Water temperature
The purpose is indicated by the temperature of the water with which the metre can be used.
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For cold water. Metres intended for use in cold water supply systems and not designed for hot water. They are blue in colour as standard. In most of these models, the maximum water temperature (Tmax) is from 30 to 40 °C — this is quite enough for traditional water supply systems. However, there are also higher rates — up to 50 °C inclusive. Such metres will be useful where the cold pipeline may be subjected to additional heating and quite warm water may flow through it. An example is a stand-alone water supply system, some of the pipes of which are exposed to direct sunlight and can become very hot.
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For hot water. Metres intended for use in hot water supply systems; traditionally painted red. The standard temperature in DHW systems with centralized water supply is 65 °C, but higher values are possible. Therefore, in such metres, the maximum water temperature (Tmax) is not lower than 90 °C, and in the most advanced models — 95 °C. Note that such devices can easily cope with cold water, so technically it is quite possible to put a hot water metre in the cold water system; however, this is not justified in fact, since constant operation in abnormal mode adversely affects accuracy. And for water supply with a very unstable temperature, it is better to use universal metres (see below).
...— For cold/hot water. Metres that are equally well suited for both hot and cold water. Like specialized models for hot water supply, they have a maximum allowable temperature of at least 90 °C; the main difference is that this variety provides almost the same accuracy at any water temperature. Such properties are useful, first of all, if the temperature in the water supply is not constant, especially if the "hot" and "cold" periods take approximately the same time intervals. However, nothing prevents the use of such metres even at a stable temperature — except that the cost of universal models will be somewhat more expensive than specialized devices of similar quality.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.
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.
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.
Dimensions
General water metre dimensions in length, depth and height. Sometimes only one size can be indicated in this paragraph — the length: it is most important during installation, it is the length that is taken into account when choosing the required distance between the ends of the pipes. At the same time, it is worth remembering that between these ends, not only the metre itself is most often located, but also other devices and parts: connecting fittings, a valve, a coarse filter, a check valve, etc. Details on this can be found in special sources; here we note that for some models, the specifications separately indicate the length with complete mounting fittings (see below). Without fittings, the length can be from
80 – 100 mm in the most compact devices to
250 mm or more in the largest.
Depth and height, in turn, allow you to estimate how much free space around the pipe is needed for the normal placement of the metre.
