Abstract: The apparatus determines the content by weight of a volatile fluid, such as water, in a sample under test where the volatile fluid is typically less than one-half percent by weight by heating the sample in a sealed chamber to evaporate the volatile fluid under test. The evaporated fluid is passed through a collector which senses and collects only the evaporated volatile fluid of interest which collection correspondingly increases the weight of the collector. Measuring the weight change of the collector and comparing it with the initial weight of the test sample provides a determination of the percent of the sensed volatile fluid in the test sample.