Description of a System for Measuring Interzonal Air Flows Using Multiple Tracer Gasses
Mass transfer due to pressure-driven air flow is one of the most important processes for determining both environmental quality and energy requirements in buildings. Heat, moisture, and contaminants are all transported by air movement between indoors and outdoors as well as between different zones within a building. Measurement of these air flows are critical to understanding the performance of buildings.
Virtually all measurements of ventilation are made using the dilution of a tracer gas. The vast majority of such measurements have been made in a single zone, using a single tracer gas. In recent years, the need to detennine ventilation rates/air flows in multizone buildings has become more acute, and many researchers have been devoting effort to the problem. In some cases clever control schemes have been utilized to allow some inferences to be drawn from a single tracer gas but a full detennination of the air flows requires multiple tracer gasses. Those multiple tracer systems that have been developed tend to suffer limitations due either to response time, number of zones, or bias.
For the past several years LBL has been developing the MultiTracer Measurement System (MTM~ to provide full multizone air flow infonnation in an accurate, real-time manner. M1MS is based on a quadrapole mass spectrometer to provide high-speed concentration analysis of multiple tracer gasses in the (low) ppm level which are injected into multiple zones using mass flow controllers. The measurement and injection system is controlled by a PC and can measure all concentrations in all zones (and adjust the injected tracer flows) within two minutes and can operate unattended for weeks. (See diagram on reverse side.) It also can measure related quantities such as weather and zonal temperature to assist in the data interpretation. M1MS uses a real-time algorithm to estimate the air flows both to improve the control and to provide realtime infonnation to the user. A more detailed off-line analysis methodology is utilized to provide more accurate estimates of the air flows.
This report will describe the system and the data it has produced. M1MS has been in field operation since January of 1988 and has been used in two to five zone dwellings. Experimental conditions have ranged from steady driving forces to rapid changes, from poorly mixed zones to forced air heating systems, and from poorly coupled apartments to tightly coupled single-family houses. Since the response time of the analysis is well under an hour, occupant effects such as door openings between zones can easily be seen; since the post-processing response-time can be made quite short, even forced-air-fan cycling can be observed. In all cases the system has proven able to maintain control and provide data from which air flows can be calculated.