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In-situ test of heat meters in routine operation


Figure 1: For the new optical procedure for in-situ measurements of flowrates on large facilities in accordance with the laser Doppler velocimetry principle, a laser is coupled to the respective pipe system via an optical window.

In practical applications, the operating conditions for heat meters in large facilities or networks often deviate from the idealized calibration conditions which are prevailing at PTB's test facility. This leads to - partly considerable - measurement deviations. A newly developed procedure now allows heat meters to be investigated under installation conditions and without service interruption.

The largest uncertainty contribution during the operation of a heat meter is furnished by the flow sensor which, for its use, has so far - as a matter of principle - been calibrated on a test facility. Due to the idealized conditions at this test facility, unknown measurement deviations of the flow sensor occur at its later place of use. A calibration under operating conditions has not been possible so far. At best, the sensors were recalibrated on a test facility. For this purpose, they had to be dismounted. The service interruption connected with this brought about high costs. To eliminate this problem, PTB has developed - in cooperation with TÜV Rheinland and the companies ILA GmbH and Optolution GmbH - an optical procedure which allows the flow sensors to be investigated on site without any service interruption.

With the aid of a drilling procedure known from district heating, an inspection glass is applied to the pipeline. After that, a laser Doppler velocimetry (LDV) system is installed, and the flow velocity is determined on a measurement path. By integrating the measured velocity via the pipe cross section, the reference volume flow is determined. Investigations carried out on the heat meter test section of PTB confirm that - under ideal conditions - a measurement uncertainty of less than 1 % can be achieved. After completion of the measurements, the laser can be removed, and the optical access can be closed with a blank flange.


O. Büker, 7.62, e-mail: Oliver.Bueker(at)ptb.de