The UIC 070 vapor pressure osmometer (VPO) is used to determine the number average molecular weight (Mn) of polymers or other non-volitile solutes in the molecular weight range of 100-25,000 Daltons (organic solvent, e.g. toluene) or 100-5,000 Daltons (aqueous). The VPO operates by measuring temperature changes with two matched thermistors in a solvent vapor filled chamber. An analyte solution and a pure solvent are injected onto the two thermistors, thereby altering the equilibrium in the chamber as the analyte sample pulls solvent vapor out of the chamber atmosphere. This results in a temperature change for the thermistor and a subsequent change in electrical resistance. This resistance change is compared to a calibration curve with known concentrations of the analyte to produce a concentration (osmolality) for the analyte.