As delivered from Finnigan MAT, the emission current of the Leybold-Heraeus model IM210 ionization vacuum gage on a model 251 isotope ratio mass spectrometer can turn on as soon as the ' PUMP ON ' switch is depressed, before a good vacuum in the mass spectrometer is attained. The filament in the source of the mass spectrometer may turn on at the same time, thereby shortening its life or burning it out if the vacuum is poor. This design flaw can be corrected by a simple modification of the electronic circuitry. (Author 's abstract)