Determination of specific gravity by lactometer

Introduction

  • A lactometer is a hydrometer especially made for the testing milk.
  • The instrument is based on the fact that when a solid body floats in a liquid it displaces an amount of liquid equal in weight to the weight of the floating body. 
  • Lactometer consists of a long narrow graduated glass stem which is enlarged below into a hollow glass bulb weighted with mercury and lead shots.
  • Numbers registered on the lactometer scale range from 10 or 15 to 40, which records Quevenne’s degrees. 

Apparatus


  • Lactometer
  • Cylinder
  • Thermometer
 








Procedure

  • The given sample of milk is thoroughly mixed and brought to a temperature between 10-21°C (50 and 70°F).
  • It is then poured into a high glass cylinder by its sides and the lactometer is slowly lowered into the fluid until it floats.
  • There it is allowed to remain in position for about half a minute.
  • The lactometer must not be allowed to touch the sides of the cylinder or its bottom.
  • When it becomes stationary one records:
  1. the scale reading
  2. temperature

  • If the temperature is above or below the standard, (15°C or 60°F) the lactometer reading is corrected according to the following rule:
  1. Add 0.1 to the lactometer reading, or 0.0001 to the specific gravity for every one degree Fahrenheit above 60°F.
  2. Subtract 0.1 from lactometer reading or 0.0001 from the specific gravity for every one degree Fahrenheit below 60°F.
  3. The specific the gravity of the sample of milk is then calculated by dividing the Quevenne’s degree by 1000 and adding one.
  4. For example, if the lactometer reads 31.9 at 63°F., the corrected lactometer reading becomes 31.9 plus 0.3, or 32.2, and by further calculation.
  5. Specific gravity equals 1+32.2/1000 or 1.0322.

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