Liquid flow switches, volumetric are devices with a switch output used for measuring the flow or quantity of a moving fluid in terms of a unit of volume per unit time, such as liters per minute.  The basis of good flow switch selection is a clear understanding of the requirements of the particular application.  Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit and taking a loss.  In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid's velocity or the change in kinetic energy.  Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit.  Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate.

Liquid flow switches, volumetric are devices with a switch output used for measuring the flow or quantity of a moving fluid in terms of a unit of volume per unit time, such as liters per minute.  The basis of good flow switch selection is a clear understanding of the requirements of the particular application.  Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit and taking a loss.  In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid's velocity or the change in kinetic energy.  Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit.  Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate.

The basic relationship for determining the liquid's flow rate in such cases is:

Q= V x A; where Q = liquid flow through the pipe V = average velocity of the flow A = cross-sectional area of the pipe. 

Other methods of flow rate determination include positive displacement, velocity meters, and true mass flow meters.  Other factors that affect liquid flow rate include the liquid's viscosity and density.  Specific applications should be discussed with a flow switch manufacturer before purchasing to ensure proper fit, form and function. A volumetric liquid flow switch’s performance can be determined by the switch’s turndown ratio. Turndown ratio is the effective dynamic or operating range of the flow switch. For example: If the 500 GPM flow rate device has a turndown ratio of 50:1 it will effectively operate and resolve flow down to 10 GPM. If the same device has a turndown of 100:1 it will effectively resolve to 5 GPM.  The operating pressure is the maximum head pressure of the process media the switch can withstand.  The maximum temperature of the media that can be monitored is usually dependent on construction and liner materials.

Volumetric liquid flow switches can have an electro-mechanical or solid-state switch construction.  The normal state can be normally open (NO), or normally closed (NC).  Liquid flow switches, volumetric can have single poles (SP), double poles (DP), and more than two poles.  The throw type on volumetric liquid flow switches can be single throw (ST) or double throw (DT). Mounting options for volumetric liquid flow switches include insertion types, in-line flanged, in-line threaded, and in-line clamp.  Insertion flow switches are inserted perpendicular to flow path. They usually require a threaded hole in the process pipe or other means of access.  In-line flanged flow switches are inserted parallel to the flow path, usually inserted between two pieces of existing flanged process pipes.  In-line threaded flow switches are inserted parallel to the flow path, and threaded into two existing process pipes.  NPT is the most common thread type.  In-line clamp flow switches are inserted parallel to the flow path, and clamped between two existing process pipes.  Common interface types include serial and parallel.