TD80™ Level Gauging and Overfill Prevention System Product Manual
Page 8 TD80 Introduction/ Rev. 2, August 4, 2015
The TD80 contains a table describing the tank depth and volumetric
characteristics. This table is programmed before installation and operation on
the tank. Liquid level is calculated from the table using maximum depth of the
tank and distance from the top to the liquid surface. The TD80 “sees” the
depth of air space in the tank and calculates the loaded volume.
Physical limitations of GWR create a dead band at the bottom of the probe
where the transmitted pulse interferes with the reflected echo. This region of
dead band is considered to be the bottom 5.5” of the tank. Any liquid level
measured at 5.5” or less is indicated by the Finch as “2 LO”, meaning “too
low”. An upper dead band exists at 7.5” from the top on the dual rod and 2.5”
on the coaxial probe. Level measurements within the dead bands are
inaccurate and unreliable.
Calculated volume is transmitted to a Finch Display by SV Bus while a
linearly scaled level is sent by the optional 4-20mA interface. The Finch is
able to display up to 4 numeric digits, including a decimal point. This numeric
display correctly indicates volume of loaded product in the tank. The 4-20mA
output is scaled linearly for a volume of 0% at an output of 4mA and 100%
volume at 20mA.
SV Bus
SV Bus is a single wire serial digital communication interface with an error
checking protocol that eliminates false communication between the TD80
transmitter and Finch Display. SV Bus is one of the three wires required for
TD80 operation with the Finch. The other two wires are power and ground.
This bus is designed for reliable communications in stationary and transport
environments.
Finch Display
The Finch Display receives a continuous stream of volume information and
alarm states from the TD80 on the SV Bus. Volume, alarm states and errors
are shown on the 4 digit numeric display. An additional alarm is controlled
directly by the Finch. Alarms and errors control three separate relays to
signal or operate external devices.
TD80 controlled relays indicate Spill, High-High and system Fail alarms to
external devices such as overfill prevention valves, lights, horns and
stationary loading controls. The Finch controlled alarm and relay indicates a
preset, increasing Fill or decreasing Fall level to warn of an approaching
operator action while loading or unloading product.
The Fill/Fall and High-High Alarms remain active until acknowledged by the
operator. The Spill alarm remains active until the spill condition is cleared by
unloading product below the spill alarm level while the system is powered
and monitoring the changing level.
Alarms are acknowledged by button presses on the Finch or a push button
connected to the Alarm Acknowledge input.
TD80 reported errors or malfunctions are indicated by the Finch showing
“E xx”, where xx is an error code and activating the Fail alarm.
Finch reported loss of communication with the TD80 is indicated by showing
“----” and activating the Fail alarm.
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