Air-Tightness Testing And Uncertainty

Marc Cowlin, Compliance Manager for Stroma Technology, a UK testing, assessment and consultancy company specialising in building sustainability, explains the principles of calibration and the evaluation of the uncertainty of measurement.

The “Fan Pressurisation Method”  standard (EN 13829) states the  frequency of calibration is  determined in accordance with  manufacturers’ recommendation,  (normally 12 months or less) or inline  with a standardised quality  assurance system.  There are three types of equipment  with the specification stated within  the standard; Fan, Manometer and  Thermometer. The fourth inferred  instrument is a barometer used to  determine air density:  1. The Fan and corresponding air  flow measurement device must  have a relative uncertainty of less  than 7% of flow.  2. The Manometer’s level of  uncertainty is stated in absolute  terms of ±2 Pa, reading in the  range of 0 – 60 Pa. We would  contend the range of measurement  stated, as the instrument should be  calibrated over the range of use,  which is explained further within  the standard as being 100 Pa. As  instruments are used to record  both pressurisation results and  depressurisation, therefore the  range should actually be ±2 Pa  from –100 to +100 Pa.  3. The Thermometer also has an  absolute limit of ±1 K. There is no  range stated for it, but should be  relevant to the climate conditions  experienced in the country of use,  in the UK the range recommended  is from -20°C to +40°C.  4. The Barometer has no  recommendation within EN 13829,  but should have an absolute  uncertainty of ±5 mbar in each  respective country’s climate; the  working UK range is 950-1050 mbar.  Significant Contributors  to Uncertainty  Uncertainty is an unavoidable part  of any type of measurement  referring to the measurement lying  within a stated range. It is  introduced because of adhoc  events such as short fluctuations in  ambient conditions.  Repeatedly taking the same  measurement will show variation  because of these variables.  There are two main elements that  dominate the uncertainty of  instruments used for air leakage  test, although there are other  contributing factors that must also  be accounted for.  The first factor is drift (the error  seen between consecutive  calibrations) which results from the  equipment use, travel and rapid  set-up and removal from one site to  the next. A means to minimise this  is to reduce the period between  calibrations i.e. a maximum period  of 12 months.  The second significant contributor  is the uncertainty associated with  the calibration itself, which may be  in the form of the uncertainty  quoted by the laboratory  undertaking the calibration, or of  the procedure/reference equipment  used to cross check; this form of  error is normally termed as  imported uncertainty.  The calibrated instruments are all  undertaken by means of intercomparison  between items of  known uncertainty (imported  uncertainty) and the respective  item under test.  Testing Uncertainty  When performing tests, a greater  uncertainty is expected than in  calibration. The test object may be  subject to changing environmental  conditions, whereas a calibration  laboratory will have controlled  environmental conditions.  Other imperfections may be  present due to a drift in the  characteristics between  calibrations for the instruments,  where the test standard is not  definitive in an aspect of the test  procedure or there is personal  bias in the measurements taken  by the tester.  EN 13829 gives an estimate of the  overall uncertainty of the  measurement of air leakage rates,  based upon the conditions, of  ±15% in calm conditions and  ±40% in windy conditions. From  Stroma Technology’s experience,  it is a reasonable estimate, once  all the individual factors of  uncertainty involved are  considered.  Stroma Technology is a UKAS  accredited testing laboratory (No.  2731) and calibration laboratory  (No. 4126) and has delivered ai  tightness testing and consultancy  for over 10,000 commercial,  domestic and public buildings.  For more information on airtightness  and other Stroma  Technology services, visit  www.stroma.com/technology/  build-performance-testing or  contact Marc Cowlin on 0845  621 2222.