Emerson LTM-300 Instrument Description Transmitter Detailed Description, Theory of Operation

SECTION 2. Instrument Description

2.0 Transmitter Detailed Description

The LTM-300 is an assembly of two major components:

•The Sensor Tube Assembly. This 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 detector electronics. The tube is made to lengths of 2-30 ft. in rigid construction.

•The Electronics Housing. The extruded aluminum housing has two separate compartments. One side contains the microprocessor board assembly and calibration push buttons. The other side contains the wiring termination board. The electronics module is connected to the detector board of the sensor tube assembly via a plug-in cable.

The main board is surface mounted component construction utilizing the latest in integrated circuit technology. It contains a high-speed micro controller with a HART modem, D/A Converters, A/D Converter (for optional temperature) and all other accessory components.

2.1 Theory of Operation

The LTM-300 Level Transmitter is based on the principle of magnetostriction first used for digital delay lines and later for precision distance or displacement in the machine tool industry. The 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 liquid level measurement application, however, a resolution of 0.01 inch is more that 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 light. Another magnetic field generated by a permanent magnet, placed near or around the wire at some distance from the point of entry of this pulse, interferes with the magnetic field of the pulse and torsional force results at the collision point.

Calibration routines are included in the software to scale the 4 and 20 mA points for any distance desired. Even reverse calibration is a simple task using the software routines. Reverse calibration is desirable if ullage instead of full level is required, or when the probe is installed with bottom mount head. See Section on Calibration for further details.

The LTM-300 transmitter can have as many as three outputs. The first is a 4-20 mA output, the second and third are digital outputs. The LTM-300 is available with the following output configurations:

All LTM-300 Units have HART as a standard.

1.Primary Level. Single output version with only one variable that will output a 4-20mA signal for level.

2.The second and third outputs are digital and can be configured to measure temperature and/or interface level. The digital outputs are read via HART.

1.Primary Level Transmitter. The most basic version of this transmitter, is that it computes the distance between the float and the detector from the elapsed time measurement. A specific time window becomes active only for a short time after the interrogation pulse is applied to the waveguide. Any feedback signal, received before and after this window, is rejected as noise. Even signals received during the active window are evaluated and filtered so that only high integrity data is accepted.

The conditioned signal is converted to a percent of full-scale number and written to the D/A Converter. The scale is defined by the calibration procedure and it corresponds to the output span (4- 20mA) of 16.00 mA.

A deadband, corresponding to approximately six inches next to the detector, is fixed in the software and the float is not permitted to enter this area. If this happens, readings may be erratic or the output may go to FAIL.

The effect of this torsion force is to twist the wire at this point producing a torsion wave traveling towards both ends of the wire. The propagation time 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.

A high-speed micro controller is utilized in the design to process and
calculate the elapsed time measurement. Accurate crystals are used
for the time base to resolve sub-microsecond timing increments. The
binary number, equivalent to the microseconds of the echo travel time,
is written to an output D/A Converter and subsequently converted to
a 4-20 mA signal proportional to the item measured. The larger the
number of microseconds there are, the greater the distance of the
float from the head of the transmitter.	2

Basic Level Transmitter Simplified Block Diagram

Facilities are provided to field calibrate the range of the 4-20 mA output using the actual position of the float and pressing a push- button on the front panel to set the 4-20 mA point.

Provision, accessed through HART or front panal, is made for a FAIL mode to High (20.8 mA), Low (3.75 mA) or “Hold last Value.”

A HART modem enables the transmitter to communicate serially over the 4-20 mA DC signal with a host computer or a handheld terminal.

2.Level/Temperature. An optional temperature sensor is embedded inside the bottom tip of the probe, and it can be calibrated to give the temperature of the liquid in the tank on the second or third

The sensor is a 1000 ohm platinum RTD type and its resistance is converted to a binary signal by a high resolution A/D Converter. The temperature range is set to order and stored in non-volatile memory before shipment.