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.