Specific gravity


The term specific gravity is used to describe the weight or density of a liquid compared to an equal volume of fresh water at 4°C (39° F). If the liquid you are comparing will float on this water it has a specific gravity of less than one (1). If it sinks into the fresh water the specific gravity is more than one. As you have already guessed fresh water at 4°C (39° F) has been assigned a value of one (1).

It is important that you do not confuse specific gravity with viscosity, which is a measurement of a fluid’s resistance to pouring. Thick liquids are said to have a high viscosity and thin liquids a low viscosity. Like specific gravity, viscosity can be altered by a change in temperature, but unlike specific gravity it can also be altered by agitation. The really important thing to keep in mind is that there is no correlation between viscosity and specific gravity.

We measure specific gravity with a hydrometer. The hydrometer is a glass cylinder with a rubber bulb on top and a float positioned inside the glass tube. The float is calibrated to float on fresh water at 4°C, so if the fluid you are testing has a higher specific gravity the float will raise in the liquid and at a lower specific gravity it will sink lower in the liquid.

This is the same instrument that we use to tell if your automobile battery is fully charged or if there is enough anti-freeze in the radiator.

OK! Now that we have all of that straight what are we going to do with this specific gravity information that we are collecting? It turns out that we use it for a lot of things. As an example:

  • It is part of the formula we use to convert pump head to pressure:

  • You will need it to calculate the hydraulic force acting on the impeller when the centrifugal pump is operating off the best efficiency point:
  • We need specific gravity to calculate the horsepower of the motor we need to operate the pump:
    • H-P needed = The HP noted on the manufacturers pump curve times the specific gravity of the fluid you are pumping.
  • NPSH and cavitation are directly related to specific gravity. The lower the specific gravity the lower the vaporization pressure.
  • Low specific gravity fluids cause a number of mechanical seal problems:
    • Low specific gravity fluid can vaporize in the stuffing box causing a big bubble and high heat at the seal faces.
    • If the specific gravity falls below 0.4 we have to change the spring load on the seal faces to a lower value, and change the hydraulic balance ratio from about 70/30 to 60/40.
    • Most low specific gravity fluids are poor lubricants that cause excessive seal face wear, slip stick vibration problems and possible color contamination of some products if carbon-graphite is used as the seal face.
    • The low specific gravity fluids can flash between the lapped seal faces. This will:
      • Cause chipping and damage to the carbon outside diameter.
      • Leave solids between the lapped faces, the last place you need them.
      • In some instances the flashing will cool the faces and freeze any oil or grease you put on the lapped face. The frozen lubricant can destroy the carbon.
  • If the fluid is a mixture of a liquid and a solid we have a useful formula to calculate the specific gravity of a slurry:
    • Sm = Specific gravity of the mixture or slurry
    • Sl = Specific gravity of the liquid phase
    • SS = Specific gravity of the solids phase
    • C= Concentration of the solids by weight
  • The formula for the concentration of solids by weight (Cw ) is:

Here is an example of a typical mixture:

You have a water and solids slurry. The water is a liquid with a Specific Gravity of 1.0 and the % solids by weight is 30%. The solids having a Specific Gravity of 2.7, then:

Oil refinery people sometimes mix oils of different specific gravity together. To determine the resultant gravity you can use the following formula:

  • D = Density or Specific Gravity of the mixture.
  • m = Proportion of oil of d1 density.
  • n = proportion of oil of d2 density.
  • d1 = Specific Gravity of “m” oil.
  • d2 = Specific Gravity of “n” oil.