Azbil SLX 110, User Manual

Page: 41 / 168

Model SLX 110/120 - Smart displacement type level transmitter 2-1
Chapter 2 : Operation
2-1 : Principles of operation
Figure 2-1 is a schematic structural drawing of the SLX.
One end of the torque tube is fixed to the torque tube housing with a fixing screw and
the other end is assembled with a torque arm supported by a knife edge fulcrum. At
the time of installation or operation, the float is suspended from the end of the torque
arm and the SLX is used with the torque tube twisted by the weight of the float.
Figure 2-2 is a signal block diagram of the SLX.
When the level of a process liquid changes, a buoyant force is produced on the float
according to Archimedes' principle (the displacement type float is heavy and the
level of the float shows little displacement even if the level of the liquid changes. In
general, the instrument is designed to meet: “buoyancy with liquid level 100% < self
weight of float”).
The buoyant force generated in proportion to the liquid level is converted to torque
by the torque arm/knife edge from which the float is suspended and transmitted to the
torque tube. The torque tube has a dual-function as sealing of the process liquid and
torsional spring and converts torque to an angle. This angle is detected by an angle
sensor and further A/D-converted and sent to the CPU as a liquid level signal.
When the torque tube contacts the process liquid, the torsional spring constant
changes because the modulus of transverse elasticity of the torque tube material
changes with the liquid temperature, producing an output shift. To compensate this
output shift by the liquid temperature, the temperature near the torque tube is detected
by the temperature sensor and further A/D-converted and sent to the CPU as a
temperature signal.
The liquid level signal and temperature signal are processed by the CPU into a digital
signal that fits into a set range. This value is converted to a 4 to 20 mA DC analog
signal by a D/A converter and then outputted.
The SLX also provides parameters that can be set by the SFC to compensate the
output shift with the liquid temperature (these are set to default values at the time of
shipment). This is because some applications cannot be compensated enough with
default values alone. See “3-3 : When output shift is large” on page 3-8, for details.