Frost-Point Measurement Error Due to a Leak in a Sampling Line

When measuring low dew-point temperatures, a leak from the sampling tubing to ambient may cause a significant error due to back-diffusion of water vapor. In the work reported in this article, the significance of this error was studied experimentally and theoretically. The effects of leaking VCR^® and Swagelok^® connectors were studied experimentally and analyzed by comparing the results to the outcomes of experiments with actual leak holes. Theoretical calculations with a simplified combined convection–diffusion model were used to evaluate the results. Calculations were successfully carried out to predict the minimum leak flow rate required to prevent any water vapor from back-diffusing into the tubing. It was concluded that, in many cases, maintaining gauge pressure in a sampling line prevents penetration of water vapor into the line. Tiny leaks, however, can be a route for harmful back-diffusing of water vapor. VCR connectors were shown to be more sensitive to leaks compared with Swagelok connectors.