The Envirotech APM 520 Flow Calibrator is designed to provide a reliable and portable flow standard for flow verification of environmental sampling systems. The APM 520 is a primary standard flow calibrator based on a glass body soap bubble meter. Photo-optics based sensing and quartz crystal clock is utilized to provide an automatic and accurate estimate of elapsed time for the soap bubble motion through a fixed volume. Each APM 520 calibrator is tested against positive displacement flow standards traceable to NIST or FCRI, maintained at Envirotech. Optionally third party certification of individual calibrators can be provided at an extra cost.