Grist describes the length of yarn per unit mass of fiber and is usually expressed as meters per kilogram or yards per pound. It does not take into account yarn structure or type of fiber. Several very different yarns can have the same grist.
A short length of yarn requires a very accurate scale and the length measured must be a representative sample. (A strongly thick and thin yarn requires a longer length to produce an accurate average.) A common kitchen scale (with one gram or tenth of an ounce accuracy) is useful for a full bobbin or spindle skein but by the time a skein is finished, it is too late to do anything about how it was spun. The McMorran Yarn Balance, a specialized device, accurately measures a fixed mass so the grist can be determined from the length by a simple calculation. With a McMorran Balance, or a very accurate scale, one can estimate the average grist frequently for a more uniform yarn.
Why measure grist?
Not every project requires a grist measurement. It is up to the spinner to determine if it is meaningful, useful or practical.
Maximum 12 points
Examiner 2 noted that the McMorran Balance is not accurate for very thick or very thin yarns. I can see how it could be a problem with thick yarns, to balance a short bit on the device and then accurately measure the length. But I don't know how it is inaccurate for fine yarns. It does measure a very precise mass and the long length of fine yarn is easier to measure, I used to do it all the time. In addition, none of this is an inherent problem with the device itself but with how it is used. I did note that any method is only an estimate if it's only considering a short length of yarn.