| Zusammenfassung |
Due to its physical properties, glycol-water-mixtures cause typical heat meters to measure with increased deviations. Both the
volume flow measurement and the fluid’s heat coefficient are affected. For the determination of the exchanged energy, for example
in cooling or solar thermal installations, heat meters which are suitable for commonly used glycol-based heat conveying media are
necessary. A joint project of the Physikalisch-Technische Bundesanstalt (PTB), the Verband der deutschen Wasser- und
Wärmezählerindustrie e.V. (VDDW) and the Arbeitsgemeinschaft Heiz- und Wasserkostenverteilung e.V. (ARGE Heiwako) aims at
gathering data and developing procedures to introduce heat meters for glycol-water-mixtures into legal metrology.
In a first step, the heat transfer and flow properties for a selection of 4 glycol-based media are investigated. Traceable investigations
of the physical properties density ρ(T), kinematic viscosity ν(T) and specific heat capacity cp(T) of the investigated fluids partially
show deviations of up to 18 % from the manufacturer's data. Heat coefficients for glycol-based fluids are calculated with an
expanded uncertainty of 1.5 % (k=2). Furthermore, preliminary investigations indicate that a review of the stability of the heat
conveying medium under conditions of use (degradation) is necessary. After performing accelerated laboratory degradation
procedures, heat transfer properties partially change to varying degrees (up to 5 %), depending on the glycol base. As the
transferability to field conditions is currently not possible, further measurements with improved measuring instruments and in-field
data acquisition is planned to confirm the experiments and further explore the field of degradation.
The second step includes the determination of the specific medium’s influence on the flow measurement of heat meters. Therefore,
a new volumetric testing facility at PTB’s laboratories is designed, built and validated. It has an expanded uncertainty (k=2) between
0.017 % and 0.36 %, based on the flow rate and temperature. Results of five mass-market suitable flowmeter types like ultrasonic
and impeller flowmeters partially show maximum deviations that are multiple times bigger than current maximum permitted
deviations (-45 % to 30 % for ultrasonic sensors; -8 % to 7 % for impeller flow sensors). However, after applying mediumspecific
corrections to the sensor, the results improve drastically (± 2 %), indicating that a legal use of those sensors in near future is
possible. More sophisticated sensors like Coriolis or Electromagnetic flow sensors measure within a narrow deviation range below ±
1 %.
Further investigations include the evaluation of in-field use of glycol-based media as well as possible challenges and testing
procedures for legal metrology purposes. Restrictions may arise due to uncertainties in the specific fluid’s composition and stability
in the field. Testing and calibration of glycol-applicable sensors with water appears feasible and will be investigated further on. |
