Level Measurement

Keywords – Pressure, Gauge and Absolute, Density, Piezoresistor, Wheatstone Bridge, Instrumentation Amplifier, Gain of Amp, Accuracy, Non linearity

1.  Define the following terms:

gauge pressure

absolute pressure

differential pressure

A tank has a diameter of 3 m and a height of 5 m.  A level sensor is installed on the wall of the tank at a height of 0.3 m above the tank floor.  What are the gauge and absolute pressures at the sensor when the tank is full of water?

2.  Convert a pressure of 8 mbar into Pascals and into psi.

What are the gauge and absolute pressures in psi from the previous question?

3.  Explain what is meant by the term piezoresistor.  How does it compare to the electrical strain gauge?

A pressure sensor made up of 4 piezoresistors in a Wheatstone Bridge arrangement is installed at the base of a tank.  The tank was filled the tank with water and measured the voltage out from the bridge at different levels to get the following data:

 Level (mm) 0 82 205 287 369 Vout (mV) 0.7 2.4 5.8 7.8 9.7

Plot the above data and from the graph determine:

Offset

Sensitivity

Maximum non linearity

The stated sensitivity of this device from the manufacturer’s data sheet is 10 mV/psi.  How does this compare to the measured sensitivity?

4.  It is required to build an instrumentation amplifier to condition the signal from the device in question 3 to give an overall sensitivity of 1 mV/mm.

(i) What should the gain of the amplifier be?

(ii) Draw a circuit diagram of the instrumentation amplifier and the Wheatstone bridge.  Show where the Wheatstone bridge is connected to the amplifier circuit.

(iii) Specify the sizes of all the resistors in the instrumentation amplifier.

5. A level measurement system consists of a piezoresistive pressure sensor (sensitivity 10 mV/psi) connected to an instrumentation amplifier which is configured to give a 0 to 5 V output for a 0 to 200 mm H2O input on rain level.  This is in turn connected to an input pin on a PIC microprocessor that performs an 8 bit ADC (analog to digital conversion).  This result is then converted to engineering units using some arithmetic in the PIC.  Draw a block diagram of this system showing:

• Units of all inputs to and outputs from each block.
• The ranges of all inputs to and outputs from each block.
• The sensitivity of each block
• The overall sensitivity (i.e show that all the individual sensitivities multiplied by each other equals 1).