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Dynamic weighing of heat quantities

Industrial flow rate measurements, e.g. for district heating and cooling supply, are usually traced back to weighings and time determinations. At PTB's new volume-flow standard measuring facilty for heating and cooling meters, a dynamic weighing method has been developed for volume flow in the range below 200 m3/h, which is particularly important for industrial applications. In comparison to the static weighing procedures commonly used up to now, the technical input and the measurement time required by this new method are significantly lower. For heat quantity measurements, the temperature must in addition be stabilized and measured precisely.

Weighing instrument for gravimetric flowrate determination; max. load: 20 t, measurement uncertainty: 100 g.

For static weighing, the volume flow, which has been regulated to a constant flowrate and a constant temperature, is led into the weighing tank for a defined time by means of a complex diverter, and the quantity is weighed. Since large volume flows must be diverted with an uncertainty of milli- seconds, the uncertainty contribution of the time measurement dominates the total measurement uncertainty of the facility. Furthermore, such selection devices are technically extremely complex, prone to defects and Ð depending on the type Ð not non-reactive.

In the dynamic weighing procedure, the water flows through the weighing tank without a diver-ter. Thereby, the weighing instrument is pre-loaded with standard weights. The measurement starts as soon as a pre-selected initial level is reached. In the course of the measurement the standard weights are removed from the weighing instrument while the tank is increasingly filled. The measurement is completed and the filling time is determined when the initial trigger threshold is reached again. At this moment, the mass of the removed standard weights is exactly balanced out by the mass of the fluid flowed in. The advantages of this method of measurement are its technical simplicity, its non-reactiveness to the volume flow and the significant time saving, which, depending on the flow rate, can reach a factor of up to 2. Since the weighing instrument is used at one distinct point only, it is not necessary to know its characteristic curve. The vibrations occurring during the filling of the weighing tank could be brought under control.

Equivalence of the two methods could be proved in a volume flow range of up to 180 m3/h. At a water temperature of 50 ¡C, for instance, an agreement of 0.04 % resulted, which lies within the relative expanded overall measurement uncertainty of the facility. Thus, a new and more economical method of volume flow determination for heat and cold meters is available for industrial use.

Contact at PTB:

Phone: +49-531-592-0