Realization of the unit of thermal energy (amount of heat)
Thermal energy is derived from the temperature difference and the volume of the flowing liquids. The temperature difference is measured by high precision resistance thermometers and the volume is determined by a high precision flow test bench. Measurement uncertainties for the thermal energy of 0,02 % can be reached under optimized conditions, because the uncertainties of material quantities have to be taken into account besides the uncertainties of temperature difference and volume.
Verification of heat meters
Heat meters are subject to mandatory verification in Austria. The first verification of a new measuring instrument is called initial verification. The verification of an unverified measuring instrument is called new verification. The verification during the period of validity of the verification is called subsequent verification.
Since 30 October 2006 the Measuring instruments directive (MID) is applied in the member states of the EU. This directive regulates the placing on the market of measuring instruments subject to mandatory verification by so called conformity assessment procedures. Heat meters are covered by the MID. They can be placed on the market for the first time without further national approval and initial verification.
The assessment of conformity is equivalent to the initial verification.
For heat meters the modules B+D, B+F or H1 are applicable as conformity assessment procedure.
Heat meters, for which this directive is not applicable, are subject to national regulations.
The new and subsequent verification of heat meters are in any case subject to national regulations and are carried out by accredited verification bodies in Austria or by verification bodies in other countries authorized and supervised by the BEV.
The interval for subsequent verification for heat meters is 5 years. An extension of this interval by 2 years is possible on the basis of a statistical control, if the heat meters are realized as compact heat meters. If the heat meters are realized as combined heat meters and if the flow sensors have been replaced, an extension by 5 years is possible.