A mains electricity (AC-powered) instrument intended to be used for the quantitative in vitro measurement of a patient's circulating blood volume, plasma volume and/or red cell volume. Measurements are typically performed on blood specimens collected at specified intervals following the introduction of a radioactive tracer (e.g., iodine isotope 131) through an intravenous line, and the dispersion or rate of dilution of the tracer solution is used to determine blood volume by comparing sequential samples to a calculated ideal blood volume based on height, weight, gender and deviation from the patient's ideal weight. It operates with automation of some procedural steps.