Patent Description:
An ultrasonic flow meter of the type described herein can be used for measuring a flow of fluid, such as an amount of liquid or gas flowing into, through or out of a fluid system at a consumer such as a household or an industrial facility.

In an ultrasonic flow meter, the measuring results follow from transmitting and receiving ultrasonic signals upstream and downstream of a flow direction in the flow meter flow tube. Various means for increasing the precision of the measurements may interfere in costs of producing, assembling or maintaining the flow meter.

It is essential for the precision of the measurements that the transducers are aligned in the flow tube to form a soundtrack between a pair of transducers. The transducers are placed in transducer housings, and the housings positioned in transducer holders in the flow tube. However, manufacturing a pair of opposite placed transducer holders to be perfectly aligned in a flow tube is difficult due to misalignments during manufacture of the transducer holders.

Document <CIT> relates to an ultrasonic flowmeter including bores with seat surfaces that are machined with precision.

Document <CIT> relates to a machining tool.

Hence, an improved alignment of opposite placed transducers holders in a flow tube would be advantageous, and in particular a more efficient and/or reliable method of manufacture of transducer holders to improve the alignment between a pair of opposite placed transducer holders would be advantageous.

It is an object of the present invention to provide an improved method for manufacturing an ultrasonic flow meter.

In particular, it may be seen as an object of the present invention to provide a ultrasonic flow meter that solves the above mentioned problems of the prior art with increased precision in manufacture of the transducer holders of the ultrasonic flow meter.

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of manufacturing an ultrasonic flow meter flow tube, wherein the flow tube comprising transducer holders, each transducer holder comprising an opening and a transducer seat for mounting of a transducer housing, and each transducer holder is forming a pair of transducer holders with an opposite placed transducer holder,
characterized by the following steps:.

The pair of transducer holders are located opposite to each other in each side of the flow tube, so that a sound track can be transmitted from a transducer housing placed in a transducer holder to the transducer housing placed in the opposite placed transducer holder.

It is essential for the precision of the flow measurements in an ultrasonic flow meter that the transducer housings, comprising the transducers, are precisely aligned to form a sound track between the transducer housings. The transducer housings therefore preferably comprising a coaxial axis with the transducer housings aligned to send an ultrasonic signal to the opposite aligned transducer housing.

It is usually quite difficult to align a pair of two opposite placed transducer holders in a flow tube on a coaxial axis. Because, when machining a transducer seat in a transducer holder, this is usually done individually before joining - by welding or by screwing - the transducer holders to the flow tube, and therefore there will usually be a misalignment because it is difficult to screw or weld a transducer holder to the flow tube and at the same time obtain a precise alignment, as the alignment of the transducer holders may by imprecise due to the mounting surfaces of the transducer seats of the pair of transducer holders are not perfectly aligned to be precisely perpendicular to the coaxial axis.

In the invention described herein, this problem is solved by using a transducer seat-machining tool, which machines the transducer seats in the transducer holders, after they have been joined to the flow tube, to align the transducer seats, such that the mounting surfaces of the transducer seats for a pair of opposite placed transducer holders are aligned to be precisely perpendicular to the coaxial axis.

Aligned on a coaxial axis means that a sound track transmitted from the transducer, placed inside a transducer housing, precisely hits the opposite placed transducer housing in a centred position. The centre of the sound track hits exactly the centre of the opposite placed transducer housing. This is ensured by that the mounting surface of the transducer seats are precisely perpendicular to the coaxial axis.

The machining tool comprising a tool head and a guide shaft. The tool head and the guide shaft are mechanical connected. They can be screwed together or pushed together and secured with a screw or similar. The transducer seat to be machined is machined by the tool head.

The transducer seat can either be machined from inside the flow tube or from outside the flow tube, depending on whether the mounting surface of the transducer seat is facing inward of outward. If the mounting surface is facing inward, the tool head is placed inside the flow tube and are mechanical connected to the guide shaft; the guide shaft is going through the opening of the transducer holder placed opposite to the one to be machined. If the mounting surface is facing outward, the tool head is placed outside the flow tube and mechanical connected to the guide shaft outside the flow tube.

To ensure the transducer holder is precisely aligned, when machining the transducer seat of one of the transducer holders, a part of the transducer seat-machining tool is guiding the tool head of the transducer seat-machining tool by engaging with the opposite placed transducer holder. That is, when machining a transducer seat of a transducer holder forming one part of a pair of opposite placed transducer holders, the opposite placed transducer holder is guiding the transducer seat-machining tool by the transducer seat-machining tool is engaging the opposite placed transducer holder. Thereby, the opposite placed transducer holder will be a guide and support for the transducer seat-machining tool, so the transducer seat being machined will be aligned precisely with the opposite placed transducer holder.

By machining the transducer seat, when the transducer holder is part of the flow tube, and using the opposite placed transducer holder as a guide and a support for the machining tool, a very high accuracy can be obtained in aligning the transducer holders to a coaxial axis.

Thereby, by engaging both transducer holders in the pair of transducer holders, the transducer seat-machining tool is aligned precisely to machine the transducer seat, so that a transducer housing mounted in the transducer seat will be aligned directly towards the opposite placed transducer housing, when mounted, having a precisely aligned coaxial axis.

When the transducer seat-machining tool is in position, engaging both transducer holders, the transducer seat-machining tool machines the transducer seat. When machining, the machining tool rotates and cuts away excess material from the transducer seat to make the surface of the transducer seat completely flat, with a surface to support a transducer housing directly aligned towards the opposite placed transducer housing.

According to an embodiment, the method comprises that each of the pair of transducer holders comprising a guide hole, and the tool head comprising a guide bushing, the method comprises the following steps:.

In the case where the transducer seat is to be machined from inside the flow tube, the tool head is placed inside the flow tube and the guide bushing is positioned in the guide hole of the transducer holder comprising the transducer seat to be machined. The guide shaft then is engaging the opposite placed transducer holder going through and being guided by the guide hole in the opposite placed transducer holder.

In the case where the transducer seat is to be machined from the outside of the flow tube, the tool head is placed outside the flow tube. The guide bushing is entering the flow tube through the opening of the transducer holder to be machined, and is going all the way through the flow tube to engage with and possible go through the guide hole of the opposite placed transducer holder.

In both cases a part of the transducer seat-machining tool is engaging the guide hole of the opposite placed transducer holder, guiding and supporting the tool head.

According to an embodiment, the method comprises that the tool head comprising a transducer seat-cutting tool, and the method further comprises the following steps:.

The tool head comprising a transducer seat-cutting tool, and the transducer seat-machining tool is positioned so that the transducer seat-cutting tool is engaging the transducer seat to be machined and the transducer seat-cutting tool is machining the transducer seat cutting away excess material.

According to an embodiment, the method comprises that machining the transducer seat comprises the following steps:.

When machining the transducer seat, the transducer seat-machining tool is rotating, and the transducer seat-cutting tool is cutting away access material of the transducer seat. A drill can be used for rotating the transducer seat-machining tool. The guide shaft has a drill connector in the second end of the guide shaft, to which a drill can be connected and the drill then can rotate the transducer seat-machining tool.

According to an embodiment, the method comprises that the tool head comprising a stop bushing, and the transducer seat-machining tool, machining the transducer seat, stops machining, when the stop bushing gets in contact with the flow tube.

To ensure the depth of the cutting of the transducer seat is correct, the tool head comprising a stop bushing. When the stop bushing hits the flow tube, it may hit the wall of the flow tube, or it may hit a part of the transducer holder. The transducer seat-machining tool can then go no further, the machining stops, and the transducer seat has the right position for mounting of the transducer housing. According to an embodiment, the method further comprises the step: placing the tool head of the transducer seat-machining tool inside the flow tube.

According to an embodiment, the method further comprises the step: placing the tool head of the transducer seat-machining tool outside the flow tube.

Either the tool head can be inside the flow tube, machining the transducer seat from inside the flow tube, or the tool head can be outside the flow tube, machining the transducer seat from outside the flow tube.

When machining the transducer seat from inside the flow tube, the guide shaft is going through the opposite placed transducer holder and is mechanically connected to the tool head inside the flow tube. In this embodiment, with the tool head engaging the transducer holder comprising the transducer seat to be machined, and the guide shaft engaging the corresponding opposite placed transducer holder, the transducer seat-machining tool is guided by and anchored in the opposite placed transducer holder.

When machining the transducer seat from outside the flow tube, the guide shaft and the tool head are mechanically connected outside the flow tube. Part of the flow head is then going through the opening in the transducer holder comprising the transducer seat being machined and continues through the flow tube to engage the opposite placed transducer holder.

According to an embodiment, the method comprises that each of the transducer holders comprising a guide hole and a guide hole wall,
the method further comprises the following steps:.

To guide the transducer seat-machining tool the transducer holders may comprise a guide hole. The guide hole is a narrowing in the opening in the transducer holder forming some kind of a throat in the opening. The guide hole comprising a guide hole wall, and the guide hole wall is machined by a guide hole machining tool. The guide hole-machining tool comprising two parts, the control rod and the guide shaft, the control rod enters the flow tube through the opening of one of the pair of transducer holders and the guide shaft enters the flow tube through the opening of the opposite placed transducer holder. The two parts are mechanical connected, for instance screwed together, when they meet inside the flow tube.

When the guide hole-machining tool has entered the flow tube and has been mechanical connected, the guide hole-machining tool is first machining the guide hole wall of the guide hole of one of the pair of transducer holders, and the guide hole-machining tool is then moved to machine the opposite placed transducer holder.

According to an embodiment, the method comprises that the guide hole-machining tool comprising a guide hole-cutting tool mounted between the control rod and the guide shaft, and machining the guide hole walls by the guide hole-machining tool,
the method further comprises the following steps:.

The guide hole-machining tool comprising a guide hole-cutting tool. The guide hole-cutting tool may be a part of the control rod or fixed to the control rod. When the control rod and the guide shaft are joined together, the position of the guide hole-cutting tool will typically be between the two parts.

When the guide hole-machining tool is mechanical connected with the control rod, going through the opening of one of the transducer holders, and the guide shaft going through the opening of the opposite placed transducer holder, the guide hole-machining tool is positioned, so that the guide hole-cutting tool is engaging the guide hole wall for one of the transducer holders. Then, when machining, the guide hole-machining tool is rotating and is cutting away material from the guide hole wall. When the machining is completed, the guide hole-machining tool is moved to a position, so that the guide hole-cutting tool is engaging the guide hole wall for the opposite placed transducer holder, and is then machining the guide hole wall of the opposite placed transducer holder.

With the guide hole-machining tool going through the openings of both transducer holders when machining, the guide hole-machining tool is supported and guided be the opposite placed transducer holder when machining the guide hole wall of a transducer holder. This ensures the guide holes will be precisely aligned to support the transducer seat-machining tool when machining the transducer seats.

When machining the guide hole, the guide hole-machining tool is rotating, and the guide hole-cutting tool is cutting away access material of the guide hole. A drill can be used for rotating the guide hole-machining tool. The guide shaft has a drill connector in the end of the guide shaft, to which a drill can be connected and the drill then can rotate the guide hole-machining tool.

According to an embodiment, the method comprises that the method further comprises the step casting the flow tube, the flow tube comprising transducer holders cast integrally with the flow tube.

The transducer holders may be cast integrally into the flow tube, so the flow tube is cast with the transducer holders. When casting the transducer holders integrally with the flow tube, the transducer seats are usually not perfectly flat and aligned and need machining.

According to an embodiment, the method comprises that the method further comprises the following steps:.

Alternatively, the transducer holders may be cast or otherwise manufactured separately and later welded into the flow tube. When welded into the flow tube, the transducer seat of the transducer holder may not be precisely aligned with coaxial axis with the opposite place transducer holder, and therefore the transducer seats needs machining, after the transducer houses is welded into the flow tube, to ensure precisely alignment to the coaxial axis.

In a second aspect, the invention relates to a ultrasonic flowmeter flow tube , wherein the flow tube comprising transducer holders, each transducer holder comprising an opening and a transducer seat for mounting of a transducer housing, each transducer holder forms a pair of transducer holders with an opposite placed transducer holder, and the pair of transducer holders holds a pair of transducer housings, the transducer seat comprising a mounting surface,.

The flow tube comprising a pair of transducer holders, in the transducer holders are mounted transducer housings, so the transducer housings forms a pair of transducer housings precisely aligned on a coaxial axis.

The mounting surface of the transducer seats are cut, so they are perpendicular to the coaxial axis.

According to an embodiment, the alignment of the pair of transducer houses deviates from the coaxial axis with less than <NUM> degrees.

It is usually quite difficult to obtain a coaxial axis between two opposite placed transducer housings in a flow tube. Because, when machining a transducer seat in a transducer holder, this is usually done individually before joining the transducer housing to the flow tube and therefore, there usually are a misalignment.

By machining the transducer seat, when the transducer holder is part of the flow tube, and using the opposite placed transducer holder as a guide and a support for the machining tool, a very high accuracy can be obtained in aligning the transducer housing to a coaxial axis.

The alignment may be so precise that the alignment of the transducer houses deviates from the coaxial axis with less than <NUM> degrees.

According to an embodiment, the flow tube comprising an additional pair of transducer holders holding an additional pair of transducer housings, and the additional pair of transducer holders comprising a coaxial axis.

A flow tube may often comprise an additional pair of transducer holders comprising an additional pair of transducer holders. This is an advantage because addition pairs makes it possible to measure flow speed different places in the flow tube giving better measurements of the flow in the flow tube. Additional pairs of transducer holders are machined by the same methods of this invention as described.

According to an embodiment, the plurality of coaxial axis for the pairs of transducer holders are parallel.

The pairs of transducer holders are having a parallel coaxial axis, so all pairs of transducer holders are precisely aligned. It is not just two transducer holders in one pair of transducer holders that are perfectly aligned on a coaxial axis, but also all pairs of transducer holders are aligned perfectly on a coaxial axis of rotation precisely parallel to the axis for the other pairs of transducer holders.

The transducer holders may be cast into the flow tube, so the flow tube is cast integrally with the transducer holders. Alternatively, the transducer holders may be cast separately and later welded into the flow tube.

The ultrasonic flowmeter flow tube and the method for manufacturing the flow tube according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

<FIG> illustrates an ultrasonic flow meter <NUM> comprising a flow tube <NUM>. The flow tube <NUM> comprising a flow passage <NUM>, an inlet <NUM> and an outlet <NUM> with a flow direction running from the inlet <NUM> to the outlet <NUM>. The flow tube <NUM> comprising a pair of opposite placed transducer holders <NUM>, <NUM>. Each transducer holder <NUM>, <NUM> comprising an opening <NUM> for mounting of a transducer housing <NUM>, <NUM>. The transducers housings <NUM>, <NUM> mounted in the pair of transducer holders <NUM>, <NUM> forms a coaxial axis <NUM> between transducer housings <NUM>, <NUM>. A gasket <NUM> between the transducer housing <NUM>, <NUM> and the flow tube <NUM> is sealing the flow tube <NUM>. The opening <NUM> comprising a guide hole <NUM> and a transducer seat <NUM> for mounting of the transducer housing <NUM>, <NUM>. The transducer seat <NUM> comprising a mounting surface <NUM>.

<FIG>, which is an enlarged section of the part encircled in <FIG>, shows one of the pair of transducer holders <NUM> with the transducer housing <NUM> mounted in the opening <NUM>. Further <FIG> shows the guide hole <NUM> and the transducer seat <NUM> and the mounting surface <NUM> of the transducer seat <NUM>. When mounted the transducer housing <NUM> rest against the mounting surface <NUM>, with the gasket <NUM> sealing the assembly.

<FIG> illustrates an exploded view of the ultrasonic flow meter <NUM> comprising a flow tube <NUM> and a pair of opposite placed transducer holders <NUM>, <NUM> with the transducer housings <NUM>, <NUM> mounted from inside the flow tube <NUM>. The transducer housings <NUM>, <NUM> are mounted in the transducer holders <NUM>, <NUM>. Each transducer housing <NUM>, <NUM> is sealed by a gasket <NUM>. When the transducer housing <NUM>, <NUM> is mounted in the transducer holders <NUM>, <NUM>, the transducer housing <NUM>, <NUM> is fixed with a nut <NUM> and a washer <NUM>. Each transducer housing <NUM>, <NUM> is mounted in the transducer seat <NUM> against the mounting surface <NUM>. Each transducer housing <NUM>, <NUM> is connected by a wire <NUM> to a controller (not shown).

<FIG> illustrates an exploded view of the ultrasonic flow meter <NUM> comprising a flow tube <NUM> and a pair of opposite placed transducer holders <NUM>, <NUM> with the transducer housings <NUM>, <NUM> mounted from outside the flow tube <NUM>. The transducer housings <NUM>, <NUM> are mounted in the transducer holders <NUM>, <NUM>. Each transducer housing <NUM>, <NUM> is sealed by a gasket <NUM>. When the transducer housing <NUM>, <NUM> is mounted in the transducer holders <NUM>, <NUM>, the transducer housing <NUM>, <NUM> is fixed with a nut (not shown). Each transducer housing <NUM>, <NUM> is mounted in the transducer seat <NUM> against the mounting surface <NUM>. Each transducer housing <NUM>, <NUM> is connected by a wire <NUM> to a controller (not shown).

The advantage of using a gasket <NUM> to seal the opening is that avoiding using an O-ring is reducing the risk of a leak over time.

<FIG> illustrates the use of the guide hole-machining tool <NUM>. The guide hole-machining tool <NUM>, also illustrated in <FIG>, comprising a control rod <NUM> and a guide shaft <NUM>. The guide hole-machining tool <NUM> also comprising a guide hole-cutting tool <NUM>. When using the guide hole-machining tool <NUM>, the first end <NUM> of the control rod <NUM>, is inserted through the opening <NUM> of the transducer holder <NUM>, and the first end <NUM> of the guide shaft <NUM> is inserted through the opening <NUM> of the transducer holder <NUM>. The guide hole-cutting tool <NUM> is mounted on the first end <NUM> of the guide shaft <NUM>, before the guide shaft is entered into the flow tube <NUM>. The first end <NUM> of the control rod and the first end <NUM> of the guide shaft are mechanically joined together inside the flow tube <NUM>. For instance the control rod <NUM> and the guide shaft <NUM> can be screwed together to form the guide hole-machining tool <NUM> with the guide hole-cutting tool <NUM> fixed between the two parts.

<FIG>, which is an enlarged section of the part encircled in <FIG>, shows an enlarged section of the transducer holder <NUM>, with the guide hole-machining tool <NUM> going through the opening in the transducer holder <NUM> ready to start machining the guide hole wall <NUM>. When the control rod <NUM> and the guide shaft <NUM> of the guide hole-machining tool <NUM> is mechanical connected, the guide hole-machining tool <NUM> is positioned in a position, wherein the guide hole-cutting tool <NUM> is engaging the guide hole <NUM> of the transducer holder <NUM>. The guide hole-machining tool <NUM> is machining the guide hole <NUM> of the transducer holders <NUM>. When finished machining the guide hole <NUM> in the transducer holder <NUM>, the guide hole-machining tool <NUM> is moved to a position, where the guide hole-cutting tool <NUM> is engaging the guide hole of the opposite placed transducer holder <NUM>, and is machining the guide hole <NUM> of the opposite placed transducer holder <NUM>.

The second end <NUM> of the control rod <NUM> is supporting the guide hole-machining tool <NUM>, when machining the guide holes <NUM> of the transducer holder <NUM> by being guided by the guide hole <NUM> of the transducer holder <NUM>. This ensures stability and precision in machining the guide hole <NUM>. When the guide hole <NUM> of the transducer holder <NUM> is to be machined, the guide hole-machining tool <NUM> is pushed through the openings <NUM> of the transducer holders <NUM> until the guide hole-cutting tool <NUM> is engaging the guide hole <NUM> of the transducer holder <NUM>. Now the stability and the precision of the machining is ensured by the second end <NUM> of the guide shaft being supported by the guide hole <NUM> or the transducer holder <NUM>.

The cutting is done by rotating the guide hole-machining tool <NUM>. The guide hole-cutting tool <NUM>, when being rotated, is cutting away excess material in the guide hole <NUM> being machined. The rotating may be done by using a drill. The guide hole-machining tool <NUM> may be connected to a drill by a drill connector <NUM> located at the second end <NUM> of the guide hole-machining tool <NUM>.

<FIG> illustrates the guide hole-machining tool <NUM> comprising the control rod <NUM>, the guide shaft <NUM> and the cutting tool <NUM>. The control rod <NUM> comprising a first end <NUM> and a second end <NUM>. The guide shaft <NUM> comprising a first end <NUM> and a second end <NUM>, and a drill connector <NUM> as part of the second end <NUM>. The cutting tool is fixed to the first end of the guide shaft <NUM>, so that the cutting tool <NUM> is placed between the guide shaft <NUM> and the control rod <NUM>. The two parts are inserted in the flow tube <NUM>, the control rod through one of the transducer holders <NUM>, <NUM> and the guide shaft through the opposite transducer holder <NUM>, <NUM>, and the two parts are then mechanical connected, when the two parts meet inside the flow tube, for instance by being screwed together.

<FIG> illustrates the use of the transducer seat-machining tool <NUM>. The transducer seat-machining tool <NUM>, also illustrated in <FIG>, comprising a tool head <NUM>, and a guide shaft <NUM>. The tool head <NUM> comprising a transducer seat-cutting tool <NUM>. In the shown embodiment, the tool head <NUM> is inside the flow tube <NUM>, entering the flow tube <NUM> through the inlet <NUM> or the outlet <NUM> before being mechanical connected to the guide shaft <NUM>.

When mechanical connecting the transducer seat-machining tool <NUM>, the first end <NUM> of the guide shaft <NUM> is inserted through the opening <NUM> of the transducer holder <NUM>, opposite to the transducer holder <NUM> to be machined. The tool head <NUM> is then, inside the flow tube <NUM>, fixed to the first end <NUM> of the guide shaft <NUM> and fixated with a screw <NUM>.

After the transducer seat-machining tool <NUM> has been mechanical connected, the transducer seat-machining tool <NUM> is positioned in a position, wherein the transducer seat-cutting tool <NUM> is engaging the transducer seat <NUM>, so that the transducer seat-machining tool <NUM> is machining the transducer seat <NUM>.

The tool head <NUM> is mechanically guided by a part <NUM> of the transducer seat-machining tool <NUM> engaging with the transducer holder <NUM> opposite placed to the transducer holder <NUM> to be machined.

When finished machining the transducer seat <NUM>, the tool is disconnected and mechanically reconnected with the guide shaft <NUM>, now inserted through the opening of the just machined transducer holder <NUM>, to make the transducer seat-cutting tool <NUM> engaging and machining the transducer seat <NUM> of the opposite transducer holder <NUM>.

The cutting is done by rotating the transducer seat-machining tool <NUM>. The transducer seat-cutting tool <NUM>, when being rotated, is cutting away excess material of the transducer seat <NUM> being machined. The rotating may be done by using a drill. The transducer seat-cutting tool <NUM> may be connected to a drill by a drill connector <NUM> at the second end <NUM> of the guide shaft <NUM>.

<FIG> illustrates the transducer seat-machining tool <NUM>, and <FIG>, which is an enlarged section of the part encircled in <FIG>, illustrates the tool head <NUM>. The transducer seat-machining tool <NUM> comprising a tool head <NUM>, and a guide shaft <NUM>. The tool head <NUM> comprising a transducer seat-cutting tool <NUM>. The guide shaft <NUM> comprising a first end <NUM> and a second end <NUM>. The first end <NUM> is inserted into the tool head <NUM>, for instance the first end <NUM> can be screwed into the tool head <NUM> and then fixed by a screw <NUM>. The second end <NUM> comprising a drill connector <NUM>, which can be used by attaching a drill to rotate the transducer seat-machining tool <NUM>, when drilling a transducer seat <NUM>. The tool head <NUM> comprising a guide bushing <NUM>.

When placing the transducer seat-machining tool <NUM> in a position to machine the transducer seat <NUM>, the guide bushing <NUM> is placed in the opening <NUM> of the transducer holder <NUM>, <NUM> to be machined. The guide bushing is supported by the guide hole <NUM>, while the guide shaft <NUM> is supported by the guide hole <NUM> of the opposite transducer holder <NUM>, <NUM>, obtaining a stable and precise processing of the transducer seat <NUM>, removing excess material from the transducer seat <NUM>.

Further, the tool head <NUM> comprising a stop bushing <NUM>. The stop bushing <NUM> ensures that the transducer seat <NUM> will not be too deep. When the transducer seat <NUM> is in the required distance from the inward end <NUM> of the transducer holder <NUM>, <NUM> the stop bushing <NUM> encounter the inward end <NUM> and can go no deeper. The inward end <NUM> of the transducer holder <NUM>, <NUM> is an integral part of the flow tube <NUM>, and the stop bushing <NUM> may, in an alternative design, be stopped by another part of the flow tube <NUM>.

<FIG>, which is an enlarged section of the part encircled in <FIG>, illustrates an alternative embodiment, where the tool head <NUM>, of the transducer seat-machining tool <NUM>, is placed outside the flow tube <NUM>. The transducer housings <NUM>, <NUM> in this embodiment is mounted from the outside, and therefore the mounting surface <NUM> of the transducer seat <NUM> is facing outward. The tool head <NUM> comprising a guide bushing <NUM>, the guide bushing <NUM> is extended to pass through both the guide hole <NUM> of the transducer holder <NUM>, comprising the transducer seat <NUM> to be machined, and the guide hole <NUM> of the opposite placed transducer holder <NUM>. A part <NUM> of the transducer seat-machining tool is engaging the transducer holder <NUM>, opposite placed to the transducer holder <NUM> to be machined, mechanically guiding the tool head <NUM>. The guide shaft <NUM> in this embodiment is used only to connect a drill to the drill connector <NUM>, to be able to rotate the transducer seat-machining tool <NUM> when machining the transducer seat <NUM>.

<FIG> illustrates a flow tube <NUM> comprising an additional pair of transducer holders <NUM>, <NUM>, so that the flow tube <NUM> comprising two pairs of transducer holders. In <FIG> is shown the transducer holder <NUM> of a first pair of transducer holders <NUM>, <NUM>, where the transducer holder <NUM> is not visible on <FIG>, but the second end <NUM> of the control rod is visible, going out of the opening of the transducer holder <NUM>. The second end <NUM> of the guide shaft is visible on the other side of the flow tube <NUM>, going out of the transducer holder <NUM> (not visible). Next to the transducer holder <NUM> another transducer holder <NUM> of a second pair of transducer holders is visible.

<FIG> shows a view of the flow tube <NUM> through the inlet <NUM>. It shows the first pair of transducer holders <NUM>, <NUM> and the second pair of transducer holders <NUM>, <NUM>. The guide hole-machining tool <NUM> is mechanical connected and going through the first pair of transducer holders <NUM>, <NUM>. When processing the guide holes of the first pair of transducer holders <NUM>, <NUM> is completed, the guide hole-machining tool <NUM> can be disconnected and moved to the second pair of transducer holders <NUM>, <NUM> and be reconnected to process the guide holes of the second pair of transducer holders <NUM>, <NUM>.

Claim 1:
A method of manufacturing an ultrasonic flow meter flow tube (<NUM>), wherein the flow tube (<NUM>) comprises transducer holders (<NUM>, <NUM>), each transducer holder (<NUM>, <NUM>) comprises an opening (<NUM>) and a transducer seat (<NUM>) for mounting of a transducer housing (<NUM>, <NUM>), and each transducer holder (<NUM>, <NUM>) is forming one part of a pair of oppositely placed transducer holders (<NUM>, <NUM>),
characterized in
that the method comprises the following steps:
- providing a transducer seat-machining tool (<NUM>) for machining the transducer seat (<NUM>) of one of the pair of transducer holders (<NUM>, <NUM>), the transducer seat-machining tool (<NUM>) comprising a tool head (<NUM>) mechanically connectable to a guide shaft (<NUM>),
- mechanically connecting the tool head (<NUM>) and the guide shaft (<NUM>),
- mechanically guiding the tool head (<NUM>) by a part (<NUM>, <NUM>) of the transducer seat-machining tool (<NUM>) engaging with the transducer holder opposite placed to the transducer holder to be machined,
- machining the transducer seat (<NUM>) of the transducer holder (<NUM>, <NUM>) to be machined with the tool head (<NUM>);
each of the pair of transducer holders (<NUM>, <NUM>) comprises a guide hole (<NUM>), and the tool head (<NUM>) comprises a guide bushing (<NUM>), the method further comprising the following steps:
- guiding the tool head (<NUM>) of the transducer seat-machining tool (<NUM>), ensuring a surface of the transducer seat (<NUM>) is perpendicular to a coaxial axis (<NUM>), by
∘ positioning the guide bushing (<NUM>) in the guide hole (<NUM>) of the transducer holder (<NUM>, <NUM>) to be machined, and
∘ positioning the guide shaft (<NUM>) or the guide bushing (<NUM>) in a position engaging the guide hole (<NUM>) of the opposite placed transducer holder (<NUM>, <NUM>).