The total pressure of pure tritium or the partial pressure of tritium or another radioactive isotope depending upon the calibration is measured utilizing a cylindrical positive electrode, an axial central thin negative electrode coaxial with the cylindrical electrode and a source having a voltage up to 200 V connected in circuits of these electrodes. The circuit also includes a picoampere meter or a high ohmic resistance whose voltage drop is monitored by a digital volt meter so that currents of the order of picoamperes in the circuit can be detected and with appropriate calibration of the unit can provide an output which represents the pressure.