A Dynamic Calibration Method for Pirani Gauges Embedded in Fluidic Networks

Abstract

This paper describes in situ dynamic calibration of microfabricated Pirani gauges, which are commonly used to measure vacuum levels in microsystems that may include pumps, valves, and other pressure modulation elements. Calibration is accomplished with a configuration where two sides of the supporting membrane of a Pirani gauge are at different pressures: an interior pressure within the microsystems and an exterior ambient pressure that can be directly controlled. This particular configuration is increasingly common as Pirani gauges are embedded in complex fluidic pathways such as those found in micro-gas chromatographs and multistage gas pumps. In the dynamic calibration the ambient pressure is rapidly modulated, while the interior pressure in the sense gap of the Pirani gauge remains relatively unchanged. The exterior pressure that is equal to the interior pressure is determined by a regression model. The dynamic calibration procedure and subsequent error analysis are illustrated by application to a 162-stage monolithic Knudsen vacuum pump. For this device, dynamic calibration improves the estimated upstream pressure from 30 (as suggested by uncorrected static calibration) to 0.9 Torr, with a 95.4% confidence interval from 0.7 to 1.1 Torr, assuming normal (Gaussian) distribution. These results demonstrate that dynamic calibration can be significantly more accurate than conventional static calibration for certain types of devices.