Emerson Transmitter LTM-300 Mode D’Emploi

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.

 

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.

 

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.

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.  

2

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:

 

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.

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.