Blog: Differences in water measurement
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Many residential buildings use both main water meters and individual apartment meters to record and bill water consumption. It is often noticeable that the sum of the consumption readings from the individual meters differs significantly from the reading of the main meter. These discrepancies regularly cause questions and uncertainty for residents and landlords. The following explains why such discrepancies are technically necessary and perfectly normal in practice, what their causes are, and how they affect water billing.

Why do discrepancies occur in water measurements?

Discrepancies between the main water meter and the apartment submeters are due to technical reasons and are common practice. The sum of the submeter readings often shows a significantly lower consumption than the main meter, frequently with a deviation of up to 30 percent. These differences arise from measurement inaccuracies, different meter ranges, and other factors. Despite these discrepancies, residents do not pay excessive water costs, as the billing takes the distribution method into account.

Each water withdrawal (dark) also results in a barely perceptible overrun (light) on the meter. However, at the main meter, every water withdrawal in the house leads to overrun and thus to a measurable quantity.

How do measurement differences arise in water meters?

  • Measuring range and installation position: Apartment water meters only register a flow rate of approximately 12 liters per hour or more, or 20 liters per hour for vertically installed meters. Main water meters register flow rates as low as 2 to 7 liters per hour. Very small amounts of water used, such as dripping taps or residual water running after installation, are often not registered by apartment meters but are totaled by the main meter.
  • Slippage quantities: Small quantities of water in the lowest measuring range are not fully detected.
  • Missing meters at public consumption points: Water sources such as garden irrigation or stairwell cleaning are often unmetered and cause discrepancies.
  • Time deviations: Different meter reading dates for main and sub-meters lead to measurement deviations.
  • Estimates for inaccessible meters: Fuel consumption figures are sometimes estimated, which causes further discrepancies.

Fig. 1: Each water withdrawal (dark) also results in a barely perceptible overrun (light) on the meter. However, at the main meter, every water withdrawal in the house leads to overrun and thus to a measurable quantity.

What role does cost-effectiveness play in measurement accuracy?

Technically, higher accuracy would be possible for apartment water meters, but such devices would be very expensive and uneconomical. Calibration regulations allow for certain measurement tolerances to ensure a compromise between accuracy and cost-effectiveness. Apartment water meters are therefore intentionally designed to be simpler than main water meters.

Fig. 2: Minimal water withdrawals, e.g., dripping taps, are not recorded by the apartment meters. However, they add up to a measurable amount at the main meter.

Minimal water usage, such as from dripping faucets, is not recorded by the apartment meters. However, it adds up to a measurable amount at the main meter.

How do measurement discrepancies affect water bills?

The total consumption as measured by the main water meter forms the basis for the costs. The sum of all apartment water meter readings serves as the basis for distributing the costs among the individual apartments. The price per cubic meter is adjusted so that any shortfall is distributed proportionally among all consumers. This ensures that no individual resident is disadvantaged – everyone pays their fair share.

Are there legal regulations regarding measurement differences?

The calibration law regulates permissible error limits and mandates the regular replacement of water meters. Court rulings confirm that measurement differences of up to 20–30 percent are tolerated. Complete agreement between main and submeters is technically and organizationally unrealistic.

Water meters in apartments are simpler in design and therefore considerably cheaper than main water meters. However, their accuracy is lower. This is a reasonable economic compromise that is tolerated by calibration law.
Fig. 3: Water meters in apartments are simpler in design and therefore considerably cheaper than main water meters. However, their accuracy is lower. This is a reasonable economic compromise that is tolerated by calibration law.
Example of the measurement error curve of a single-jet impeller counter
Fig. 4: Example of the measurement error curve of a single-jet impeller meter. Especially in the lowest range with minimum flow rate (Qmin), water withdrawal leads to a reduced reading on the water meter.
Measurement tolerances for cold water meters and hot water meters
When apartment water meters are installed vertically, the impeller axis is horizontal. This reduces the start-up sensitivity by a few percent compared to the main meter, which is almost always installed horizontally.
Fig. 5: When apartment water meters are installed vertically, the impeller axis is horizontal. This reduces the start-up sensitivity by a few percent compared to the main meter, which is almost always installed horizontally.
Apartment water meters are typically dry-running meters with a magnetic coupling (left). In contrast, the main meters of water supply companies are wet-running meters (right).
Fig. 6: Apartment water meters are generally dry-running meters with a magnetic coupling (left). In contrast, the main meters of water supply companies are wet-running meters (right). Wet-running meters are more accurate, but cannot be used in apartments because exposure to light (sunlight) would inevitably lead to algae growth in the sight glass. This light exposure is prevented in apartment water meters by a protective cover that blocks light.
Unmetered water outlets lead to discrepancies in meter readings. Garden and garage water lines are typical examples. However, if the consumption at these outlets is low, meter installation is not required.
Fig. 7: Unmetered taps lead to discrepancies. Garden and garage lines are typical examples. However, if the consumption at these taps is low, meter installation is not required.
House water meters are more accurate than apartment water meters. However, it is hardly possible to install these devices in apartments because they would be far too large and expensive for this purpose.
Fig. 8: House water meters are more accurate than apartment water meters. However, it is hardly possible to install these devices in apartments because they would be far too large and also too expensive for this purpose.
Even though water meters display cubic meter quantities, they are used as distribution devices. This ensures that each user contributes to any measurement discrepancies and that water costs are distributed fairly.
Fig. 9: Even though water meters display cubic meter quantities, they are used as distribution devices. This ensures that each user contributes to any measurement discrepancies and that water costs are distributed fairly.

Further reading on the topic of measurement discrepancies in water meters: Special features of consumption-based water cost billing, reprint from "Die Heizkostenabrechnung" (The Heating Cost Bill), May 2000, Arbeitsgemeinschaft Heiz- und Wasserkostenverteilung eV (Working Group for Heating and Water Cost Distribution), Bonn-Bad Godesberg. Tenancy law issues in the installation and operation of heat and water meters, WM Wohnungswirtschaft und Mietrecht (WM Housing Industry and Tenancy Law), February 1998, Assessor Dietmar Wall, Cologne. Measurement discrepancies in water meters, Sanitär-, Heizungs-, Klima und Klempnertechnik SBZ (Sanitary, Heating, Air Conditioning and Plumbing Technology SBZ), issue 17/95, Dipl.-Ing. Manfred Jutte, TWS-Stuttgart.

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Title page of the handbook on heat cost accounting

Quelle: Handbuch der Wärmekostenabrechnung | Frank Peters Minol Messtechnik W. Lehmann GmbH & Co. KG, 2019

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