Emerson Transmitter 350 Mode D’Emploi

A high-speed microcontroller is utilized in the 

design to process and calculate the elapsed time 

measurement.  Accurate crystals are used for the 

time based to resolve sub-microsecond timing 

increments.  The binary number, equivalent to the 

microseconds of the echo travel time is used to 

calculate the distance of the float and a 

corresponding digital signal is output.  A basic block 

diagram describing the operation is shown below.

Figure 7.  Basic Transmitter Block Diagram

Calibration routines are included in the software to 

the 0% and 100% points for any distance desired.  

Even reverse calibration is a simple task using the 

software routines.  Reverse calibration is desirable if 

ullage instead of level is required, or when the 

probe is installed with bottom mount electronics.  

The LTM-350 transmitter has four output 

configurations. 

1. Primary Level – 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 

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 

a number representing the distance and output 

as a digital signal. (LTM-250/350)

2. Primary Level and Interface Level – A second 

float may be added below the first, and the 

second output will be calibrated automatically.  

The second time interval is timed in the same 

manner as the first one and added to the first to 

derive the position of the heavier float. The two 

floats require a separation of approximately 

three inches.  The float size, geometry, and 

magnetic strength all play a factor in how close 

the two floats can be without interfering with 

each other.

3. Primary Level and Temperature – An optional 

temperature sensor is embedded inside the 

bottom tip of the probe, and it is configured to 

be the third digital output of the transmitter, 

and comes factory calibrated for the operating 

range of  -50C to 149C (-58F to 300F)

Temperature – This options is called a ‘full-

blown” unit and offers all three possible 

outputs.

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