To verify the accuracy of their cold atom vacuum standard (CAVS) for measuring ultralow vacuum pressures, NIST researchers built a high-performance version of a traditional pressure metrology setup known as a dynamic expansion system. In this system, they injected gas at a flow rate of roughly 10 billion to 100 billion molecules per second into the top chamber. The gas moved from the upper chamber to the lower chamber, which is evacuated by a large pump at a known rate through a precisely dimensioned orifice. A set of gauges measured the pressure ratio between the top and bottom chambers to correct for imperfections. Using the gas inflow rate and the rate that gas moves between the two chambers, the researchers calculated the pressure in the top chamber, which the CAVS independently measures. The researchers found agreement between this known pressure value and the readings from the CAVS sensors, thereby validating their new method. Credit: NIST
A vacuum chamber is never perfectly empty. A small number of atoms or molecules always remain, and measuring the tiny pressures they exert is critical. For instance, semiconductor manufacturers create microchips in vacuum chambers that must be almost entirely devoid of atomic and molecular contaminants, and so they need to monitor the gas pressure in the chamber to ensure that the contaminant levels are acceptably low. Now, scientists at the National Institute of Standards and Technology (NIST) have validated a new approach to measuring extremely low gas pressures called CAVS, for cold atom vacuum standard. They have established that their technique can serve as a “primary standard” — in other words, it can make intrinsically accurate measurements without first needing to be calibrated to reference pressure readings.
