Mechanical
Features of the design of a mechanical water metre (see "Principle of operation").
— Turbine. Metres in which water during operation passes through a turbine — a wheel with blades, the axis of rotation of which is parallel to the direction of flow. This mechanism is somewhat more expensive than the impeller (see below), but it allows you to effectively cope with the intense flow of water while providing good accuracy. Therefore, in mechanical metres with a nominal diameter (see "Diameter (DN)") of more than 50 mm, only turbines are installed; in 50 mm models, turbines are found along with impellers, and in devices of a smaller diameter they are not used at all. Also, note that all models with this type of metre are industrial (see "Type").
— Impeller. Water metres in which water during operation passes through an impeller — a wheel with blades, the axis of rotation of which is directed perpendicular to the flow of water. This design is simpler and cheaper than the turbine design (see above), but it has lower accuracy and is not well suited for large volumes of water. Therefore, only models with a nominal diameter (see "Diameter (DN)") of 50 mm or less are equipped with impellers; these are, in particular, all household appliances (see "Type") with a mechanical principle of operation, as well as some industrial metres of low productivity.
— Single jet. Metres in which water enters the measuring mechanism in a continuous stream, without divid...ing into separate jets. Compared to the multi-jet devices described below, such devices are much simpler, cheaper and more compact, but they are more prone to errors associated with uneven flow. This is not a serious drawback for domestic use but is unacceptable for accurate calculations. Therefore, only household metres are made single-jet (see "Type").
— Multi-jet. In metres with this feature, the water flow entering the impeller or turbine (see above) is pre-cut into several jets. Due to this, the most uniform effect on the measuring mechanism is ensured and the turbulence that occurs in the pipeline is compensated, which significantly increases the accuracy of measurements. The main disadvantages of multi-jet devices are the complexity of the design and higher price than that of single-jet ones. Thus, it makes no sense to use such metres for household measurements; but in the industrial sector (see "Type"), where accuracy is key, they are extremely common.
— Dry. Dry-running metres are called metres in which the counting mechanism is completely isolated from the water flowing through the device. Unlike wet-running devices, where this mechanism is in contact with water, in dry-running models, most of the hardware is separated from the water-measuring section by a sealed partition, and rotation is transmitted through a special magnetic coupling. This arrangement complicates and increases the cost of the design; on the other hand, metres are more reliable, resistant to dirt and durable than wet metres. In addition, they can provide some special features — for example, disconnecting the measuring mechanism without removing the entire device.
— Combined. Combined models are actually two metres in one case, connected in parallel. One of these metres is designed for small volumes of water, the second for intensive consumption; switching between them is carried out automatically — by a special valve that reacts to the flow rate. This design is not cheap, but it allows you to significantly expand the effective range of measurements and achieve high accuracy at both low and high flow rates. It makes sense to use combined models where the intensity of water consumption can vary over a very wide range, which cannot be covered by a conventional metre.
Max. pressure
The maximum water pressure that the water metre is designed for — that is, the highest pressure under which it can work correctly indefinitely. Traditionally reported in megapascals; 1 MPa corresponds approximately to 10 bar (10 atm).
The water pressure in the pipe to which the water metre is connected must not exceed the permissible maximum — otherwise, the device may be damaged and even depressurized with a flood. At the same time, when buying a device for domestic water supply, this indicator can be completely ignored. The fact is that even the simplest household metres have an allowable pressure of 1 MPa (10 bar), and in the most reliable models it can reach 1.6 MPa (16 bar); at the same time, the standard pressure in the water supply system is 4-6 bar, and it rises to 10 bar only occasionally, during tests of hydraulic systems. So you need to pay attention to the maximum pressure only in cases where the water metre is selected for a non-standard water supply system, where the pressure can exceed 1 MPa — an autonomous circuit with self-selected pumps, a production line, etc. Detailed information on choosing for such cases can be found in special sources.
Minimum water flow (Qmin)
Minimum water flow for this metre model.
The minimum flow rate is the smallest flow rate at which the counting mechanism can provide measurement with an acceptable error of ±5%. This deviation is higher than the counter error in the standard mode (from Qt to Qn, see below for details), it is considered undesirable, but generally acceptable. But when the flow rate drops below Qmin, the error increases to unacceptable values, and there is no question of acceptable measurement accuracy. So, ideally, it is worth choosing a metre in such a way that its Qmin is not higher than the water consumption at the minimum intensity of consumption. Detailed recommendations for estimating the actual water consumption for different water supply systems can be found in special sources.
Transitional water flow (Qt)
Transitional water consumption for water metre model.
The transitional flow rate is the flow rate at which the maximum measurement error changes — namely, decreases: in the range from Qmin (see above) to Qt it is ±5%, and at the Qt level and above it drops to ±2%. In other words, Qt is the smallest flow rate at which the device gives not just an acceptable, but a minimum error. Thus, the optimal consumption intensity for any metre is in the range between Qt and Qn (see below), and it is this range that is best to focus on when choosing.
Detailed methods and recommendations for estimating water consumption for a particular water supply system can be found in special sources.
Rated water flow (Qn)
Nominal flow rate for this metre model.
It is the highest flow rate at which the device can work indefinitely (during the entire service life) without failures, malfunctions and exceeding the maximum allowable error (± 2%). For a short time, a higher flow rate is also allowed (for more details, see “Maximum water flow (Qmax)”), however, the regular mode is still the mode in which this rate does not exceed Qn. So this parameter is the main one when choosing a device. Ideally, the actual water flow should always be in the range between the nominal and transitional (see above) flow.
Max. water flow (Qmax)
Maximum water flow for water metre .
The maximum flow rate is considered to be the highest flow rate at which the device can operate for a short time (less than 1 hour per day and less than 200 hours per year) without failures, malfunctions and exceeding the maximum allowable error (± 2%). For several reasons, this figure is usually twice the nominal flow rate Qn (see above). When choosing a metre for maximum flow, you need to take into account the peak consumption of the system that it serves — that is, the flow rate when all consumers are turned on maximum at the same time: this rate should not exceed Qmax, otherwise the metre will not be able to cope with its task normally. If the system is constantly operated in the maximum consumption mode, then it is worth choosing not by Qmax, but by Qn.
Max. water temperature (Tmax)
The highest water temperature allowed for water metre.
This parameter is directly related to the purpose (see above): in devices only for cold water, Tmax is usually in the range of 30...40 °C heating water in a pipeline exposed to the sun). For metres that can be used with hot water, this figure is 90...95 °C.
Of course, in no case should the maximum water temperature be exceeded — this can lead to damage to the metre and even an accident with a flood” Putting a metre with Tmax at a level of 90 °C and above in a cold water supply system makes sense only if the device was originally designed for both hot and cold water; see "Suitable for" for details.
Division value
The division value on the measuring scale of the water metre.
The division price is the minimum measurement step, the difference between two adjacent smallest units, in other words, the smallest difference in readings that the metre can display. The smaller this difference, the more detailed data the device produces. For example, a value of 0.01 m³ allows you to display data with an accuracy of the second decimal place — that is, such a metre will show, for example, a change in the amount of water consumed from 0.02 m³ to 0.03 m³, but will not perceive the difference between 0.022 m³ and 0.028 m³ (readings will remain at the level of 0.02 m³). However, modern water metres — even household ones — for the most part, have a lower division price, from 0.001 m³ and below.
Sensitivity threshold, less than
Sensitivity threshold for this water metre model.
The sensitivity threshold is the lowest flow rate at which the device begins to respond to the movement of water and record the flow; at a lower speed, the measuring mechanism simply does not distinguish between flow and still water. The lower this indicator, the less likely it is that, at low consumption, water will be consumed without accounting. In household models (see "Type") the sensitivity threshold does not exceed 40 L/h, there are also much lower figures — 10 L/h, or even 5 L/h. In industrial metres designed for large industries, there are values of hundreds of litres per hour.
