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Natural gas – billed correctly

Determining the measurement trueness of state reconstruction systems used by gas utility companies

PTBnews 3.2018
Especially interesting for

regional gas utility companies

verification authorities

manufacturers of state reconstruction systems


Extensive uncertainty analyses and innovative methods for uncertainty determination allow state reconstruction systems to be used for commercial transactions in the gas grids of regional utility companies in order to determine the energy content of natural gas. PTB has provided the preconditions for the use of these systems.

Processed and conditioned biogas is an important part of the German Federal Government’s energy concept in pursuit of low-carbon energy generation. (Photo credit: Ökobit GmbH)

Supplying biogas to the grid, adding hydrogen, importing LNG, transitioning from L-gas (low calorific gas) to Hgas (high calorific gas), and diversifying supply sources – all these factors cause an increasing variability in the quality of the natural gas provided to the end user. In order to guarantee fair trade conditions in accordance with the Measures and Verification Act and to enable fair and correct billing, natural gas is traded and billed according to its energy content (calorific value × volume). In contrast to volume measurement, measuring the calorific value at each exit point requires considerable technical and financial resources; the gas supply grid is therefore divided into billing areas, each of which has a single calorific value that is traceable to verified measurements. In these regional distribution grids, gas-grid state reconstruction systems are being increasingly used; if the quantity of gas flowing into and out of the grid and the topology of the gas grid are known, it is possible to track an amount of gas on its way through the grid. Based on the gas quality measured at the entry point of the gas grid, the gas quality – and thus the calorific value – can be determined at each exit point.

To increase the acceptance of these reconstruction systems among gas utility companies, consumers and verification authorities, PTB has developed technical testing specifications. These specifications allow the abstract requirements of the Measures and Verification Act to be transposed into specific technical requirements that apply to these computing systems, which are not subject to legal verification. Moreover, each system is tested individually by PTB before being used for commercial transactions.

Within the scope of these tests, particular attention is paid to the determination of the measurement trueness by means of comparison measurements and modern uncertainty determination methods. The complexity of reconstruction systems means that the only method that can be applied is that of Monte Carlo simulations. Exhaustive analyses and practical experience have shown that these reconstruction systems can achieve a measurement uncertainty comparable to that of conventional calorific value determination devices; as a result, the errors in service are certain to be within the permissible limit of 2 %. Further items that are part of the testing include security against intentional and unintentional tampering, the system’s stability in the event of changes in the topology or in the entry/exit status, the clear and unmistakable presentation of the results, the verifiability of the system by the verification authorities, and the processing of the computed results. The test results are reported by means of a PTB certificate that attests to the gas utility company and the verification authority that the requirements have been met.


Stefan Sarge
Department 1.4
Gas Flow
Telephone: +49 531 592-3310
Opens window for sending emailstefan.sarge(at)ptb.de

Scientific publication

S. M. Sarge: Ermittlung von Gasbeschaffenheitskenngrößen für Erdgas im geschäftlichen Verkehr mittels Gasnetz-Zustandsrekonstruktion. PTB-Mitteilungen 1.2017, 23–31