48 FEATURES
<< Continued from page 47 piston or disk moves a magnet that drives the register. PD meters are generally very accurate at the low-tomoderate flow rates typical of residential and small commercial users, and commonly range in size from
⅝ " to 2 ". Because displacement meters require that all water flows through the meter to‘ push’ the measuring element, they generally are impractical in large commercial applications requiring high flow rates or low-pressure loss. PD meters normally have a built-in strainer to protect the measuring element from rocks or other debris that could stop or break the measuring element. PD meters normally have bronze, brass, or plastic bodies with internal measuring chambers made of moulded plastics and stainless steel.
Velocity water meters A velocity-type meter measures the velocity of flow through a meter of a known internal capacity. The speed of the flow can then be converted into volume of flow to determine the usage. Several types of meters measure water flow velocity, including jet meters( single-jet and multi-jet), turbine meters, propeller meters, and magnetic flow meters( commonly referred to as mag meters). Most velocity-based meters have an adjustment vane for calibrating the meter to the required accuracy.
Multi-jet meters Multi-jet meters are very accurate in small sizes and are commonly used in ⅝ " to 2 " sizes for residential and small commercial users. Multi-jet meters use multiple ports surrounding an internal chamber to create multiple jets of water against an impeller, whose rotation speed depends on the velocity of water flow. Multi-jets are very accurate at low flow rates, but there are no large size meters since they do not have the straight-through flow path needed for the high flow rates used in large pipe diameters. Multi-jet meters generally have an internal strainer element that can protect the jet ports from getting clogged. Multi-jet meters normally have bronze alloy bodies or outer casings, with internal measuring parts made from modern thermoplastics and stainless steel.
Turbine meters Turbine meters are less accurate than displacement- and jet meters at low flow rates, but the measuring element does not occupy or severely restrict the entire path of flow. The flow direction is generally straight through the meter, allowing for higher flow rates and less pressure loss than displacement-type meters. They are the meter of choice for large commercial users and fire protection, and as master meters for the water distribution system. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system. Turbine meters are generally available for 1½ " to 12 " or higher pipe sizes. Turbine meter bodies are commonly made of bronze, cast iron, or ductile iron. Internal turbine elements can be plastic or non-corrosive metal alloys. They are accurate in normal working conditions but are greatly affected by the flow profile and fluid conditions.
• Fire meters are a specialised type of turbine meter meeting the high flow rates requirements for fire protection. They are often approved by underwriters laboratories( UL) or factory mutual( FM) for use in fire protection.
• Fire hydrant meters are a specialised type of portable turbine meter attached to a fire hydrant to measure water flowing out of the hydrant. The meters are normally made of aluminium to keep their weight low and usually have a 3 " capacity. Utilities often require them for measuring water used on construction sites, for pool filling, or where a permanent meter has not been installed yet.
Compound meters A compound meter is used where high flow rates are necessary, but where at times there are also smaller rates of flow that need to be accurately measured. Compound meters have two measuring elements and a check valve to regulate flow between them. At high flow rates, water is normally diverted primarily or completely to the high flow element. The high flow element is typically a turbine meter. When flow rates drop to where the high flow element cannot measure accurately, a check valve closes to divert water to a smaller element that can measure the lower flow rates accurately. The low flow element is typically a multi-jet or PD meter. By adding the values registered by the high and low elements, the utility has a record of the total consumption of water flowing through the meter.