Volumetric Liquid Flow Switches

Volumetric liquid flow switches measure the flow or quantity of a moving fluid in terms of a unit of volume per time, such as liters per minute. These flow sensing devices have a switch output. Pipe size and pipe style are important specifications to consider when selecting volumetric liquid flow switches. There are three main parameters: pipe diameter, mounting style, and end fittings. The pipe diameter is specified in English units such as inches (in) or metric units such as centimeters (cm). There are three choices for mounting type or style: in-line, insertion, and non-invasive. End fittings for volumetric liquid flow switches include clamps as well as compression, flanged, plain end, threaded, and tube end fittings. Socket welds or unions are also used. 

Some volumetric liquid flow switches are used with differential pressure (DP) meters or positive displacement (PD) meters. DP meters include orifice plates, venturi tubes, flow nozzles, cone types, Pitot tubes, target meters, elbow tap meters and rotameters. With differential pressure meters, the basic operating principle is that the pressure drop across the meter is proportional to the square of the flow rate. By contrast, positive displacement meters are used to take direct measurements of liquid flow. PD meters divide the liquid into specific increments which are moved on. The total flow is an accumulation of the measured increments. Typically, volumetric liquid flow switches count these increments by mechanical or electronic techniques. 

Volumetric liquid flow switches are also used with velocity meters and true mass flow meters. Velocity-type meters are devices that operate linearly with respect to the volume flow rate. Because there is no square-root relationship (as with differential pressure devices), their range ability is greater. Examples of velocity meters include turbine meters, electromagnetic meters, vortex meters, and ultrasonic meters. True mass flow meters directly measure the mass rate of flow. They include thermal meters and Coriolis meters. Thermal meters use a heated sensing element isolated from the medium flow path. The flow stream conducts heat from the sensing element, which works in conjunction with volumetric liquid flow switches. 

Product specifications for volumetric liquid flow switches include liquid volumetric flow rate range, operating temperature and operating pressure. Switch specifications include switch type, normal state, and number of poles and throws. Most devices have either solid state or electromechanical switches. There are two normal states: normally open (NO) and normally closed (NC). Volumetric liquid flow switches are also designated as single pole (SP) or double pole (DP), and single throw (ST) or double throw (DT).

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