(1) fluid type
Although compressed air flowmeter can be used to measure fluid volume flow, it is often necessary to measure mass flow in actual production. Therefore, different types of fluids need different measurement ranges of compressed air flowmeter.
(2) the temperature of the fluid
Fluid parameters, such as density and viscosity, vary with the temperature of the fluid. In order to ensure the measurement accuracy of compressed air flowmeter in measuring mass flow, it should be selected according to the fluid temperature.
(3) Analog signal
The compressed air flowmeter has both pulse signal output and analog signal output. The selection of analog signals should be determined according to whether the recorder or display used receives 0 ~ 10ma or 4~20mA signals.
(4) Inner diameter of fluid pipeline
The inner diameter of the measured fluid pipeline determines the maximum flow through the pipeline. The range of compressed air flowmeter is different with different inner diameter of pipeline. The inner diameter of pipeline is a factor that can not be ignored in type selection.
(5) Fluid composition
The compressed air flowmeter requires that the measured fluid does not contain large-particle solid dust or fibrous solid matter, nor does it contain oil pollution and condensate, so as not to change the shape of the vortex generator and bring errors to the measurement. In practical use, the measured fluid often contains these impurities. Although a filter can be installed in front of the vortex flowmeter, the harm of these impurities to the vortex generator is inevitable. When the flow is not allowed to be interrupted, in order to facilitate the regular inspection and cleaning of the vortex generator, a compressed air flowmeter with a ball valve should be selected.
(6) Fluid flow state
After comprehensive consideration of the above factors, compressed air flowmeter can be selected. However, the compressed air flowmeter requires the measured flow to be in a turbulent state, so it requires enough straight pipe sections upstream and downstream of the flowmeter (upstream straight pipe section ≥ 15D, downstream straight pipe section ≥5D, where d is the inner diameter of the fluid pipeline). If a long enough straight pipe section cannot be found in the production site, a rectifier should be selected to obtain enough turbulent flow field.
Second, the principle of compressed air flowmeter:
The basic principle of compressed air flowmeter is Carmen vortex street principle, that is, "the vortex separation frequency of vortex street is proportional to the flow rate", and the diameter of the transmitter circulation body is basically the same as the nominal diameter of the instrument. An approximately isosceles triangle cylinder is inserted into the fluid, with its axis perpendicular to the flow direction of the measured medium and its bottom facing the fluid.
When the measured compressed air flows through the cylinder, vortices are alternately generated on both sides of the cylinder, and the vortices are constantly generated and separated, forming two staggered vortices downstream of the cylinder, that is, "vortex street". Theoretical analysis and experiments show that the frequency of vortex separation is proportional to the velocity of the medium at the tower side. The staggered separation of vortices produces pulsating pressure in the wake on both sides of the cylinder and behind the cylinder. The detection probe arranged inside the cylinder (or behind the cylinder) is affected by this tiny pulsating pressure, which makes the piezoelectric crystal elements buried in the probe subject to alternating stress and generate alternating charge signals.