Textiles are often sold on a weight basis and consequently it is natural to express the size of “thickness” of a yarn in terms of weight (or mass). There are two basic ways in which this may be done. These are: (a) by saying how much a given length of yarn weighs, or (b) by saying what length of yarn one would have in a given weight. Generally these are known as the direct and indirect systems of yarn numbering, respectively. In other words:
Direct yarn number = Weight / unit Length
Indirect yarn number = Length / unit weight
It will be noted that one is the inverse of the other. In the first case, the number gets larger as the yarn or strand gets coarser. In the second case, the number gets smaller as the yarn or strand gets coarser.
Each system has its advantages and disadvantages and each has found areas in which, by custom, it is used. It so happens that because long, thin strands are usually involved, the length figures are usually large and the weight figures are small. Consequently, the yarn numbers would get impossibly large or impossibly small unless special units are used. The following paragraphs will explain a selection of the most important sets of units used. A summary is listed in Table 1
- DIRECT SYSTEMS
The technical name used to describe the yarn size in the direct system is linear density,* and this is always expressed in terms of weight/unit length. In commerce, the technical name is used less frequently than in the fiber industry or the scientific community. Often the name of the particular unit, such as denier or tex is used instead. Sometimes the term “yarn number” is used, but this is confusing unless the system is quoted.
With the usual range of linear densities found in yarns, one pound of yarn if stretched out straight could extend up to 20 miles in length. Clearly if one were to express the linear density in lb/yd, the numbers would be impossibly small and cumbersome to use. Since scientists tend to use the metric system [grams (g) and meters (m) for weight and length, respectively] one could consider g/m as a unit, but in this case, it turns out that the number is too large to be handled conveniently.
In practice there are two major subsections, which refer to yarn. In one case, the logically minded scientists have chosen a length unit of 1000 meters, whereas the technologists have chosen 9000 meters. The reason for this latter choice is obscure.
The scientific subsystem uses the unit “tex,” where 1 tex is the weight in g of 1000 meters (1 km) of yarn and the number gets bigger as the yarn gets fatter. Thus, a 50 tex yarn weighs 50 g for every kilometer of yarn. The normal metric prefixes of kilo, deci, etc., can also be applied to the unit tex. Hence 1 decitex is 1/10 tex and 1 kilotex = 1000 tex.
The fiber industry tends to use the unit “denier” where 1 denier is the weight in g of 9000 meters of yarn. Thus a 450 denier yarn weighs 450 g for every 9000 meters. A moment’s reflection will show that the two examples given refer to the same yarn size. Denier is very often used to describe the size of the fiber; hence, a 1½ denier filament weighs 1½ g per 9000 m of that filament. In passing it might be noted that if the 450 denier yarn is made up of 1½ denier filaments, there will be 450/1½ = 300 filaments making up that yarn.
As will be shown later, there are intermediate products, such as sliver, to which the direct system of measurement can be applied. A sliver is a rope-like material and a typical linear density (or “sliver weight”) is 50 grains/yard. (A grain is 1/7000 lb. and care should be taken when handwriting the units to distinguish clearly between grain and gram.)
In this bulletin, the symbol “n” will be used to denote linear density, and in every case, the appropriate units should be quoted.
- INDIRECT SYSTEMS
It will be remembered that the indirect system is in terms of length per unit weight. Once again, there has to be special units, but there is a large variety of systems which are a legacy of the ancient crafts, and there is no discernible logic in the choice of units. Generally, all the subsystems in this category are referred to as yarn count or yarn number, and it is necessary to specify the subsystem if confusion is to be avoided. It is normal to specify the yarn count in hanks/lb where a hank contains a specified length of yarn.
Unfortunately, each of the subsystems specifies a different length.
In cotton processing technology (and those technologies which have evolved from that technology), the units developed in England during the industrial revolution are still used in the USA. In this case, a hank is specified as containing 840 yds* of yarn. Thus, if the count of a singles yarn is 20 hanks/lb. (usually written as 20s or 20/1), there will be 20 x 840 yards in a pound of yarn. The symbol used in this bulletin will be Ne where the subscript refers to “English” and distinguishes it from Nm, which refers to the metric count (meters/gram). In the case of long staple yarns, whose technology is derived from one of the processes for making yarn out of wool, a hank is often defined as 560 yds. of yarn, and in this case, the symbol NW will be used to describe count. There are many others, and a list of some specified hank lengths is given in Table 1.
With the indirect system, the number gets larger as the yarn gets finer. In the English cotton system, a 4s yarn is very coarse whereas 40s yarn is fine.
In normal practice, it is unnecessary to go through such a calculation each time a conversion is required, and generally a conversion factor can be used (see Table 2). In the case of converting from one direct subsystem to another, one merely multiplies the known linear density by the conversion factor to get it into different counts, similarly, when converting from one indirect subsystem to another. When converting from indirect to direct, or vice versa, then the factor must be divided by the known quantity.