Low-frequency measurement of leakage in enclosures

Abstract

A wide variety of enclosed structures either require or cannot entirely prevent leakage from their interior space to the outside. Existing methods for measuring such leakage have important disadvantages. We have developed a device and technique that permits leakage areas to be measured from within or without the enclosure without causing unacceptable disturbance. The apparatus uses low-frequency (1 Hz) acoustic monopoles to generate an internal pressure signal which is then analyzed syncronously to provide a measurement of leakage area. We have successfully· applied this technique to measuring air tightness in residential houses, and believe it can be easily adapted for use in field, laboratory, or classroom applications. We are currently evaluating why the values we obtained were, on average, 14% lower than those obtained through conventional methods and we are investigating the apparent inability of the device, as presently designed, to measure large leaks.

AC pressurisation: Fourier analysis and the effect of compressibility of air

“…The present paper discusses Fourier analysis in greater detail. 4 Simplified model of the process A simplified model of the AC pressurisation process was formulated, and implemented as a simulation computer pro- air in leak represented as prism having cross sectional area equal to leak area, and finite length greater than or equal to zero -inside-outside pressure difference assumed to be algebraic sum of pressure differences required to overcome instantaneous friction loss and produce instantaneous acceleration of prism of air in leak. Earlier authors have reported that at low frequencies the pressure response is dominated by leakage through the envelope, and the effects of compressibility of the air and flexing of the envelope are small.…”

“…The method is known as AC pressurisation because the building envelope is subjected to an alternating pressure difference, or as the infrasonic method because it operates at infrasonic frequencies (of the order of 0.1 to 1 Hz). Card et al (1,2) and Sherman and Modera (3,4) have described the AC pressurisation method, using various equipment arrangements for producing and measuring the motion of the drive unit and measuring the air pressure variations, and also various methods of deriving the building's airtightness characteristics from the measured data. 2 Effect of frequency on pressure amplitude At low frequency (of the order of 0.1 Hz) the effects of compressibility of the air and flexing of the envelope are small, the flow rate through the envelope is approximately the same as the rate of change of volume of the drive unit, and the inside pressure fluctuation depends mainly on the air flow resistance of the envelope.…”

AC pressurisation: Analysis by non-linear optimisation

“…Thus the effective acoustic length/diameter for a leak of the type used in these experiments is about 0.7 greater than its geometrical length/diameter. 4.b Estimates of enclosure flexingK The estimated flexing coefficient of the test chamber was about 25 cm3 Pa-I from steady-state pressurisation measurements(l4), while the estimated compressibility of the air in the chamber was about 200 CM3Pa-I assuming isentropic compression of dry air. Thus the chamber was much stiffer than the air, and AC pressurisation could not be expected to yield an accurate estimate of the enclosure flexing coefficient.…”

“…Motion of the form (sin (a + k cos 2ca) was better, as it gave equal velocities in both directions and therefore little offset in mean inside pressure. 4 Laboratory experiments 4.1 General Experiments were performed with simple artificial leaks attached to a test chamber. The leaks comprised circular orifices in flat plates with diameters of 50, 80 and 100 mm, and straight 80 mm diameter tubes up to 400 mm long.…”

Single-Zone Stack-Dominated Infiltration Modeling

“…In addition to being measured from a fan pressurization test, the leakage parameters can be found from more advanced techniques (Sherman and Modera, 1986;Sherman and Modera, 1988).…”