The Trouble with Measurements in Classical Formulas
An enthusiastic student of Zhang Zhongjing's formulas just asked me once again about the precise meaning of 兩 (liang), the basic unit of measurement. So I dug around yet again and yet again return to what continues to be my answer to this question:
First off, I remembered having done research on this issue for my doctoral dissertation, with the result that I had found a table of modern equivalents in an old Japanese-Chinese dictionary that listed the changing amount from one dynasty to another. But ultimately, for my dissertation, I had decided to simply not list modern equivalents because the formulas included by Sun Simiao could have come from any of the previous dynasties or been edited in the meantime, so there was no way of knowing what the meaning of 1 liang in a particular formula was. I did check my gynecology book and sure enough, it does have a table that lists 1 liang as equal to 3 g. But it also lists 1 zhu 銖 as 0.3 g, so I knew something was not right. The modern equivalents were added by my publisher for the sake of making the book clinically relevant and more useful to TCM practitioners. And modern Chinese conversions just don't work for classical literature.
The only facts we know for certain are as follows: According to the Shuo Wen Jie Zi, the standard suspects of other classical writings, and Sun Simiao's own statements in the first volume, 1 liang was equal to 24 zhu, and 16 liang were equal to 1 jin 斤. This remained constant throughout the classical age.
A little background may be helpful if you want to count millet grains: The measurement liang is based on the weight of the Huang bronze bell, a big ceremonial bell used in the Warring States courts to communicate with the spirits: 1 pitch pipe (龠 yue) has the capacity to hold 1,200 millet grains and weighs 12 zhu. Doubling that, you arrive at 1 liang. So the weight of 1 liang is equal the weight of two pichpipes for a Huang bronze bell. Aren't you glad you asked, at this point?
This is much is pretty solid. Now the problem is that Chinese measurements varied throughout time in their precise definition of what weight they referred to.
According to a table that lists the modern gram equivalents of liang from one dynasty to the next, which I found in an old Japanese dictionary of ancient Chinese (新字源), 1 liang was equal to 16 g in the Zhou to early Han periods (tenth c. BCE to ca. 1st c. CE), 13.92 g in the 1st. to third c CE, 41.76 in the 6th-7th c., and 37.3 in the 7th to early 20th c., and then dropped to 31.25 in modern China.
This gives you a range to work with. The problem with the Jingui and Shanghanlun is that these texts were revised and edited in each dynasty, and we don't have a manuscript from the Han dynasty, when they were first written. So any textual version we have is from a later date, and it's quite possible that the ingredient amounts were updated to reflect this change in the meaning of reference. We just don't know, would be my careful answer as a critical historian. So I avoid modern equivalents like the plague and just use the ancient measurements. In any case, I think the key aspect is the proportions between different ingredients, which the skillful physician must then adapt to the individual patient anyway.
Lothar von Falkenhausen wrote a fantastic book on them a long time ago that I studied and dearly loved in graduate school, if you want to read more on the subject.