Dave,
If that's the case, you need a little cheat on the camera. While the scene is not a complete darkness (with 1 or 2 EV luminance is OK), you can manipulate by increasing the film's ISO/ASA setting to let the camera do the metering. For instance, when you have ASA100 film in the cartridge, you can manually overwrite the ISO/ASA setting to 800 (3 stops) or 1600 (4 stops) to get the exposure then change back the ISO to the original and manual expose by 2 empowered by the stop value in exposure time (with same aperture of course), which means if you get 1 second in ASA 800 setting, you need 8 seconds, plus compensation, for your ASA100 film exposure.
For any shutter time longer than 1 second, I believe, you need exposure compensation due to the failure of reciprocity. Usually it differs from film to film and from b&w to color film. The general rule is: +1/2 stop for 1 second, +1 stop for 10 seconds, +1.5 stops for 100 seconds; and the compensation for b&w should be doubled. Pls refer to the Kodak website for a search of the subject.
Last but not least, for safety you need bracketing the exposures for important shots. I hope that can be of help.
Rgds, David