Flow rate measuring device

A flow rate measuring device, comprising:

• a connector portion, which is arranged outside a pipe flowing a fluid to be measured, and is configured to transmit and receive a signal to and from an exterior;
• a main body portion extended from the connector portion so as to be located in the pipe;
• an internal flow passage, which is formed inside the main body portion, and is configured to cause part of the fluid to be measured flowing through the pipe to flow therethrough;
• and
• a flow rate detection element, which is installed in the internal flow passage, and is configured to detect a flow rate of the fluid to be measured,
• wherein the internal flow passage includes:
• a main flow passage configured to extend from an inflow port configured to introduce the part of the fluid to be measured flowing through the pipe to an outflow port configured to return the introduced part of the fluid to be measured to the pipe;
• and
• a sub-flow passage branching from the main flow passage to bypass the main flow passage,
• wherein the sub-flow passage includes:
• a flow detection element-side flow passage in which the flow rate detection element configured to detect the flow rate of the fluid to be measured from a flow rate of a measurement fluid is installed;
• and
• connection flow passages configured to connect the main flow passage and the flow rate detection element-side flow passage to each other,
• wherein the main flow passage includes:
• an introduction portion configured to guide the fluid to be measured, which has flown into the main body portion from the inflow port, into the main body portion;
• a small flow passage sectional area portion having a flow passage sectional area smaller than a flow passage sectional area of the introduction portion;
• and
• an exit portion, which has a flow passage sectional area larger than the flow passage sectional area of the small flow passage sectional area portion, and is configured to guide the fluid to be measured to the outflow port,
• wherein the connection flow passages include:
• an upstream-side connection flow passage connected to the main flow passage at a position on the inflow port side with respect to the small flow passage sectional area portion;
• and
• a downstream-side connection flow passage connected to the main flow passage at a position on the outflow port side with respect to the small flow passage sectional area portion, and
• the connection flow passages are configured to introduce part of the fluid to be measured flowing through the main flow passage via the upstream-side connection flow passage as a measurement fluid to cause the measurement fluid to flow through the flow rate detection element-side flow passage and return the measurement fluid to the main flow passage via the downstream-side connection flow passage,
• wherein the main flow passage and the flow rate detection element-side flow passage are formed so as to be symmetric with respect to a plane having a flow direction of the fluid to be measured flowing through the pipe as a normal, and
• wherein the main flow passage has a portion at the plane of the symmetry as the small flow passage sectional area portion.