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Thursday, 30 November 2006 |
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Problem: Measure the level of fluid in an opaque metal tank with reasonable accuracy and with no moving parts.
Solution: Using a standard piezo resistive pressure transducer sealed onto an open ended pipe that is immersed into the fluid to be measured. Because the trapped air in the pipe will be compressed by the head of fluid a proportional reading can be made.
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The fluid will rise up the sensor tube until the
pressure of the trapped air is in equilibrium with the pressure head
caused by the fluid level, in this respect this is like a 'U' tube
manometer with one end closed and pressure applied to the open end.
The hydrostatic pressure caused by a column of liquid is given by the following equation:
P = pgh
where P is pressure in Pascal's
p is density of liquid ( fuel oil is 890 kg/m3 )
g is gravitational constant ( 9.81 ms-2 )
h is the height of the column ( in meters )
..so for a column of 450mm we get a pressure of about 3900 Pa which is approximately 0.56 psi.
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The sensor is a resistive bridge type and requires 10v excitation and outputs about 0.035v for 0.5psi, the sensor I bought has a positive offset of about 9mV and 44mV full scale for 0.5psi, so as the electronics needs to run from a single 5V supply the following circuit was devised, the -5V is generated on-board to provide the supply for the bridge and an op amp is used to generate a single 0 to 5V (3.5v with the given resistors ) signal from the differential output of the pressure sensor.
Pin connections
X1-1 +5V
X1-3 Level Signal
X1-4 0V Signal Ground
J1-1 Sensor Vs(+)
J1-2 Sensor Output(+)
J1-3 Sensor Ground(-)
J1-4 Sensor Output(-)
Artwork & Circuit Layout
Here's the completed sensor module minus a housing but with a 1/4 inch enots compression fitting
Here's the results of the preliminary testing, without any temperature compensation, the results look very promising.
PCB Artwork, print using Adobe Acrobat at 1:1 scaling
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Last Updated ( Saturday, 09 December 2006 )
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