Clarification of Tolerances on Declared Values for Radiation Properties
The harmonised European Standards for the glass products incorporate as essential characteristics the ‘radiometric properties’, i.e. the light transmittance/reflectance and solar energy characteristics.
The declared values for these characteristics are given without any explicit tolerances being quoted. The implicit expectation is that the tolerance taken be the industry norm, i.e. ± 0,03. This value only applies to monolithic glass products, i.e. soda lime silicate glass, thermally treated glasses, etc.
Note For composite products, i.e. laminated glass, insulating glass units, due to their complex nature, no tolerances have been specified.
EN 1096 – 4: Glass in building – Coated glass – Part 4: Evaluation of conformity/Product standard is the only standard that quotes tolerances on radiometric properties. According to Annex A – Tables A.1 and A.2; Section 3 – Product control; Ref.3 – Radiometric properties, there is a requirement for the measurement of the following:
|τe||Solar direct transmittance|
|ρe||Solar direct reflectance|
The requirements are as follows, i.e. measured value equals declared value + tolerance
τv,m = τv,d ± 0,03
ρv,m = ρv,d ± 0,03
τe,m = τe,d ± 0,03
ρe,m = ρe,d ± 0,03
That is to say that the declared values have a tolerance of ± 0,03. However, if the declared value is quoted as a percentage then the tolerance is ± 3.
Examples τv,d is given as either 0,60 or 60 %.
Measured values τv,m will be:
0,60 ± 0,03, i.e. between 0,57 and 0,63, or
60 % ± 3 %, i.e. between 57 % and 63 %.
The tolerance on all monolithic glass products shall be the same as for the coated glass