PULSE is a pioneering approach to air-tightness testing which releases a low-pressure air pulse for realistic and accurate measurement of airtightness in seconds
The solution has been developed by a consortium of specialists and academics in the energy efficiency industry. In joining forces with The University of Nottingham as well as The National Energy Foundation, Absolute Air and Gas and Elmhurst Energy; BTS hopes to tackle the problems associated with current methods of air tightness testing.
We are pioneering the new low air pressure method to bring huge benefits to developers, testers, building product manufacturers and building occupants.
For more information on how PULSE can help you, contact us
Key Benefits of Pulse
Uncontrolled air leakage can increase a building’s heating demand by as much as 50%. It can reduce the efficiency of ventilation systems and adversely affect occupant health and comfort.
QUALITY ASSURANCE TESTING
PULSE is a low-skill test that site operatives can use regularly during construction. PULSE can:
• Track changes in airtightness throughout the build process, bringing contractor accountability
• Reduce the risk of non-compliance, defects and the cost of remedy
• Give indication of build quality
• Raise on-site awareness of airtightness
We are working with Government to bring PULSE into Part L1a of the Building Regulations, allowing for rapid, reliable, low-cost compliance testing
It is estimated that around 65% of English homes need energy efficiency improvements. Sometimes these improvements can cause their own problems with moisture control and ventilation. Testing the airtightness of existing homes can inform retrofit decisions and prove retrofit post-installation performance. PULSE’s low disruption, ease of use, and speed of testing makes it ideal for use in occupied homes.
In critical environments like refrigeration chambers, cleanrooms and laboratories, the quality of air must be kept to an incredibly high standard: Airtightness prevents contaminants entering or escaping unintendedly
PULSE offers bespoke systems for quick, low-skill and non-disruptive testing in these environments, improving efficiency and safety.
Pressure Pulse FAQ
|The purpose of testing airtightness is commonly perceived as a benchmarking comparison between buildings, however in reality the airtightness obtained is used to determine total ventilation rates for energy calculations in SAP (and other energy modelling tools), and to a lesser extent for indoor air quality. As such, determining the correct airtightness is important for more than simple benchmarking and if incorrect can lead to errors in SAP/energy assessments and also incorrect HVAC system design/setup.|
|The PULSE technique works by subjecting a building envelope or enclosure to a known volume change and measuring the pressure response. The known volume change is introduced by releasing compressed air into the test space for a period of 1.5 seconds from an air tank which generates a flow rate through any adventitious openings whilst in turn creating a pulse in the internal pressure. During this pulse, a period of quasi-steady flow is established that directly gives the leakage characteristic at a specified level of pressure (where in buildings, our advantage over a blower door fan is that PULSE can work at far lower pressures – 3 to 10 Pascal’s – which are more representative of typical atmospheric pressures. After adjustment to still air conditions and a small correction for the effective flow rate arising from the compressibility of air, the result can be plotted or read in the same way as the blower door fan technique.|
|Yes, the pulse of air is uniformly distributed very quickly. We have measured the internal pressure in the middle of buildings and also at multiple points throughout them (up to three storeys high) without noticeable differences. We have also done numerical analysis (using Fluent CFD, fluid dynamic simulation software) that shows the pressure across the test space equalises almost instantaneously and certainly by the time of the analysis period. This is due to the fact that the pulse spreads at the speed of sound (approx. 340 meters per second) and is a long wavelength infrasonic frequency. Just as with a conventional blower door test, the only requirement is that internal doors are kept open in order to ensure full and equal pressurisation.|
|No, the pulse does not generate a noticeable localised pressure but it does require a sufficient volume of air to be released into the enclosure in order to be detectable; sometimes necessitating multiple PULSE units in particularly large buildings. The pulse spreads at the speed of sound (approximately 340 meters per second) and is also a long wavelength infrasonic frequency that simply doesn’t get absorbed or slowed down within typical building environments in the same way shorter acoustic wavelengths do. The pressure transducer currently used for measurement within the PULSE device samples at 20Hz, giving us 30 readings in the 1.5 second pulse period (120 readings in the full 6 seconds of the test) and it is very clear in all of our data that the enclosure pressurises almost instantaneously, quickly and consistently reaching a quasi-steady state before then decaying. Although the pulse or air does not get weakened by the building form, the volume of air released is relative to the size of the enclosure and its airtightness. For equal pressure distribution within very large buildings we can therefore tether multiple units evenly throughout the building and release them simultaneously.|
|Q4, (the air leakage flow rate at 4 Pa), is obtained in the same way the blower door test obtains the Q50 (leakage rate at 50 Pa). A curve fit is made to the plot of leakage against pressure difference, and then the leakage corresponding to 4 Pa is calculated. This can be done using either the power law or quadratic equation, and in practice (with experimental data) is simply a case of flipping the axes to convert between them. While the quadratic equation has been shown both numerically and experimentally to provide a better model of low pressure leakage, in practice it doesn’t really matter that much which is used for fitting measured data as long as it isn’t extrapolated outside the measurement range.|
|In order to make a direct comparison at the same pressure level, either one or the other has to be extrapolated and significant uncertainties are introduced to the results as a consequence of this forced extrapolation (see comments below on using a single ‘n’ value).
In deciding whether it is best to extrapolate PULSE up or the blower door down, it is necessary to question what is the point of airtightness testing – essentially it is to try and infer the infiltration that happens under natural conditions, of which 4 Pa is considered a typical pressure difference. Therefore, a comparison at 4 Pa is the most relevant as the use of 50 Pa is simply a former compromise made as there was previously no technically accurate way of measuring it reliably.
However, if it is accepted that the blower door cannot give accurate results below 10 Pa (either directly or through extrapolation) then comparing them at 4 Pa could rightly be seen as biased towards PULSE. Therefore, perhaps the fairest comparison is to see which technique is more accurate at measuring a known opening added to the building envelope. Tests conducted throughout the development of the PULSE technique on this exact basis suggest that PULSE is able to give better agreement with the known opening than the blower door. Also, external literature suggests using the blower door with standard infiltration models routinely over-predicts the directly measured infiltration (using the tracer gas technique). We are continuing to conduct more extensive testing of this, using controlled openings and tracer gas, to further validate how this relates to different building types.
|We fully recognise how important a direct comparison is, particularly for demonstrating compliance with building codes where all existing conventions are geared toward the blower door and for compliance values to be cited at relatively high pressures (e.g. 50 Pa in the UK). Whilst we would ideally like the building industry to fully adopt an approach of accurately measuring infiltration at more natural pressures, we are also currently working on a ‘step PULSE’ concept where three pulses will be fired in succession to enable results to be spread across a wider pressure range. This will give us more confidence in extrapolation and allow PULSE to cite the air leakage rate at pressures beyond the current 3-10 Pa range.|
|Unfortunately not and it is incorrect to say ‘n’ is a building property and will be constant. The flow coefficient ‘n’ is a function of both the opening geometry and the flow regime through it. As such ‘n’ does actually change with flow rate (and therefore pressure difference). This isn’t apparent when fitting the power law to test results as it is a function of the measured points themselves, however if you extrapolate outside the measured data range it can be expected to incur significant uncertainty (either up or down) as the flow regime is unknown.|
|A high level of repeatability is a critical factor in demonstrating a high level of accuracy. We see excellent repeatability with a relative percentage difference from the mean (RPD) falling comfortably within +/- 5% for all tests. If required, the repeatability can be improved even further, simply by using a larger tank. The operative also will have a far lesser impact on repeatability, with tests conducted at the single push of a button. Unfortunately we are not aware of any studies that have looked specifically at the repeatability of the blower door test. The Zero Carbon Hub ‘Design vs As Built’ project reported upon in July 2014 did however conduct a ‘round robin’ experiment, sending 5 different companies to perform standard blower door air tests on six different plots split evenly across two development sites. Despite being conducted in close succession, the largest variation recorded was 66%. Unfortunately the study was unable to pin point the exact causes for such levels of variation but it illustrates that despite using calibrated equipment and being trained competent operatives, considerable margins for error remain and there is a strong argument for the way in which PULSE simplifies the test procedure.|
|In short, we have identified a period of quasi-steady flow between the peak of the pulse and the closing of the valve, by matching theoretical curves with experimental ones, and have shown that by using the same equipment configuration we can rely on a certain time range to always give quasi-steady flow. In practice this can actually be seen in a plot of the measured PULSE pressure against time, as a period where the driven pressure decay is relatively small (some change in pressure is actually desirable, in order to give a range of values).
The theoretical curves mentioned are obtained using a model based on the quadratic equation. An advantage of the quadratic relationship is you can represent the steady and unsteady elements of the flow separately, which is impossible with the power law as it assumes one flow type as representative of the total. During the analysis period the steady flow is typically around 99% of the total flow (hence quasi-steady) for openings up to approximately 3.5 m in length, at which point the inertia in the openings starts to increase the unsteady component.
|One advantage of the current fan based blower door air leakage testing technique is in its ability for pressurisation or depressurisation to be sustained so that a smoke pen and/or thermal imaging camera can be used to visualise air leakage paths. The PULSE cannot achieve the pressures required to undertake this type of diagnostics and therefore requires a small, cheap uncalibrated fan to be mounted in an open window in order to conduct the same test. The PULSE device will never be able to be used to pinpoint leakage paths but we are however continuing to undertake work to test how much information about the leakage characteristics may be inferred from the PULSE shape itself – similar in some ways to how the power law flow coefficient can indicate if there is a large unrestricted opening or lots of smaller, narrower gaps and cracks.
Our general view on the matter of leakage diagnostics is that despite a blower door fans capability to be used to trace leaks, such a test is often not conducted until the property or renovation works is completed and ready for handover. By which point it is often too late to do anything meaningful other than to use mastics and expanding foams which only offer a temporary fix. Our hope is that with the advent of PULSE, airt tightness testing can become far more commonplace, with PULSE tests conducted at regular intervals throughout the works phase by both skilled and unskilled operatives alike. In this digital age, test results for different house types at different construction phases would soon paint a very comprehensive picture as to what steps can be taken to either improve air tightness or reduce risks associated with under ventilation.
|The PULSE equipment configuration comprises of a rocking piston compressor, a composite air tank, a reference pressure tank, two solenoid valves that open and close the tanks, a nozzle, two pressure transducers and the control box with firmware and a user interface. The pressure transducers are the key pieces of instrumentation that will require annual calibration. However, as the PULSE unit is currently not yet a formally recognised means of demonstrating compliance with building codes we have yet to confirm the full protocol with regards to calibration/swap-out the transducers but it essence, the procedure would be very quick and simple. The remainder of the equipment is very low maintenance and would simply require servicing on a periodic basis. Some ask about building site dust clogging up the air release nozzle but the main function of this device is as a pneumatic silencer and the diffusion of air as part of the test process will keep the component sufficiently clear.|
|The time it takes to conduct an airtightness test can have a direct impact on the income that a tester might expect to earn and can equally be critical for customers in terms of the level of downtime or disruption caused.
Whilst it is often claimed anecdotally that the blower door test takes only 15 minutes, the UK blower door competent persons schemes cite 30-45 minutes, which is in agreement with what we have observed on numerous blower door tests witnessed. Performing the blower door test too quickly is highly likely to add considerable uncertainty (and arguably is evidenced by the poor repeatability found in blind ‘round robin’ tests).
If taking 30 minutes as a full blower door test; 10 minutes is used to measure and prepare the building, 5 minutes to set up the equipment, 10 minutes to conduct the test itself with the remaining 5 minutes used to pack-up. The PULSE test on the other-hand can be switched on to charge (5-10 minutes depending on tank size) whilst the tester measures up and prepares the building. A PULSE test can then be fired in a matter of seconds, with results instantly captured within the control box and the whole test procedure wrapped up and completed within 15 minutes, half the time of a blower door test. This time benefit is increased greatly as the complexity of a given testing situation grows.
Incidentally, there are also a number of things not included in the present UK technical standard that could be done to improve the certainty of blower door results, namely: take an average external pressure reading from all sides of the building; take repeated average readings at a given pressure with a zero reading taken before and after each pressure change; take the average of both pressurization and de-pressurisation results; when making a curve fit do so with greater weighting on higher pressure readings (otherwise errors in the lower pressure readings have a disproportionally large impact on the Q-P relationship.). Implementing many of these aspects would likely lead to the blower door test taking even longer than it currently does whilst the PULSE negates the need for all of this extra work whilst continuing to give accurate and repeatable results.