Abstract: The disclosed invention, a gasometer, measures the total dissolved gas pressure in a fluid in relation to the ambient atmospheric pressure, thus determining the relative saturation of dissolved gases in the liquid. In prior devices, bubbles have caused inaccurate readings, thus requiring that an operator be present during operation to manually clear bubbles. This gasometer is an improvement over previous devices in that it allows continuous and automatic monitoring of the dissolved gas pressures in the fluid by greatly reducing or eliminating the formation of bubbles in the apparatus. The formation of bubbles is prevented by selectively pressurizing the testing chamber using inlet and outlet valves at either end to selectively vary the pressure. The gasometer containing a pressurized chamber is inserted into the fluid transport system. Located in the interior of the chamber is semipermeable tubing which allows gas to pass through to its interior, but which excludes the fluid.