Sop for Operation and Calibration of Refractometer

This Document describes (Standard Operating procedure) Sop for Operation and Calibration of Refractometer

Sop for Operation and Calibration of Refractometer

I. Purpose & Scope:

  • The Purpose of this SOP is to lay down the procedure to lay down a procedure for Operation, Cleaning & Calibration of Refractometer.
  • This procedure is applicable for Operation, cleaning & Calibration of Refractometer used in Quality Control Department

II. Responsibilities:

  • All Quality Control personnel shall be responsible to follow and implement this SOP.

III : Introduction and Procedural Part :


  • Clean the Machine with a clean dry cloth every day. Occasionally wet cloth dipped in dilute soap solution may be used.
  • A precaution has to be taken to clean the instrument immediately with dry cloth to remove the moisture.


  • Remove the instrument from the wooden cabinet ,by unfastening the knob on the top of the instrument and remove all packing
  • To ensure safety of the reflector, move it away from underneath the prism box
  • To check liq. Sample, simply open the prism box by shifting the direction of the lock .Now place few drop of sample on the lower prism & simultaneously close the prism box so that a thin layer of sample is squeezed between the prisms.
  • Position the reflector should underneath the prism box. Using a table lamp expose light on the reflector. Now looking in the cross-wire eyepiece, adjust the reflector for better illumination.
  • After the sample is placed in the prism box or the test piece has been attached to the upper prism the refraction creates a shade in the cross-wire. The shade is brought in the center of cross- wire, by rotating the control knob in either direction
  • Rotate the dispersion scale in either direction to eliminate the colour bands visible on the edge of the shade, to attain sharp borderline.
  • Simply read the refractive index of the sample in the scale eyepiece.
  • After testing or calibration apply a small amount of ethanol to a cotton swab & gently clean the prism. Now using dry cotton wipe the prism. Make sure not to scratch the prism.


  • Ensure the cleanliness of instrument before doing calibration.
  • Temperature should be carefully adjusted and maintained since the refractive index varies significantly with temperature.
  • Place the instrument on a table where adequate light can be reflected through the mirror.
  • Put two to three drops of sample, close properly. Use following liquids for calibration.
Reference Liquid Standard  RI Tolerance
Distilled water at 25º C 1.3325 ± 0.0001
Carbon Tetrachloride at 20°C 1.4603 ± 0.0001
Toluene at 20°C 1.4969 ± 0.0001
α-Methylnaphthalene at 20°C 1.6176 ± 0.0001
  • Allow the liquid to smear on the lower prism surface, quickly close the prism.
  • Note down the reading from the scale with the help of the magnified lens of the instrument.
  • Record the observations in a format as per Annexure I – Record for calibration of Refractometer
  • If any discrepancy observed during operating the instrument report to QC Manager & notify the defect to Maintenance Manager to rectify the defect. Affix ‘Under Maintenance Label’ on the instrument.
  • Handle the out of calibration results as per specified SOP.
  • Maintain the instrument usage log book as per SOP Handling of instrument usage log book.
  • Calibration of refractometer activity should be record in instrument usage log book.

Calibration Frequency:

  • Once in three months and after each maintenance job.


Annexure I      :  Instrument Calibration Record





   Name of Instrument : Refractometer Make :
   Instrument ID No : Model :
Calibration Done on: Calibration Due on:

 A) Cleanliness of the Instrument:

   Observation: Clean / Not Clean

1) Any other maintenance:

Observation: Required / Not required

B) Refractive index of Reference liquid                                                            

Sr.No. Reference Liquid Observation Acceptance criteria
1 2 3 4 5 Mean
1 Water at 25°C 1.3324 to 1.3326
2 Carbon Tetrachloride  at 20°C 1.4602 to 1.4604
3 Tolune at 20°C 1.4968 to 1.4970
4 α-Methylnaphthalene  at 20°C 1.6175 to1.6177


Remark: The instrument is calibrated and Complies / does not comply with the acceptance criteria


Calibrated By:                                      Checked By:                                  Approved By:

 Date :                                                      Date :                                            Date :

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