to achieve that the float will change its density to the same extent as the density of the investigated fluid. This will allow you to get away from the need to measure density at a well-defined temperature (15 ° C or 20 ° C).

A series of experiments were conducted to determine the presence of gasoline in Nissan SAE 5W40, Toyota SAE 10W40, Lukoil SAE 15W40 oils by their density.

We took a container with a volume of 22 cm with a piston and a tube, placed in it a flat float weighing 0.0457 g with a small amount of air inside. The experiment was carried out at room temperature - 22 ° C.

Completely lowered the piston. The end of the tube was placed in Nissan SAE 5W40 oil with a density of 849 kg / m (at 22 ° C), then the piston was fully extended, the oil began to flow into the tank. The float increased in volume and began to float on the surface of the oil. When the oil reached the level of 19 cm ^{3, the} float began to sink (the volume of the float decreased, the average density of the float became equal to the density of the oil).

After that, a similar experiment was carried out at a temperature of 40 ° C. The float also began to sink when it reached 19 cm.

Then conducted similar experiments with a mixture of oil Nissan SAE 5W40 (97% wt.) And gasoline (3% wt.). Both at 22 ° C and at 40 ° C the float began to sink when the oil reached the level of 17 cm (instead of 19 cm, as for pure oil), which indicates a decrease in density. This was confirmed by measuring the density of the mixture at 22 ° C using a hydrometer: ρ _mixture = 847 kg / m ³ .

Thus, by changing the liquid level, it was determined that the oil density in the experiments with the second oil sample became lower, which indicates the presence of gasoline in the oil, and the temperature change did not affect the measurement results.

Claims (9)

1. A method of measuring the density of a liquid using a float, characterized in that the float, which changes its volume when the pressure changes, is placed in the test liquid in a sealed container, after which the pressure in the container is smoothly changed, and the density is determined by the pressure value at which the float changes his condition - pops up or drowns. 2. The method of measuring the density of a liquid according to claim 1, characterized in that a gradual increase in pressure is achieved as a result of creating a vacuum and the gradual flow of the investigated fluid into the container through the tube. 3. The method of measuring the density of a liquid according to claim 2, characterized in that the pressure in the vessel is determined by the level of the liquid under investigation. 4. The method of measuring the density of a liquid according to claim 3, characterized in that when measuring the density of a liquid that may be contaminated, a magnet and / or filter are installed in front of the measuring capacity to clean the liquid and determine the level of contamination, depending on the type of contamination of interest. 5. The method of measuring the density of a liquid according to claim 1, characterized in that a gradual decrease in pressure is achieved as a result of creating excess pressure and the gradual outflow of the test fluid from the tank through the tube. 6. The method of measuring the density of a liquid according to claim 5, characterized in that the pressure in the vessel is determined by the level of the liquid under investigation. 7. A device for measuring the density of a liquid, characterized in that it contains a sealed, graduated container with a piston and a tube, as well as a float that changes the average density when the pressure changes, which by selecting the size, shape, material, amount of air, wall thickness, filler is reduced to the required range of density measurement and the desired temperature range. 8. The device according to claim 7, characterized in that a filter is installed on the tube in front of the container to assess the degree of contamination of the liquid by changing the time it takes to fill the container with liquid. 9. The device according to claim 7, characterized in that a magnet is mounted on the tube in front of the container to assess the degree of contamination of the liquid with magnetized products by changing the time it takes to fill the container with liquid.

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