Patent Number: 
Section: claims

1. Apparatus for monitoring pressure in a chamber, comprising, an ion source which produces an ion beam;  an ion collector which collects the ion beam to produce a beam current;  an ammeter for continuously; measuring the beam current;  a pressure gauge which measures the pressure of the chamber in a series of discrete measurements; and  a data processor which combines the measured beam current and the measured gauge pressure to continuously determine the pressure in the chamber. 2. The apparatus of  claim 1 , wherein the data processor combines the measured beam current and the measured gauge pressure by using the most recently measured gauge pressure as a base pressure and using the beam current to determine deviation from the base pressure. claim 1 3. The apparatus of  claim 1 , further comprising orthogonal acceleration means for deflecting the ion beam to perform time-of-flight mass spectrometry. claim 1 4. The apparatus of  claim 1 , wherein the series of discrete measurements are taken at a frequency not greater than about 100 Hz. claim 1 5. The apparatus of  claim 1 , wherein the series of discrete measurements are taken at a frequency not greater than about 1 Hz. claim 1 6. The apparatus of  claim 1 , wherein continuously monitoring beam current comprises measuring beam current at a frequency of at least 1 kHz. claim 1 7. The apparatus of  claim 1 , wherein continuously monitoring beam current comprises measuring beam current at a frequency of at least 10 kHz. claim 1 8. The apparatus of  claim 1 , wherein continuously monitoring beam current comprises measuring beam current at a frequency of at least 50 kHz. claim 1 9. The apparatus of  claim 1 , wherein the ion beam is a component of a mass spectrometer. claim 1 10. A method of monitoring pressure in a chamber having a pressure gauge, an ion source and an ion collector, the ion source producing an ion beam which is collected by the ion collector to produce a beam current, comprising, monitoring the pressure gauge to determine a discrete series of base chamber pressure measurements;  continuously monitoring the beam current; and  using the beam current and the base chamber pressure measurements to continuously determine a corrected chamber pressure measurement. 11. The method of  claim 10 , wherein the corrected chamber pressure measurement is determined by combining the measured beam current and the measured gauge pressure by using the beam current to determine deviation from the most recently measured base chamber pressure. claim 10 12. The method of  claim 10 , further comprising using the determined corrected chamber pressure measurement as input to an automated process control system. claim 10 13. The method of  claim 10 , wherein the series of base chamber pressure measurements are taken at a frequency not greater than about 100 Hz. claim 10 14. The method of  claim 10 , wherein the series of base chamber pressure measurements are taken at a frequency not greater than about 1 Hz. claim 10 15. The method of  claim 10 , wherein continuously monitoring beam current comprises measuring beam current at a frequency of at least 1 kHz. claim 10 16. The method of  claim 10 , wherein continuously monitoring beam current comprises measuring beam current at a frequency of at least 10 kHz. claim 10 17. The method of  claim 10 , wherein continuously monitoring beam current comprises measuring beam current at a frequency of at least 50 kHz. claim 10