This invention relates generally to flow measuring apparatus, and more particularly to a Karman vortex flowmeter having a relatively large diameter (i.e., greater than 250 m.m.).
It is well known that under certain circumstances the presence of a vortex shedder in a flow conduit will give rise to Karman vortices. The number of such vortices formed per unit of time, i.e., the frequency of Karman vortices, is a function of the flow velocity or flow rate of the fluid following through the conduit. Thus the flow rate can be determined by measuring the frequency of the Karman vortices.
A significant advantage of a Karman vortex flowmeter, apart from the fact that it is of simple construction, is that there exists an excellent proportional relationship between the frequency of the vortices and the velocity of flow, but only if the probe is ideally positioned and the flow velocity profile assumes an ideal form. In actual practice, however, the flow velocity in a cross-sectional plane in the flow tube does not assume an ideal profile wherein the fluid velocity is at its maximum value at the central axis of the tube and diminishes in value as one approaches the inner surface thereof.
For example, when the flowmeter is installed downstream directly adjacent an elbow in the flow line, the elbow acts to disturb considerably the velocity profile. Consequently, if the detected vortices are converted into a flow velocity reading on the basis of the ideal velocity profile condition, this conversion results in a substantial error.
In the case of flowmeters of large diameter installed in culverts, other problems arise that are dictated by the nature of the installation. The culvert is usually placed within an underground pit that is only accessible through a manhole. In an installation of this type, the probes of the flowmeter are subject to contamination from particulate matter carried by the fluid being metered and, in some instances, the probes become clogged with contaminants. But because of the confined pit space within which the flowmeter is installed, with existing types of flowmeters it is very difficult if not impossible to remove the contaminated probes for inspection, cleaning or replacement.