THE PYCNOMETER AT 20 C ( 3 ) BY MULTIPLYING THE MASS OF WATER AT T C CONTAINED IN THE PYCNOMETER AT T C BY THE FACTOR F GIVEN IN TABLE I . THIS TABLE GIVES THE RECIPROCALS OF THE DENSITIES OF WATER AT T C CORRECTED FOR THE EXPANSION OF THE PYREX PYCNOMETER BETWEEN T AND 20 C .
THE KNOWLEDGE OF THIS VOLUME IS NECESSARY FOR THE MEASUREMENT OF THE DENSITIES OF LIQUIDS .
IF THE SPECIFIC GRAVITY IS TO BE CALCULATED , IT IS NECESSARY TO KNOW THE MASS OF THE WATER AT 20 C CONTAINED IN THE PYCNOMETER AT 20 C . THIS IS OBTAINED BY MULTIPLYING THE VOLUME AT 20 C BY 0 * 998203 , THE DENSITY OF WATER AT 20 C .
THE CALIBRATION OPERATION SHOULD BE CARRIED OUT IN TRIPLICATE TO CHECK THAT THERE IS NO ERROR IN THIS FUNDAMENTAL OPERATION ( 4 ) .
THE TARE OF THE EMPTY PYCNOMETER AND THE VOLUME AT 20 C ( AND CONSEQUENTLY THE MASS OF THE WATER AT 20 C ) ARE CHARACTERISTIC OF THE EQUIPMENT . BECAUSE A TARE OF THE SAME VOLUME AS THE FULL PYCNOMETER IS USED IT IS NOT NECESSARY TO DETERMINE THEM AT EACH MEASUREMENT . IT IS RECOMMENDED , HOWEVER , TO CHECK THEM PERIODICALLY , E.G . ANNUALLY .
( 1 ) THE CORRECTION FOR THE BUOYANCY OF THE AIR IS THUS MADE ONCE AND FOR ALL .
( 2 ) STRICTLY SPEAKING , IT WOULD BE NECESSARY TO NOTE THE TEMPERATURE OF THE PYCNOMETER AND THE ATMOSPHERIC PRESSURE AND TO CALCULATE THE DENSITY OF THE AIR UNDER THESE CONDITIONS . IN PRACTICE SUCH A CALCULATION WOULD BE NECESSARY ONLY WHEN WORKING AT AN ALTITUDE HIGHER THAN 1 400 M AND AT A TEMPERATURE ABOVE 30 C . A VARIATION OF 10 % IN THE DENSITY OF THE AIR PRODUCES AN ERROR OF 5 * 10-6 IN THE SPECIFIC GRAVITY OF A WINE DISTILLATE AND A MAXIMUM ERROR OF 0 * 005 IN THE ALCOHOLIC STRENGHT MEASURED ; THIS IS NEGLIGIBLE .
( 3 ) THE VOLUME SO CALCULATED IS NOT STRICTLY ACCURATE , AS THE BUOYANCY OF THE AIR ON THE WEIGHTS RECORDED HAS NOT BEEN TAKEN INTO ACCOUNT , THESE WEIGHTS GENERALLY NOT BEING CORRECTED FOR THIS BUOYANCY . HOWEVER IT IS PREFERABLE TO MAINTAIN THIS NON-CORRECTED , IF FICTITIOUS , VALUE SO AS TO AVOID INTRODUCING A CORRECTION OF THE SAME NATURE BUT INVERSE IN THE ACTUAL MEASUREMENTS . THE REAL VOLUME CAN BE CALCULATED BY MULTIPLYING THE FICTITIOUS VOLUME BY 0 * 999857 .
( 4 ) IT IS POSSIBLE TO AVOID ANY CALCULATION BY CALIBRATING THE PYCNOMETER FILLED WITH WATER AT 20 OR 15 C , THE TEMPERATURE READING BEING MADE BY MAGNIFYING GLASS TO THE NEAREST 0 * 02 C INCLUDING ANY CORRECTION FOR THE THERMOMETER SCALE .
TABLE I
F FACTORS
BY WHICH THE MASS OF THE WATER CONTAINED IN THE PYREX PYCNOMETER AT T C HAS TO BE MULTIPLIED