CALIBRATION^ OF GERBER MILK BUTYROMETERS. 



By C. O. Williams, B.Sc, A.R.C.8., 

 Lecturer in Chemistry, School of Agriculture, Cedara, Natal. 



Read July 17, 1920. 



{Abstract.) 



The object of the investigation was to ascertain if the 

 centrifug-al method of calibrating butyrometers, given by Day 

 and Grimes in voL xlviii of the " Analyst," is sufficiently 

 accurate when making* use of t)rdinar\' commercial paraffin oil 

 and centrifuging' the butyrometers to the same degree as is 

 done when testing" milk samples. 



The butyrometers taken for this investigation were first 

 calibrated by the standaxd gravimetric method, using mercurv', 

 and the eiTor per unit graduation ascertained in each case. 

 The calibration was repeated by the centrifugal or paraffin 

 method, and the imit enor in each case obtained, as in the 

 previous method. 



It was noticed that the difference between the unit errors 

 obtained by these two methods was approximately a constant 

 amount (0017 per cent.) for each but\Tometer calibrated. 

 This constant difference is apparently due to the iucompleto 

 separation of the water and paraffin columns during the 

 process of centrifuging. Therefore, when adopting the 

 centrifugal method of calibration, it is necessary to multiply 

 this constant by the obsei-ved volume of the paraffin column 

 and subtract this pioduct from the observed total error. 



On testing a further lot of butyrometers by both methods 

 and apiilying the correction in the case of the paraffin method, 

 results Avere obtained that were very concordant with those 

 obtained by the standard mercurv^ method. 



Lastly, it is pointed out that the correction obtained in 

 this investigation Avould i)robably only apply M'hen using the 

 same brand of paraffin and centrifuging to the same degree, 

 so that each investigator should work out a similar method 

 and calculate the correction for himself. 



