The Oxalic acid standard which was developed is no longer commercially available. In the early 1980's, a group of 12 laboratories measured the ratios of the two standards.Another standard, Oxalic Acid II was prepared when stocks of HOx 1 began to dwindle. The ratio of the activity of Oxalic acid II to 1 is 1.29330.001 (the weighted mean) (Mann, 1983). There are other secondary radiocarbon standards, the most common is ANU (Australian National University) sucrose.D14C is calculated using: Figure 1: Decay curve for C14 showing the activity at one half-life (t/2).The terms "%Modern", or "pm C" and D14C are shown related in this diagram along with the Radiocarbon age in years BP (Before 1950 AD).You can get an idea of the relationship between C14 and age at the Carbon Dating calculator page. 1950 was chosen for no particular reason other than to honour the publication of the first radiocarbon dates calculated in December 1949 (Taylor, 19). Ninety-five percent of the activity of Oxalic Acid from the year 1950 is equal to the measured activity of the absolute radiocarbon standard which is 1890 wood.
One key to understanding how and why something happened is to pinpoint when it happened.
The ratio of the activity of sucrose with 0.95 Ox was first measured by Polach at 1.50070.0052 (Polach, 1976b:122).
Later inter-laboratory measurements put the ratio at 1.5081 (Currie and Polach, 1980).
In an earlier section we mentioned that the limit of the technique is about 55-60 000 years.
Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced.