The ratios are consistent among species, and the slight (1-3%) differences can also be calculated from the ratio of C) decreases as the radiocarbon decays. Libby determined, one gram of pure carbon should produce about 14 (13.56) radioactive decays per minute.
The Beta-counting method detects the rate at which purified carbon decays. A rate of 7 decays/gram/minute would indicate an age of one half-life, or 5730 years old.
In fact, the natural production of radiocarbon has varied as well.
Before the industrial revolution, from 1800 - 1400 AD, the natural production of radiocarbon was high, so dates are "too young." From 1400 AD to 300 BC they are "too old," and prior to 300 BC , they are too young.
Since the carbon in these fuels was ancient, it contained no radiocarbon.
Therefore, prior to atmospheric bomb testing, the proportion of radiocarbon to C was relatively low, giving relatively old ages.
The production of radiocarbon has not varied wildly through time, but the changes produce consistent differences from calander ages.
In this study, we present a new validation for a published 14C dating method for ice cores.
Previously 14C-dated horizons of organic material from the Juvfonne ice patch in central southern Norway (61.676°N, 8.354°E) were used as reference dates for adjacent ice layers, which were 14C dated based on their particulate organic carbon (POC) fraction.
Therefore, radiocarbon dates are calculated to a "pre-bomb" age of 1950 A. Material which died after 1950 has such high amounts of radiocarbon its age is reported as "percent modern (1950)" (example 180% modern).
This bomb radiocarbon has been gradually removed from the atmosphere by by natural processes, but the "bomb spike" can be shown through the dating by means such as comparing the bottle date and radiocarbon age of wines.