Urine specific gravity test
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest (at 4 °C or °F); for gases, the reference is air at room temperature (20 °C or 68 °F). The term "relative density" is often preferred in. Calculating volume of a fluid: The fluid’s volume can be calculated using the specific gravity of the fluid and the weight. Conversely the weight can be calculated if the volume is known. Other applications of specific gravity include fluid mechanics, buoyancy and the brewing industry.
The temperature and pressure of both the material and water need to be the same as these factors influence the density and hence the specific gravity. Specific gravity is unique to every material and has a very wide range of application. In more general terms specific gravity is the ratio of the density of a material to that of any standard substance, although usually this is water at 4 degrees Celsius or By definition, whaf has a density of 1 kg per litre at this temperature.
The specific gravities of othdr usually are compared to dry air which generally has a density of 1. The specific gravity of all other materials is compared to water as a fraction heavier lighter or heavier density.
The reference density of water at 4 o C 39 o F is used as the reference as these are the conditions of maximum density. There is a wide range of instruments designed to measure the graviy gravity of a material. The hydrometer can be used to measure the specific gravity of any liquid.
The device is designed to float freely at the liquid surface with a protruding stem giving a reading corresponding to the whaat gravity of the liquid.
Other instruments to measure specific gravity are the Pycnometer, and digital density meters based on oscillating U-tubes. What is the nfl super bowl trophy made of Gravity has a wide range of applications including: Pharmaceuticals: The specific gravity is used to find out the purity of a drug since each of the constituents has a distinct specific gravity.
Determining the additives used in a base material: Specific gravity is used to find out the amount of additives used in a base material which might affect the performance and stability of the base material.
Urine Specific Gravity: The Urine Specific gravity USG is measured and used mostly in veterinary sciences to understand whether water is excreted or conserved in an appropriate fashion.
Conversely the weight can be calculated if the volume is known. Other applications of specific gravity include fluid mechanics, buoyancy and the brewing industry. Bookmark this page in your browser using Ctrl and d or using one of these services: opens in new window Pin Diigo Facebook Reddit Tweet What are these?
How to Prepare for the Test
Apr 02, · Urine osmolality is a more specific test for urine concentration. The urine specific gravity test is easier and more convenient, and is usually part of a routine urinalysis. The urine osmolality test may not be needed. The normal range for urine specific gravity is to Normal value ranges may vary slightly among different laboratories. Oct 22, · One uses a device called a hydrometer to measure the specific gravity of liquids. There are a variety that are used for specific purposes such as: the charge of a car battery, alcoholic content of water-alcohol mixtures, freezing point of radiator fluid in a car, etc. what does specific gravity mean the specific gravity of an object is the density of that object divided by the density of water the density of water is kilograms per meter cubed for instance the density of gold is kilograms per meter cubed so the specific gravity of gold is the density of ketchup is 1, kilograms per meter cubed so the specific gravity of ketchup is note.
Relative density , or specific gravity ,   is the ratio of the density mass of a unit volume of a substance to the density of a given reference material. The term "relative density" is often preferred in scientific usage. If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference.
If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then a substance with a relative density or specific gravity less than 1 will float in water.
For example, an ice cube, with a relative density of about 0. A substance with a relative density greater than 1 will sink. Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm Where it is not, it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry.
In British brewing practice, the specific gravity, as specified above, is multiplied by Relative density RD or specific gravity SG is a dimensionless quantity , as it is the ratio of either densities or weights. Relative density with respect to air can be obtained by.
Where M is the molar mass and the approximately equal sign is used because equality pertains only if 1 mol of the gas and 1 mol of air occupy the same volume at a given temperature and pressure i. Ideal behaviour is usually only seen at very low pressure. For example, one mol of an ideal gas occupies Those with SG greater than 1 are denser than water and will, disregarding surface tension effects, sink in it. Those with an SG less than 1 are less dense than water and will float on it.
In scientific work, the relationship of mass to volume is usually expressed directly in terms of the density mass per unit volume of the substance under study. It is in industry where specific gravity finds wide application, often for historical reasons. The apparent specific gravity is simply the ratio of the weights of equal volumes of sample and water in air:. The density of water varies with temperature and pressure as does the density of the sample.
So it is necessary to specify the temperatures and pressures at which the densities or weights were determined. It is nearly always the case that measurements are made at 1 nominal atmosphere But as specific gravity usually refers to highly incompressible aqueous solutions or other incompressible substances such as petroleum products , variations in density caused by pressure are usually neglected at least where apparent specific gravity is being measured.
For true in vacuo specific gravity calculations, air pressure must be considered see below. Here, temperature is being specified using the current ITS scale and the densities  used here and in the rest of this article are based on that scale.
As the principal use of specific gravity measurements in industry is determination of the concentrations of substances in aqueous solutions and as these are found in tables of SG versus concentration, it is extremely important that the analyst enter the table with the correct form of specific gravity.
In the sugar, soft drink, honey, fruit juice and related industries, sucrose concentration by weight is taken from a table prepared by A. Brix , which uses SG Given the specific gravity of a substance, its actual density can be calculated by rearranging the above formula:.
Occasionally a reference substance other than water is specified for example, air , in which case specific gravity means density relative to that reference. The density of substances varies with temperature and pressure so that it is necessary to specify the temperatures and pressures at which the densities or masses were determined.
It is nearly always the case that measurements are made at nominally 1 atmosphere For true in vacuo relative density calculations air pressure must be considered see below. Here temperature is being specified using the current ITS scale and the densities  used here and in the rest of this article are based on that scale. The temperatures of the two materials may be explicitly stated in the density symbols; for example:. Relative density can also help to quantify the buoyancy of a substance in a fluid or gas, or determine the density of an unknown substance from the known density of another.
Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones.
Water is preferred as the reference because measurements are then easy to carry out in the field see below for examples of measurement methods. As the principal use of relative density measurements in industry is determination of the concentrations of substances in aqueous solutions and these are found in tables of RD vs concentration it is extremely important that the analyst enter the table with the correct form of relative density.
In the sugar, soft drink, honey, fruit juice and related industries sucrose concentration by mass is taken from this work  which uses SG Relative density can be calculated directly by measuring the density of a sample and dividing it by the known density of the reference substance. The density of the sample is simply its mass divided by its volume. Although mass is easy to measure, the volume of an irregularly shaped sample can be more difficult to ascertain. One method is to put the sample in a water-filled graduated cylinder and read off how much water it displaces.
Alternatively the container can be filled to the brim, the sample immersed, and the volume of overflow measured. The surface tension of the water may keep a significant amount of water from overflowing, which is especially problematic for small samples. For this reason it is desirable to use a water container with as small a mouth as possible. When these densities are divided, references to the spring constant, gravity and cross-sectional area simply cancel, leaving.
Relative density is more easily and perhaps more accurately measured without measuring volume. Using a spring scale, the sample is weighed first in air and then in water. Relative density with respect to water can then be calculated using the following formula:. This technique cannot easily be used to measure relative densities less than one, because the sample will then float.
W water becomes a negative quantity, representing the force needed to keep the sample underwater. Another practical method uses three measurements. The sample is weighed dry. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the displaced water has overflowed and been removed.
Subtracting the last reading from the sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method allows the use of scales which cannot handle a suspended sample. A sample less dense than water can also be handled, but it has to be held down, and the error introduced by the fixing material must be considered.
The relative density of a liquid can be measured using a hydrometer. This consists of a bulb attached to a stalk of constant cross-sectional area, as shown in the adjacent diagram. First the hydrometer is floated in the reference liquid shown in light blue , and the displacement the level of the liquid on the stalk is marked blue line.
The reference could be any liquid, but in practice it is usually water. The hydrometer is then floated in a liquid of unknown density shown in green. In the example depicted, the hydrometer has dropped slightly in the green liquid; hence its density is lower than that of the reference liquid. It is, of course, necessary that the hydrometer floats in both liquids.
The application of simple physical principles allows the relative density of the unknown liquid to be calculated from the change in displacement. In practice the stalk of the hydrometer is pre-marked with graduations to facilitate this measurement. Since the floating hydrometer is in static equilibrium , the downward gravitational force acting upon it must exactly balance the upward buoyancy force.
The gravitational force acting on the hydrometer is simply its weight, mg. From the Archimedes buoyancy principle, the buoyancy force acting on the hydrometer is equal to the weight of liquid displaced. Setting these equal, we have. This equation allows the relative density to be calculated from the change in displacement, the known density of the reference liquid, and the known properties of the hydrometer.
A pycnometer is usually made of glass , with a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus.
This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury , using an analytical balance.
If the flask is weighed empty, full of water, and full of a liquid whose relative density is desired, the relative density of the liquid can easily be calculated.
The particle density of a powder, to which the usual method of weighing cannot be applied, can also be determined with a pycnometer. The powder is added to the pycnometer, which is then weighed, giving the weight of the powder sample. The pycnometer is then filled with a liquid of known density, in which the powder is completely insoluble. The weight of the displaced liquid can then be determined, and hence the relative density of the powder. A gas pycnometer , the gas-based manifestation of a pycnometer, compares the change in pressure caused by a measured change in a closed volume containing a reference usually a steel sphere of known volume with the change in pressure caused by the sample under the same conditions.
The difference in change of pressure represents the volume of the sample as compared to the reference sphere, and is usually used for solid particulates that may dissolve in the liquid medium of the pycnometer design described above, or for porous materials into which the liquid would not fully penetrate. When a pycnometer is filled to a specific, but not necessarily accurately known volume, V and is placed upon a balance, it will exert a force. The bottle is, of course, filled with air but as that air displaces an equal amount of air the weight of that air is canceled by the weight of the air displaced.
Now we fill the bottle with the reference fluid e. The force exerted on the pan of the balance becomes:. If we subtract the force measured on the empty bottle from this or tare the balance before making the water measurement we obtain.
The bottle is now emptied, thoroughly dried and refilled with the sample. The force, net of the empty bottle, is now:. The ratio of the sample and water forces is:.
This is called the Apparent Relative Density, denoted by subscript A, because it is what we would obtain if we took the ratio of net weighings in air from an analytical balance or used a hydrometer the stem displaces air.
Note that the result does not depend on the calibration of the balance. The only requirement on it is that it read linearly with force. Nor does RD A depend on the actual volume of the pycnometer.