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3–303
Motorola Sensor Device Data
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Open Pipe Before
Calibration
Control Module
Pressure
Sensor
Air
Liquid Level
in the Pipe
Electrical
Pump
AC Line
Reference
Level
H
Figure 3. Liquid Level Monitoring
LEVEL SENSING THEORY
If a pipe is placed vertically, with one end dipped into a liquid
and the other end opened, the level in the pipe will be exactly
the same as the level in the tank. However, if the upper end of
the pipe is closed off and some air volume is trapped, the
pressure in the pipe will vary proportionally with the liquid level
change in the tank.
For example, if we assume that the liquid is water and that
the water level rises in the tank by 10 mm, then the pressure
in the pipe will increase by that same value (10 mm of water).
A gauge pressure sensor has one side connected to the
pipe (pressure side) and the other side open to ambient (in this
case, atmospheric) pressure. The pressure difference which
corresponds to the change in the tank level is measured by the
pressure sensor.
PRESSURE SENSOR CHOICE
In this example, a level sensing of 10 mm of water is desired.
The equivalent pressure in kilo pascals is 0.09806 kPa. In this
case, Motorola’s temperature compensated 0–10 kPa,
MPXM2010GS is an excellent choice. The sensor output, with
a pressure of 0.09806 kPa applied, will result in 2.0 mV/kPa
x 0.09806 = 0.196 mV.
The sensing system is designed with an amplifier gain of
about 1000. Thus, the conditioned signal voltage given by the
module is 1000 x 0.196 mV = 0.196 V with 10 mm – H2O
pressure.
Table 1. Liquid Level Sensors
METHOD
SENSOR
ADVANTAGE
DISADVANTAGES
Liquid weight
Magnetoresitive
Low power, no active electronic
Low resolution, range limited
Magnetoresitive
Very high resolution
Complex electronic
Ultrasonic
Easy to install
Need high power, low accuracy
Liquid resistivity
No active electronic
No active electronic
Low resolution, liquid dependent
String potentiometer
Potentiometer
Low power, no active electronic
Poor linearity, corrosion
Pressure
Silicon sensors
Inexpensive good resolution, wide
range measurements
Active electronic, need power
F
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