Specific Gravity (S) (Relative Density): Definition, Formula, and Applications

Definition:

Specific Gravity is defined as the ratio of the weight density (or Density) of a fluid to the weight density (or density) of a standard fluid. For liquids, the standard fluid is taken water and for gases, the standard fluid is taken air. 

Specific gravity is also called Relative Density. 

                ·  For Liquids: The standard fluid used for comparison is water.

                ·  For Gases: The standard fluid used for comparison is air.

It is dimensionless quantity and is denoted by the symbol 'S'.

Material	Specific Gravity
Dry Air	0.0013
Alcohol	0.82
Carbon dioxide	0.00126
Cast Iron	7.20
Petrol	0.72
Rubber	0.96
Wood Oak	0.77
Nylon	1.12

 

Factors Affecting Specific Gravity:

Specific gravity is not always constant. It is heavily influenced by:

Temperature: Changes in temperature alter the volume of a fluid, which changes its density. Water reaches its maximum density at 4 °C.

Pressure: Particularly for gases, an increase in pressure increases density, thereby increasing the specific gravity relative to the standard.

        Standard temperature and pressure are used in the calculation of specific gravity. If failed to regulate the outside influences, then the specific gravity will change.

Applications of Specific Gravity:

Specific gravity has a wide range of applications, following are a few of the applications:

· Gemology: Professionals determine the gem purity by comparing its specific gravity with the already measured high purity level of another gem. This allows the gem’s value to be determined very quickly.

· Mineralogy: Specific gravity is used by mineralogists and geologists to determine the mineral content of the rock.

· Urinalysis: Urinary specific gravity is a measure of the concentration of solutes in the urine. It helps Doctors to evaluate kidney function and hydration levels. The test is a routine part of a urinalysis.

· Buoyancy and Stability: It tells us if an object will sink or float. Any substance with a specific gravity less than 1.0 will float on water.

· Pressure Calculations: It is used to calculate pressure head and manometric heights in hydraulic systems.

· Automotive: It is used to check the state of charge in lead-acid batteries by measuring the specific gravity of the electrolyte.

Frequently Asked Questions on Specific Gravity (FAQ)

Q1:  What does a Specific Gravity value tell us?

The specific gravity of a substance indicates whether a substance will sink or float in the reference fluid. For example, if an object's specific gravity is less than 1.0, it will float on water.

 

Q2: What is the difference between specific gravity and specific weight?

Specific weight (ω) is the ratio of the weight of an object to the object’s volume (ω = ρg) measured in N/m³. In the case of Specific Gravity (S), it is the ratio of an object’s density to the density of water.

 

Q3:  Which metal has the highest specific gravity?

Lead has the highest specific gravity.

 

Q4: Which metal has the lowest specific gravity?

Lithium has the lowest specific gravity.

 

Q5: Which instrument is used to measures the specific gravity of liquids?

A Hydrometer is used to measure the specific gravity of liquids.

Q6: Is specific gravity the same as mass density?

 No, mass density (ρ) is the mass per unit volume and has units like kg/m³.

 Specific gravity (S) is a dimensionless ratio and does not have any units.

 

Q7: What is a urine specific gravity test?

Urine specific gravity test compares the density of urine with the density of water. The test gives information about how well the kidneys concentrate the urine and a person’s hydration status.

 

Q8: Which substance is denser Mercury or Water?

 

Mercury is 13.6 times denser than Water. Since water's density is 1000 kg/m³, the density of mercury is 13,600 kg/m³.

 

Q9: What is specific gravity in a gemstone?

The specific gravity in a gemstone is the ratio of the density of the gemstone to the density of water.

 

Q10: At What temperature water has the highest density?

4 °C is the temperature at which water has the highest density.

 

Q11:  What is the specific gravity of Petrol & rubber?

The specific gravity of petrol is 0.72 kg/m³ and rubber is 0.96 kg/m³.

Q12: What standard fluids are used for comparison?

 For liquids, the standard fluid is always taken as water.

              For gases, the standard fluid is taken as air.

 

Q13: What is the main application of specific gravity?

The concept of specific gravity is utilized by geologists and mineralogists to analyse the mineral constituents of rocks.

 

 

 

 

 

TO DRAW A PERPENDICULAR TO A GIVEN LINE FROM A POINT OUTSIDE IT (AWAY FROM IT)

 (A) When the point is nearer the centre

(i) Let AB be the line and P be the point.

(ii) With centre P and any convenient radius draw an arc cutting AB at C and D.

(iii) Take any radius greater than half the length of CD in compass, and with centres C and D draw the arcs intersecting each other at E.

(iv) Draw a line connecting P and E, this line cuts the line AB at Q. 

(v) Then PQ is the required perpendicular to AB line.

(B) When the point is nearer to the end of line

(i) Let AB be the line and P be the point.

(ii) With centre A and radius equal to AP, draw an arc cutting AB at C.

(iii) with centre C and radius equal to CP, draw an arc cutting previously drawn arc at D.

(iv)  Draw a line joining P and D and intersecting AB at Q.

(v) then PQ is the required perpendicular.

 

TO DRAW PERPENDICULARS TO A LINE

  1) To draw a perpendicular to a given line from a point within it.

Method 1:- 

(A) When a point P is near the middle of the line.

(i) Let us consider AB be the given line and P the point on it.

(ii) with P as centre and any convenient radius R1 draw arcs cutting AB at point C and D.

(iii) with any radius R2 greater than R1 and centres C and D, draw arcs intersecting each other at O.

(iv) by using scale draw a line connecting points P and O. Then this is the required perpendicular to the given line.

 

Method 2:- 

(B) When a point P is near the end of the line.

(i) Let us consider AB be the given line and P the point on it.

(ii) Mark a point O, with O as centre OP as radius, draw an arc greater than the semi-circle cutting AB at C.

(iii) Draw a line joining C and O, and extend it up to arc to cut it at Q,

(iv) Draw the line joining P and Q. Then PQ is the required perpendicular to the given line.

 

Method 3:- 

(C) When a point P is near the end of the line.

(i) Let us consider AB be the given line and P the point on it.

(ii)  with P as centre and any convenient radius, draw an arc greater than the semi-circle cutting AB at C.

(iii) With the same radius cut the arc into two equal divisions CD and DE.

(iv)  With the same radius drawn an arc from D and cut this arc by drawing same radius arc from E.

(v) Mark this intersecting point as Q. 

(vi) Draw a line joining P and Q, This is the perpendicular to the line AB.

Representing scale in engineering drawing

          The proportion between the drawing and the object can be represented by two ways as follows: 

a) Scale:- 1cm = 1m (or) 1cm = 100cm (or) 1:100

b) Representative Fraction:- (RF) = 1/100 

"The ratio of the length of the object represented on drawing to the actual length of the object represented is called the Representative Fraction (i.e., RF)".

There are three types of scales depending upon the proportion it indicates as 

1. Full Scale: Some times the actual dimensions of the object will be adopted on the drawing then in that case it is represented by the scale and RF as 

Scale:- 1cm = 1cm (or) 1:1 and by R.F =1/1 (equal to one)

2. Reducing scale: When the dimensions on the drawing are smaller than the actual dimensions of the object. It is represented by the scale and RF as 

Scale:- 1cm = 100cm (or) 1:100 and by R.F=1/100 (Less than one)

3. Enlarging scale: In some cases when the objects are very small like inside parts of a wrist watch, the dimensions adopted on the drawing will be bigger than the actual dimensions of the objects then in that case it is represented by scale and RF as 

Scale:- 10cm = 1cm (or) 10:1 and by R.F =10/1 (Greater than one)

Note: The scale or R.F of a drawing is given usually below the drawing. If the scale adopted is common for all figures under the title of the sheet.

PRACTICAL HINTS ON DIMENSIONING

 1) Dimension lines should be drawn at least 8 mm away from the outlines and from each other.

2) Dimensions in a series may be placed in any one of the following two ways:

a) Continuous or Chain dimensioning:

Dimensions are arranged in straight line. An overall dimension is placed outside the small dimension. Least important dimensions is generally omitted.

b) Progressive or parallel dimensioning:

All dimensions are calculated from a common base line. Cumulative error is avoided by this method. This method is preferable compared to other.

3) Smaller dimensions should be placed nearer the view and the larger further away that so that further away so that extension lines do not cross dimension lines. Extension lines may cross each other or the outlines.

4) When a number of parallel dimension lines are to be shown near each other, the dimensions should be stagged.

5) Dimension should be placed where the shape is easily identified.

UNIT OF DIMENSION AND GENERAL RULE FOR DIMENSIONING

UNIT OF DIMENSION:

          As far as possible all dimensions should be given in millimeters, omitting the abbreviation mm. Even when it is not convenient to give dimension in millimeters and another unit is used, only the dimension figures are written. But a foot note such as "All dimensions are present in centimeters/meters" is inserted in a prominent place near the title block. The decimal point in a dimension should be quite distinct and written in line with dimension line. A zero must always precedes the decimal point when the dimension is less than unity.

GENERAL RULE FOR DIMENSIONING:

1) Every dimension of a part/segment must be given, but none should be given more than once.

2) Every dimension should be given so completely that further calculation or assumption of any dimension, or direct measurement from the drawing is not necessary.

3) A dimension should be placed on the view where it use is shown more clearly.

4) Dimension should be placed outside the views, unless they are clearer and more easily read inside.

5) Dimensioning between hidden lines and mutual crossing of dimension lines should be avoided. 

6) Dimension lines should not cross any other line of the drawing.

7) An outline or centre line should never be used as a dimension line. A centre line may be extended to serve as an extension line.

8) In general we use two types of dimensional systems (i.e, Unidirectional and aligned), in those Aligned dimensioning system is recommended.

 

 

   

LINES in Drawing

Various types of lines used in general engineering drawing are shown below  

Types of Lines:

Solid Lines:

          These are the most commonly used lines in Engineering Drawing. They represent the edges, surfaces, or Outlines of an object, Solid lines are typically used to depict the visible parts of an object.

Thin Lines:

          These are used for various purposes, such as dimension lines (showing the size of an object), Extension lines (extending a line on an object to aid in dimensioning), and leader lines (connecting a dimension number or note to a feature).

Dashed Lines:

Also known as hidden lines. Dashed lines are used to represent edges or surfaces that are not visible in a particular view. For example, they might show the edges of an object that are hidden behind other parts.

Dotted Lines:

          Often used for center lines or symmetry lines, dotted lines indicate the geometric centre of an object or a part. They can also be used to represent the path of movement for moving parts.

Phantom Lines:

          These are used to indicate alternate positions of a moving part or adjacent positions of related parts.

Centre Lines:

          Parts with holes and symmetrical features can be shown by using centre lines. Symmetry can reduce the number of dimensions in a drawing and make it more visually appealing, making it easier for the reader to comprehend.

Dimension Lines:

          Extension lines are used to describe the data being collected. Two arrow heads separate the extension lines on the dimension line from the measurement above (or inside, as shown in the image).

Cutting plane Lines:

          The cutting plane lines illustrate the path of the cutout in a cutout view. The A-A cutting line may be seen here bringing both types of holes into view.