It will be seen that we have two possible main factors of variation:
(a) Distance between partitions, causing visible differences in thickness of coagulum.
(b) Dry rubber content of latex, causing differences in the density (e.g., hardness or softness) of the coagulum.
The effect of variation in (a) will be clear. Even when latex of a standard of 11⁄2 lbs. per gallon is employed the resulting sheet may be either too thin or too thick.
Similarly, as already argued, the use of too low or too high a standard of dilution (when the factor of distance between partitions is not allowed to vary) is capable of causing much difficulty. While this is correct, broadly, it is found in the experience of some estates that their requirements are satisfied by a slightly lower standard than 11⁄2 lbs. per gallon. Thus it is not uncommon to note the adoption of a standard equivalent to 1 lb. 4 ozs. or 1 lb. 6 ozs. dry rubber per gallon. Experience dictates, however, that for the recognised standard measurements of modern tanks the practical limits of satisfactory density of latex lie between 11⁄4 lbs. and 11⁄2 lbs. per gallon.
Standardising Instruments.—For standardising latex by dilution all that is required is an instrument which will preserve a perpendicular position while floating in latex, will be sufficiently sensitive to indicate fairly small differences in density of latex, and has one mark on its aerial portion accurately indicating a density corresponding to the required standard. On scientific grounds it can be demonstrated that such an instrument as employed in common practice would not be strictly accurate.[6] It is not proposed, in this section of the book, to discuss such considerations.
[6] De Vries, “Archief voor de Rubbercultuur.”
Instruments of this nature are represented by the “Metrolac” (originating from the Rubber Growers’ Association) and other similar recorders. They generally consist of a submersible bulb with a projecting stem which is graduated. The “Metrolac” differs from others in that the bulb is of torpedo form (thus reducing “skin friction”), and the graduations on the stem indicate actual weight of dry rubber per gallon instead of the ordinary specific gravity figures.
Theoretical considerations to the contrary, it is found in actual practice in Malaya and Ceylon that, although such instruments are naturally delicate and require careful manipulation, they are of considerable practical value and satisfy a definite requirement. Until an instrument of greater accuracy and equal simplicity can be discovered all estates should regard the possession of a few “Metrolacs” as essential.
The nature of their construction and the average conditions under which they are used (and abused) make it impossible to rely upon their accuracy indefinitely or for any long period. It is always recommended, therefore, that there should be at least two instruments available, one of which may be in daily use, while the other is kept in safe custody and only employed, say, once a week for purposes of checking the accuracy or degree of inaccuracy of the other. This can be done with reasonable approximity by placing both instruments in a tall vessel containing well-mixed and diluted latex. Instruments showing a marked degree of inaccuracy should not be preserved; but in cases of necessity “Metrolacs” from estates belonging to company members of the Rubber Growers’ Association may be sent to the laboratories for repair and adjustment.[7]