Method of dry-calibrating vortex flow sensors

Details Method of dry-calibrating vortex flow sensors Method of dry-calibrating vortex flow sensors

1999-05-10

2001-10-23

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

By measuring vibrations or acoustic energy

C073S001160

Reexamination Certificate

active

06305232

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method of dry-calibrating vortex flow sensors, hereinafter called “vortex sensors” for short.
BACKGROUND OF THE INVENTION
The operation of conventional vortex sensors, as is well known, is based on the utilization of periodic pressure fluctuations in a Karman vortex street. Such a vortex street is formed when a fluid in a measuring tube flows against an obstruction, particularly a bluff body. From the downstream side of the body, vortices are shed, which form the vortex street. The frequency of vortex shedding is proportional to the volumetric flow rate of the fluid.
A conventional vortex sensor comprises the aforementioned bluff body and a measuring tube of predetermined length through which the fluid to be measured flows during operation. The measuring tube has an axis, an internal surface, an inlet end, an outlet end, a bore size corresponding to a nominal bore ordered by a customer, and a wall thickness suitable for a permissible pressure.
The bluff body has a horizontal cross-sectional area with a geometrical shape that can be selected by the manufacturer. It further has a first end and a second end which are connected with the wall of the measuring tube along a first fixing zone and a second fixing zone, respectively. The bluff body has a surface facing fluid flow and having a first and a second vortex-shedding edge, and is commonly disposed along a diameter of the measuring tube. The bluff body may have further vortex-shedding edges, particularly a third and a fourth vortex shedding edge.
A sensing element is fitted in the bluff body or is mounted downstream of the bluff body on the internal or external surface of the wall of the measuring tube or in this wall. The electric signals generated by the sensing element are processed by evaluation electronics and indicated and/or passed on to further electronics in the usual manner.
A characteristic feature of bluff bodies is that they have a surface facing fluid flow, at which the fluid is “dammed up”. On the downstream side, the bluff bodies taper so as to obtain at least the first and second vortex-shedding edges and favor the vortex shedding.
The pressure variations associated with the vortices are converted into electrically processable signals by means of the sensing element mounted in or downstream of the bluff body, which may be a capacitive, inductive, or piezoelectric device, but also an ultrasonic transducer, for example. The frequency of these signals is directly proportional to the volumetric flow rate in the measuring tube.
Due to variations in the geometrical dimensions of produced vortex sensors, however, each of these devices must be calibrated individually, i.e., each device is measured in a calibration facility using a standard fluid, generally water. For this calibration measurement, the term “wet calibration” has come into use.
This is usually done by presetting several precisely known flow-rate values (“calibration values”) by means of the calibration facility and registering the associated values indicated by the respective vortex sensor via the associated evaluation electronics. The deviation of the registered values from the precise values yields a calibration factor characteristic of the respective flow sensor.
The calibration factor is used, inter alia, to adjust a variable-gain amplifier stage in the evaluation electronics of the respective vortex sensor so that the indicated flow-rate values of all produced vortex sensors are equal to one another and to the above-defined calibration values.
The wet calibration described is complicated, time-consuming, and expensive. In the literautre, cf. “Bulletin of NRLM”, Vol. 45, 1996, pages 174 to 179, a concept is described for dry-calibrating the produced vortex sensors based on determined physical dimensions and an experimentally optimized geometry of the individual bluff bodies. This optimization consists of determining that bluff body geometry for which the dependence of the Strouhal number on the Reynolds number is as linear as possible.
However, this method of dry calibration is not accurate enough since manufacturing tolerances of the bluff bodies and of other parts of the flow sensors are not taken into account. Furthermore, this method does not allow the use of bluff body shapes that may be necessary for other reasons. Moreover, present-day accuracy requirements, which are of the order of 0.75% of the measured value, cannot be met with the prior-art method.
It is therefore an object of the invention to provide a method of dry calibration which is much more accurate than the prior-art method.
To attain this object, the invention provides a method of dry-calibrating vortex sensors each comprising:
a measuring tube of predetermined length having a lumen
through which a fluid whose volumetric flow rate is to be measured flows during operation, and
which has an axis,
an internal surface,
an inlet end, which forms a contour line with the lumen,
an outlet end,
a bore size corresponding to a nominal bore, and
a wall thickness suitable for a permissible pressure of the fluid;
a bluff body
which has a cross-sectional area with a geometrical shape selectable by the manufacturer,
which has a first end connected with the wall of the measuring tube along a first fixing zone and
a second end connected with the wall of the measuring tube along a second fixing zone,
which has a surface facing fluid flow and having a first and a second vortex-shedding edge, and
which is disposed along a diameter of the measuring tube; and
a sensing element
which is fitted in the bluff body or
which is mounted downstream of the bluff body on the internal or external surface of the wall of the measuring tube or in said wall,
said method comprising the steps of:
producing, by means of a high-resolution electronic camera located on the axis in front of the measuring tube, in the direction of fluid flow, a digitized, two-dimensional overall image of the internal surface of the measuring tube in the area of the bluff body, the bluff body, the two fixing zones, and the contour line of the inlet end;
dividing the overall image into a first, a second, and a third partial image,
the first partial image containing virtually only information about the inlet end and the internal surface,
the second partial image containing virtually only information about the bluff body without the fixing zones, and
the third partial image containing virtually only information about the fixing zones;
extracting from the first partial image
contour information about the contour line and
first surface defect information relating to the internal surface of the measuring tube;
extracting from the second partial image
first edge information about the first vortex-shedding edge of the bluff body,
second edge information about the second vortex-shedding edge of the bluff body, and
second surface defect information relating to the surface of the bluff body facing fluid flow;
extracting from the third partial image
first shape information about the first fixing zone of the bluff body,
second shape information about the second fixing zone of the bluff body,
third surface defect information relating to the surface of the first fixing zone, and
fourth surface defect information relating to the surface of the second fixing zone;
forming from the first and second edge information
distance information and
angle information relating to the deviation of the vortex-shedding edges of the bluff body from parallelism;
forming from the distance information
first roughness information relating to the not exactly straight course of the first vortex-shedding edge,
second roughness information relating to the not exactly straight course of the second vortex-shedding edge,
mean-value information for all distances between the vortex-shedding edges along the bluff body, and
weighting information using a weighting function characteristic of predetermined flow profiles of the fluid;
forming first cross-correlation information from the first shape information and from first ideal information c

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