Volumetric Gas Flow Switches

Volumetric gas flow switches have a switch output and are used to measure the flow or quantity of a moving gas in terms of a unit of volume per unit time, such as cubic feet per minute. They differ in terms of switch type, normal state, poles, and throws. There are two types of volumetric gas flow switches: electromechanical and solid-state. For each type of device, the normal state can be normally open (NO) or normally closed (NC). In terms of poles, a single pole (SP) switch has a single set of contacts for switching just one electrical line. A double pole (DP) switch has two sets of contacts for switching two lines simultaneously. Volumetric gas flow switches with more than two sets of contacts are also available. Single throw (ST) volumetric gas flow switches are simple, on/off devices. With double throw switches (DT), a common contact is switched between two contacts.  

Selecting volumetric gas flow switches requires an analysis of performance specifications and media capabilities. Performance is determined by the turndown ratio, the effective dynamic or operating range of the device. For example, a 500 SCCM volumetric gas flow switch with a turndown ratio of 50:1 will operate effectively and resolve flow down to 10 SCCM. Similarly, a volumetric gas flow device with a turndown ratio of 100:1 will resolve effectively to 5 SCCM. Performance specifications for volumetric gas flow switches also include gas volumetric flow rate, operating temperature, and operating pressure. Media capabilities are determined by meter technology and may include gas, liquid, steam, and suspended solids or slurries. Media temperature range is dependent on construction and liner materials.   

Parameters for volumetric gas flow switches include pipe diameter, mounting style, end fittings, electrical outputs, and interface options. There are three basic mounting styles: in-line, insertion, and non-invasive. In-line products are installed directly in the process line. Insertion-type devices are inserted perpendicular to the flow path and usually require a threaded hole in the process pipe. Non-invasive volumetric gas flow switches do not require mounting directly in the process flow and can be used in closed piping systems. Fittings can be flanged, threaded, or compression-style devices. Clamps, plain ends, socket welds, tube ends, and hose nipples are also available. In terms of electrical outputs, choices include: analog current, analog voltage, frequency, and switch. Some volumetric gas flow switches provide signal outputs in serial, parallel, Ethernet, or other digital formats. Others format output signals according to industrial fieldbus, networking, or industrial automation protocols.  

Volumetric gas flow switches differ in terms of meter technology and features. There are four choices for meter technology: differential pressure (DP), positive displacement (PD), velocity, and true mass. In terms of features, volumetric gas flow switches that can measure temperature, density, or level are commonly available. These products may include audible or visual alarms, averaging and controller functions, programmability, and recorder or totalizer functions.  

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