DE 43 30 745 Cl describes an ultrasonic transducer having a cylinder-shaped housing with a plane bottom on which the piezo-electric ceramic disc bears. The bottom of the housing has a circle-shaped groove along the housing wall for admis- sion of a soldering for electrical connection to one of the terminals of the piezo-electric ceramic disc. The fact that the groove extends along the wall causes that the piezo- electric ceramic disc can be placed without considering the placing of the soldering point. However, the solderings will put a limit to the maximum temperature, which the transducer can stand.

From DK 93 00 409 U4 is known an ultrasonic transducer in which a piezo-electric disc is separated from the medium pressure only by a thin metal film, thus transmitting the me- dium pressure to the piezo disc. For this purpose a support of the rear of the disc is required, as here the support will be imperfect due to the need for a clearance for the electric cables. Even small medium pressures may damage the piezo disc, causing not only a stop of the measuring function but also the risk of medium escaping through the transducer.

It is the purpose of the invention to produce a simple and cheap ultrasonic transducer, which can resist high medium pressures and which functions perfectly over a large tempera- ture interval, the ultrasonic transducer comprising only a few components.

This task can be solved with an ultrasonic transducer of the type described in the introduction, if it is made so that in a non-connected state the spring presses the conducting means to bearing against a locking means with contact to the hous- ing, thus short-circuiting the piezo-electric ceramic disc, whereas at connection of a cable the conducting means is pressed free of the locking means, thus forming an electrical connection between the cable and the piezo-electric ceramic disc.

This causes that the electrical connection to the piezo- electric ceramic disc takes place without soldering, and the ultrasonic transducer is short-circuited when the cable is removed, and high voltage cannot be built up over the piezo- electric ceramic disc. Thus, circuits for the protection against high voltage become unnecessary, and any damages of connected electronics are avoided. Also the risk of electric shocks on the electrician fixing the cables on the transducer is eliminated.

The short-circuiting between the conducting means and the locking means can be effected with a conducting sealing clos- ing the transducer tightly towards the surroundings. This en- sures that foreign matters are prevented from reaching the inside of the transducer, when the cable holder is dis- mounted. Thus the transducer housing can be welded onto a pipe construction long before fitting the rest of the flow transmitter.

The thickness of the housing bottom can be adapted to the ul- trasonic wave length of the housing material. Thus the piezo disc oscillates on a resonant frequency of the housing bot- tom, so that transmission of oscillations through the bottom takes place with small losses.

The cable can be mounted in a holder screwed into the hous- ing, said holder having means for cable force release and forming the first electrical connection from the first con- ductor of the cable to the housing by bearing of a flange on the locking means, whereas the second conductor of the cable forms the second electrical connection through a central pro- jection on the holder, the projection of the holder and the centre electrode pressing the disc away from the locking means while screwing in the holder at simultaneous compres- sion of the spring. This gives a simple and efficient cable connection without the use of soldering. Avoiding soldering in the ultrasonic transducer makes it useful at high tempera- tures in which solderings would melt.

The projection of the holder can be made by an isolator disc with a central opening for admission of the second conductor of the cable and of a pressure shoe, which is conducting and which also has a central opening for the admission of the second conductor of the cable. This gives a direct connection from the second conductor of the cable to the conducting means and on through the spring to the piezo disc.

The flange of the holder can be made with a neck with an opening for admission of the second conductor of the cable and the cable isolation, whereas the first conductor of the cable extends externally on the neck of the flange, whereby the flange neck and the first conductor can be surrounded by a pipe.

Advantageously, the cable has a first section, which is high temperature resistant, said first cable section being con- nected with a second cable section by way of a cable assem- bly, which contains an electronic circuit. This means that the electronic circuit can be removed from the ultrasonic transducer, which can then be placed in very high ambient temperatures.

Advantageously the electronic circuit of the cable assembly comprises a coil for impedance adaptation and for adaptation of a resonant system. Thus the piezo-electric ceramic disc together with the coil forms an oscillation circuit, whose resonant frequency coincides with the signal with which the oscillation circuit is supplied.

The electronic circuit can have means for both amplification and impedance conversion, in the direction of both the trans- ducer and the connection cable. Thus the length of the cables between the electronic unit and the transducer is uncritical.

Drawing description: In the following the ultrasonic transducer is described on the basis of the drawings, showing: Fig. 1 a section through a transducer housing Fig. 2 a section through a transducer with fitted cable Fig. 3 cable connection and cable assembly of the transducer.

Fig. 1 shows a transducer housing 2 with a bottom 3 forming a window for ultrasound, which is generated by a piezo-electric ceramic disc 4, which is retained to bearing against the bot- tom 3 by a compression spring 5. The spring 5 is led into a spring holder 6, and opposite to the piezo-electric disc 4 the spring 5 is bearing on a conducting means 7 pressed to bearing against a locking means 8 by the spring.

The first electrical connection to the piezo-electric ceramic disc 4 is formed by means of the housing 2 and the bottom 3.

The second electrical connection to the piezo disc 4 is formed by means of the spring 5, which extends in the isolat- ing spring holder 6.

As shown in Fig. 1 there is bearing between the conducting means 7 and the locking means 8, the piezo disc 4 thus being short-circuited. This prevents the formation of high voltage over the piezo-electric ceramic disc, which would occur at temperature changes. A short-circuiting during transport and storing ensures that the electrician, who will eventually fit the cable, will not get an unpleasant and dangerous surprise in the form of an electric shock.

Fig. 2 shows the ultrasonic transducer with fitted holder 13.

The cable 9 has a first conductor 10, an isolation 11 and a second conductor 12. While screwing in the holder 13 a spring washer 18 presses the first conductor 10 to bearing against a conducting flange 14. The first conductor can also be fixed by means of a pipe 15, which is clamped around the cable 9 after fitting. The second conductor 12, which is isolated, passes through the flange 14 and on through an isolator 16 to contact with a thrust pad 17, the second conductor 12 here shown tucked away. The cable is further retained by means of a bush 19 clamped around the cable 9 by means of a union nut 20.

While screwing in the holder 13, the thrust pad 17 and the tucked away end of the second conductor 12 will form a bear- ing against the conducting means 7. Continued screwing in of the holder 13 will cause a flexion of the spring 5. This re- moves the short-circuiting between the conducting means 7 and the locking means 8, and the ultrasonic transducer gets func- tional.

Fig. 3 shows the cable holder 13 in the dismounted state- the cable 9 is fixed on the flange 14, the spring washer be- ing shown in a not tightened state, as the spring washer is only tightened and clamping the first conductor of the cable against the flange 14 while screwing in the holder 13. A sub- sequent tightening of the union nut 20 will cause that the cable bush also tightens the cable 9. The cable 9 leads to a cable assembly 21, which may comprise an electronic circuit 22. From the electronic circuit 22 and the cable assembly 21 another cable 23 leads on towards the electronic calculation circuit, with which the ultrasonic transducer co-operates.

The electronic circuit 22 may consist of a coil for impedance adaptation and for obtaining an oscillation circuit between the coil and the piezo-electric ceramic disc 4. The elec- tronic circuit may also contain circuits for amplification and impedance conversion, both against the cable 9 and against the cable 23. The cable 23 can supply the required current for the electronic unit 22 simultaneously with the transfer of signals. By making the final signal treatment close to the ultrasonic transducer, the most expedient trans- mission form can be chosen via the cable 23, which can thus obtain a very large length, if required.

The ultrasonic transducer is particularly suited for high pressures, as the bottom 3 together with the housing 2 can resist very high ambient pressures, and the embodiment of the ultrasonic transducer may make it particularly suited for ex- tremely high temperatures, as at no time the cable connection requires soldering. The upper temperature limit will be de- termined by the cable used. Thus, ultrasonic transducers can work at temperatures of 400 to 500°, if the cable 9 is a spe- cial high temperature cable, whereas the cable 23 can be of an ordinary type, as this cable is assumed to be used where the ambient temperatures are relatively low.