Zero flow pressure differences are sometimes called static pressures, fan off pressures and baselines but they are basically the pressure that’s present in the building that is being created by the wind and to a lesser extent the stack effect. They are basically a measure of the base pressure present in the building before you turn the fan on.
Why do we record Zero Flow Pressure Differences?
The averages of the zero flow pressure differences apply an adjustment to your building pressure readings and either increase them, when the zero flow pressures are working against you, or reduce them, when the zero flow pressure differences are working with you.
What we mean by this is that if you are doing a depressurisation test and your average zero flow pressures are a negative number then the zero flow pressure differences are assisting you in the depressurisation of the building and as such your fan is inducing a lower pressure in the building than the number you are reading on your manometer. For example, with a building pressure of -50Pa and a zero flow pressure difference average of -2Pa your fan inducing a pressure in the building of -48Pa.
If you are depressurising the building and your zero flow pressure difference average is a positive number then the building is being pressurised before you turn on the fan so it has to work that little bit harder to achieve a building pressure. For example, with a building pressure of -50Pa and a zero flow pressure average of 2Pa your fan is inducing a pressure in the building of -52Pa.
How many readings do you need to take?
Prior to the release of ISO 9972:2015 the minimum requirement for zero flow pressure was to take zero flow pressure difference readings over a period of at least 30 seconds which came from BS EN 13829:2001. It was not specified how many zero flow pressure readings needed to be recorded within this test standard. With the new test standard in 2015, which superseded BS EN 13829:2001, this led to the older test standard being withdrawn.
ISO 9972:2015 requires that a minimum of 10 zero flow pressures are required to be recorded both before and after the test and that these readings are required to be recorded over a total duration of at least 30 seconds. Just to confirm that’s all 10 readings over a total duration of a least 30 seconds, so roughly 3 seconds each to get them done quickly, rather than 30 seconds per reading.
With BS EN 13829:2001 being withdrawn, which the ATTMA test standards were based upon and the introduction of the new test standard ISO 9972:2015, ATTMA TSL1:2010 was updated with the changes and released for use in 2016. ATTMA TSL1:2016 therefore includes the requirement to take 10 zero flow pressure difference readings both before and after the test. This has been carried over to the most recent TSL1 document which can be found here
The reason for taking numerous readings is that it allows you to capture all the peaks and troughs as the zero flow pressure differences change rather than having an average which may smooth out the peaks and troughs as you take them over an extended period.
Taking fewer readings can mask that a test has zero flow pressure averages outside of the acceptable range and that a test is invalid. As a tester you will experience difficulties taking readings under adverse weather conditions and by taking 10 zero flow pressure difference readings you will show this on your test due to the fluctuating values rather than a smoothed out average.
What is the acceptable range for zero flow pressure readings?
The simple answer is there is no acceptable range for zero flow pressure readings specified in the test standard. The reason for this is that the range of acceptability is relative to the averages rather than the individual readings.
For example, if you take 10 zero flow pressure difference readings and 9 of them are 0.1Pa then technically you can have 1 of them at 49.1Pa as that gives you an average of 5Pa which is a perfectly valid test. It would obviously be a very odd day if this occurred but as an extreme example it highlights the misconception regarding acceptable zero flow pressure readings that testers have.
Therefore you need to look at the 3 averages before the test and the 3 averages after the test and ensure that these are within the +/- 5Pa range as tests with values outside of this are invalid. It’s the average of the positive and negative values before and after the test that need to be checked as these will always either be higher or equal to the average of all the values.
For reference any 0 values are only included in the average of all the zero flow pressure readings and not the average of the positive or negative values.
