Measuring head for an ultrasonic flowmeter

A measuring head for an ultrasonic flowmeter, in which an ultrasonic coupling element and a measuring head housing are cast together and form, along with casting material, a contact surface E. The surface E allows an effective transfer of heat to the measuring head. In order to compensate manufacturing tolerances, the distance a between the measuring head housing and the ultrasonic coupling element is at least 1 mm in the area of the contact surface E.

FIELD OF THE INVENTION

The invention relates to a coupling element for an ultrasonic flowmeter.

BACKGROUND OF THE INVENTION

Ultrasonic flowmeters are used often in process- and automation-technology. They permit in simple manner contactless determination of volume flow rate in a pipeline.

Known ultrasonic flowmeters operate on the basis of either the Doppler principle or the travel time difference principle.

In the travel time difference principle, the different travel times of ultrasonic pulses are evaluated relative to the flow direction of the liquid.

For this, ultrasonic pulses are transmitted both in the direction of flow and opposite thereto. From the travel time difference, one determines the flow velocity, and, with that, knowing the diameter of the pipe section, the volume flow rate.

In the Doppler principle, ultrasonic waves of a known frequency are coupled into the liquid, and the ultrasonic waves reflected from the liquid are evaluated. Using the frequency shift between the waves coupled in and the reflected waves leads likewise to the flow velocity of the liquid.

Reflections in the liquid occur, however, only when small air bubbles or impurities are present therein, so that this principle finds application primarily in the case of contaminated liquids.

The ultrasonic waves are produced and received in a measuring head. The particular measuring head is fixed on the wall of the pipe section of concern. Recently, clamp-on ultrasonic measurement systems have become available. In these systems, the measuring head is only pressed with a clamp on the pipe wall. Such systems are described e.g. in EP-B 686,255, and the U.S. Pat. Nos. 4,484,478 and 4,598,593.

The essential elements of a measuring head are the measuring head housing, a piezoelement with connection unit, and an ultrasonic coupling element made of plastic. The ultrasonic waves are produced in the piezoelement and guided to the pipe wall through the ultrasonic coupling element. From there, they are conducted into the liquid.

In a known measuring head, the ultrasonic coupling element extends somewhat beyond the measuring head housing, so that only the ultrasonic coupling element contacts the measurement pipe. Due to the low heat conductivity of the ultrasonic coupling element, only a small heat transfer from the pipe wall to the measuring head is possible. For this reason, temperature gradients can arise in the measuring head, gradients which negatively affect the measurement accuracy.

An additional disadvantage of known measuring heads is that measuring head housing and ultrasonic coupling element are adhesively bonded. This requires very small tolerances in the manufacture of the measuring head housing. Cast parts, which have greater tolerances, therefore require expensive finishing operations.

SUMMARY OF THE INVENTION

An object of the invention is to provide a measuring head for an ultrasonic flowmeter, which measuring head lacks the above-mentioned disadvantages, makes a rapid, homogeneous temperature distribution possible in the measuring head and permits manufacturing tolerances in the production of the measuring head housing, and which can be produced simply and with favorable costs.

This object is solved by a measuring head for an ultrasonic flowmeter having a partially open measuring head housing, in which an ultrasonic coupling element with piezoelement and connection unit is fixed, characterized in that the ultrasonic coupling element and measuring head housing are cast, or potted, together and form with the cast, or potting, material a contact plane, wherein the separation between measuring head housing and ultrasonic coupling element in the area of the contact surface amounts to at least 1 mm.

The essential idea of the invention resides in that the casting together enables compensating of the manufacturing tolerances of the measuring head housing and simultaneously with placement of the measuring head on the measurement pipe an effective thermal equilibrium is possible by way of the entire contact surface.

For simplicity, the contact surface is flat.

In a further development of the invention, for reasons of cost, the measuring head housing is not filled completely with cast material.

For reliable fixing of the coupling element in the measuring head housing by the cast material, anchoring elements (e.g. lugs or grooves} are provided on the measuring head housing.

In advantageous manner, the cast material has a thermal conductivity>1 W/mK, in order to enable a rapid thermal equilibrium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows, in greatly simplified manner, an ultrasonic flowmeter having two measuring heads2,3, which are arranged on the outer wall of pipe1and displaced from one another in a direction parallel to the axis. The liquid is flowing in the pipe1in the direction of arrow F.

The measuring head pair2,3can be driven in two different ways. Either measuring head2acts as the transmitter and measuring head3as the receiver, or measuring head3is the transmitter and measuring head2the receiver, whereby measurements are taken alternating between in the flow directing and opposite to the flow direction.

The two measuring heads2,3are connected over connection lines23,33, respectively, with a measuring circuit100. The two connection lines23,33carry the electrical pulses. Suitable measuring circuits100are known and are not the subject matter of the invention.

FIG. 2shows, approximately to scale, a measuring head ofFIG. 1in cross section. Essential elements of the measuring head2,3are measuring head housing10, piezoelement20with connection unit30and ultrasonic coupling element40made of plastic (e.g. PEEK, PI, PEI) as well as the cast material50. The connection unit30is composed of a flexible cable-pair32and a plug connection34for the connection lines23or33. The measuring head housing has an opening14to its interior12, where the ultrasonic coupling element40is fixed.

Ultrasonic coupling element40, measuring head housing10and casting material50form a contact surface E in the area of the opening14.

In the region of the contact surface E, the distance a between ultrasonic coupling element40and the inner wall16of the measuring head housing10is at least 1 mm.

For supplemental fixing, anchoring elements80are provided on the measuring head housing10. Shown is a groove; however, other means, such as lugs, etc. can be used.

The measuring head2,3is suited for a temperature range from minus 20° to plus 80° Celsius.

Operation of the measuring head of the invention will now be described in more detail for an ultrasonic flowmeter.

In use, the measuring head2,3lies with the entire contact surface E against the outer wall of the pipe1. A temperature change in the liquid to be measured leads to a temperature change on the outer wall of the pipe1. This temperature change is transferred to the measuring head2,3. Because the heat transfer from the pipe1occurs over the entire contact surface E, a rapid temperature equilibrium is possible. The heat transfer occurs not only from the pipe1to the ultrasonic coupling element40, but also from the pipe1to the casting material50and to the measuring head housing10. Through the good thermal conductivity of the casting mass50and the measuring head housing10, which is normally made of metal, the temperature gradients are rapidly equaled out. This assures a reliable measuring.

Because the ultrasonic coupling element40and the measuring head housing10are cast together, the manufacturing tolerances in the manufacture of the measuring head housing10can be easily compensated. The contact surface E is primarily responsible for this compensation. It has been found that a separation a between measuring head housing10and ultrasonic coupling element40of 1 mm in the area of the contact plane E is sufficient for compensating expected manufacturing tolerances.

It makes sense for reasons of cost not to fill the entire interior12with casting material50. Even a partial casting assures a reliable fixing of the ultrasonic coupling element40.

During the casting, the casting material50enters into the groove80and, following solidification, prevents release of the ultrasonic coupling element40from the measuring head housing10.

The high thermal conductivity of the casting material50contributes to an improved thermal equilibrium.