Page:Colorimetry104nime.djvu/45

and the color of the oil specimen is designated by a number defined from the disc used and the required depth of oil. Table 15 gives this definition [5] in terms of Saybolt number, Union color, u, land ASTM color, a.

In spite of the fact that the prism eyepiece brings the fields to be compared into juxtaposition and so facilitates the detection of differences in brightness, the change in luminous transmittance with thickness of these refined oils is so slight that it is much less easily detected than the corresponding chromaticity change. The settings of the Saybolt chromometer therefore probably depend essentially on nearest chromaticity match. If the oil sample is turbid, however, not even an approximate match can be obtained, and the method may be inapplicable. In these cases a thickness of the specimen yielding a chromaticity match is much darker than the standard, and no reliable setting of depth of sample yielding nearest match can be found.

Fig. 28.Chromaticities of the Saybolt color standards compared to that for Union No. 1 color, and to those of the yellowish wedge of the chromaticity-difference colorimeter (arrows).

Table 15. Saybolt numbers and corresponding Union color, u, and ASTM color, a, defined in terms of specimen depth and disk number

Number of disks n	Deptli of oil in	Saybolt number	Union color u	ASTM color a
½	20.00	+30	0.030	0.015
½	18.00	+29	.033	.017
½	16.00	+28	.037	.019
½	14.00	+27	.042	.022
½	12.00	+26	.049	.025
1	20.00	+25	.059	.030
1	18.00	+24	.066	.034
1	16.00	+23	.074	.038
1	14.00	+22	.084	.043
1	12.00	+21	.098	.051
1	10.75	+20	.110	.056
1	9.50	+19	.124	.064
1	8.25	+18	.143	.074
1	7.25	+17	.163	.084
1	6.25	+16	.189	.097
2	10.50	+15	.225	.116
2	9.75	+14	.243	.125
2	9.00	+13	.262	.135
2	8.25	+12	.286	.147
2	7.75	+11	.305	.157
2	7.25	+10	.326	.167
2	6.75	+ 9	.350	.180
2	6.50	+ 8	.364	.187
2	6.25	+ 7	.376	.194
2	6.00	+ 6	.393	.202
2	5.75	+ 5	.411	.211
2	5.50	+ 4	.430	.221
2	5.25	+ 3	.450	.231
2	5.00	+ 2	.474	.243
2	4.75	+ 1	.498	.256
2	4.50	0	.526	.270
2	4.25	−1	.557	.286
2	4.00	−2	.592	.304
2	3.75	−3	.631	.324
2	3.625	−4	.652	.335
2	3.50	−5	.677	.347
2	3.375	−6	.701	.360
2	3.25	−7	.797	.374
2	3.125	−8	.758	.388
2	3.00	−9	.789	.405
2	2.875	−10	.823	.422
2	2.75	−11	.860	.441
2	2.625	−12	.901	.462
2	2.50	−13	.947	.486
2	2.375	−14	.996	.511
2	2.25	−15	1.05	.539
2	2.125	−16	1.11	.671

Figure 28 shows a portion of the ($x,y$) chromatcity diagram on which have been plotted the rectangles corresponding to the chromaticity tolerances for Saybolt half disks and whole disks (see above). It will be noted that the tolerances are fairly wide so that variations of ±10 percent of the chromaticity change caused by introduction of the disk into the daylight beam are permissible. The relation between the Union color numbers, u, and the Saybolt color expressed in terms of number of whole disks, n, and depth, D, in inches,

$u={\frac {1.182n}{D}}$

may be derived by reference to figure 28. Two vectors, each corresponding to 300 wedge units define the steps from source C to Saybolt ½ disk to Saybolt whole disk, or 600 wedge units in all. The chromaticity point representing Union Color No. 1 does not fall exactly on the extended line. The intersection of the dotted line and the extended line gives the closest chromaticity match, and shows that the chromaticity of Union Color No. 1 approximates 1.10 Saybolt disk. An oil of Union Color No. 1 will therefore match 1.00 Saybolt disk in a thickness of 33.0/1.10 or 30.0 mm (1.182 in) and will match two whole disks in a thickness of 60.0mm (2.365 in).

The corresponding relation for ASTM color number, a, is