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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.
Configuration options must be chosen at
quoting stage.
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)
LTM-350, via HART protocol only.
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)
4. Primary Level, Interface Level, and
Temperature
– This options is called a ‘full-
blown” unit and offers all three possible
outputs.
A deadband of approximately three inches,
next to the detector, is fixed in the software
and the float is not permitted to enter this
area. If this happens output readings maybe
erratic or go to fail mode.
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