Emerson 250, 350 Transmitter Description, Detailed Description, Technology Theory of Operation

Section
3	Transmitter Description

3.0 Detailed Description

The LTM-Series is an assembly of two major components:

The Sensor Tube Assembly: This is a 5/8” diameter stainless steel probe, sealed on one end, with the magnetostrictive waveguide in its center. In addition to the magnetostrictive waveguide, the tube also houses the optional temperature sensor and the sensing elements. The tube is made to lengths of 2-30ft. in rigid construction.

The Enclosure and Electronics: The extruded aluminum housing has two compartments. The enclosure is rated NEMA 4X and 7. One side contains the microprocessor board assembly and calibration push buttons. The other side contains the field wiring termination board. The electronics module is connected to the detector board of the sensor tube assembly via a plug-in cable. The electronics module houses printed circuit boards (PCB) that encompass surface mount component construction utilizing the latest integrated circuit technology.

Magtech also has a stainless steel enclosure that can be utilized. Please contact factory for further details.

3.1 Technology – Theory of Operation

The LTM-250/350 series level transmitters are based on the principle of magnetostriction, first used for digital delay lines and later precision distance or displacement in the machine tool industry. This principle, if designed and applied properly, has potentially very high measurement resolution, typically better than 0.001 inch. In the machine tool industry such a high resolution is desirable. In the level measurement application, however, a resolution of 0.03 inch is more than adequate.

In a brief description, the magnetostrictive principle consists of a wire extruded and heat treated under carefully chosen conditions to retain desired magnetic properties, which is pulsed by a circuit with a relatively high current pulse. The high current pulse produces a circular magnetic field as it travels down the wire at the speed of sound. Another magnetic field generated by a permanent magnet (the float), placed near or around the wire at some distance from the point of entry of this pulse interferes with the magnetic field of the current pulse and a torsional force results at the collision point.

Figure 6. Principle of Operation

The effect of this torsion force is a twist to the wire at this point producing torsion wave traveling towards both ends of the wire. The propagation time (or time-of-flight) of this wave is measured precisely and if the wire properties remain stable, it is very repeatable at about 5-10 microseconds per inch, which is approximately the speed of sound in that medium. By measuring the exact number of microseconds it took the torsion wave to reach a designated termination point of the wire, the distance to the magnet from this termination point can be easily calculated.

5