Some Notes on the Baume Hydrometer

George H. Taber
1920 Journal of Industrial & Engineering Chemistry  
After the name crude oil and the names of some of its products, the first thing likely t o come t o the attention of anybody engaging in the petroleum business, or studying i t theoretically or practically, is a reference t o what is called the "Baume gravity" of the oils; b u t i t is a conservative assertion t h a t a very large majority of the users of the Baume hydrometer and its readings are under some one or more misapprehensions in regard t o the instrument, its principles or their
more » ... ples or their application. It is quite likely, as some readers may point out, t h a t the present writer shares in some of these misapprehensions, but, if so, they are self-inflicted and of long standing. Baume hydrometers are of the constant-weight, variable-volume type. As a n aid t o understanding what the Baume instrument is, and what it is not, it will be well first t o consider the constant-weight, variable-volume specific gravity hydrometer, which it is precisely like except in its stem divisions and numbers. The basic principle of all hydrometers is t h a t a floating body displaces an amount of liquid equal t o its own weight. As the weight of the hydrometer under consideration is constant, the same weight of all liquids is displaced by i t when floating freely in them, and the specific gravities of the liquids vary inversely as the volumes displaced, these volumes being indicated by marks on the stem of the hydrometer, accompanied by numbers giving the corresponding specific gravities. Since t h e numbers marked on the stem delimit equal increments of specific gravity, and the volumes immersed vary as the reciprocals of these numbers, the successive volumes will necessarily form a harmonic series, the differences between the successive terms of which will increase as the specific gravities decrease; t h a t is, the spaces between the divisions showing equal increments of specific gravity increase in width from the bottom t o the top of the stem. The same thing may be stated in another way: namely, if the successive specific gravities marked on the stem be considered as abscissas of a rectangular hyperbola, t h e constant k of whose equation equals the volume immersed when the hydrometer is floating in water, the successive total volumes immersed will correspond t o the ordinates of the hyperbola. It will be seen from the preceding paragraph t h a t if, reversing the usual practice, the stem of the specific gravity hydrometer were graduated with equispaced divisions t o show the volume immersed, the corresponding specific gravities would not decrease by equal decrements, b u t these decrements would decrease in amount from the bottom t o the top of the stem. Disregarding the principles upon which the graduation and markings of t h e stems of Baume hydrometers were originally Eased, and the object of their adoption, and without going into details as t o the differences in scales which have prevailed in the past-some of which still persist-it suffices t o state here t h a t Baume A N D E N G I N E E R I N G C H E M I S T R Y 593 scales are now based on mathematical formulas, and t h a t the formula which has the most authority in this country for liquids heavier t h a n water is and for liquids lighter t h a n water-145 being what is called the modulus of the former scale, and 140 t h a t of the latter. Examining the formula for liquids heavier t h a n water, we see t h a t Be.: = the reciprocal of the specific gravity multiplied by 145, and the product subtracted from 145; and t h a t the greater the specific gravity, the greater the equivalent Be., degrees. From the formula for liquids lighter t h a n water, we see t h a t Be.: = the reciprocal of the specific gravity multiplied by 140, and the product diminished by 140 less IO; and t h a t the greater the specific gravity, the smaller the equivalent Be., degrees. The deduction of I O from the modulus, which is in effect deducting the modulus and adding I O t o the result, shows t h a t water is 10' by this scale, instead of o o as by the heavy scale. Since, as previously stated, the specific gravity varies inversely as the volume of liquid displaced, if the stem of a specific gravity hydrometer were divided t o show equal increments of immersion, the corresponding specific gravity numbers would form a harmonic series; and since the equivalent Baume degree numbers are reciprocals of a harmonic series of specific gravities, modified only by being multiplied by a constant and added t o another constant, they form a n arithmetic progression, and if these Baume degree numbers were applied t o such a scale, we should have an equispaced scale with equicrescent numbers, or in fact a Baume scale,-heavy or light according t o the conversion formula used. The following are some frequently misunderstood points: I-The name is Baume, not Beaume. 2-Specific gravity numbers are ratios, not degrees. Baume numbers are degrees, not ratios. 3-The Baume scale is not inaccurate, and it is not unscientific. From its equispaced divisions, it is likely t o be more correctly divided in the making, the correctness of its graduations can be more easily checked, and almost anyone can interpolate its markings readily by the eye, whereas correct, rapid interpolation between specific gravity graduations is practically impossible. Of course, in significance, Baume degrees are inferior t o specific gravity ratios, and it requires one more step in calculation t o convert them into pounds per gallon or other absolute density standards; however, the Baume scale has become so fastened t o the petroleum business t h a t in all prob-
doi:10.1021/ie50126a028 fatcat:kok3xxvgofh2tcs3rsf3zmpq6e