Download Design of an electronic barometer

Design of an electronic barometer
Design Team: Department of Physics, Univerisity of Zimbabwe
Names:
Dr X F Carelse
Dr E Mashonjowa
Mr J Muganda
The Aneroid Barometer
An aneroid barometer uses a small, flexible, evacuated metal box called an aneroid cell (capsule). The
evacuated capsule is prevented from collapsing by a strong internal spring. Small changes in external air
pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that
the tiny movements of the capsule are amplified and displayed on the face of the dial and forms the basis of the
aneroid barometer.
The aneroid barometer is accepted as an industry standard for monitoring atmospheric pressure. It is small,
robust and easily transported. However, the mechanism to measure the movement of the walls of the capsule is
probably the most expensive part of the barometer and is also the part that is most likely to wear out and
become faulty.
In our design, the movement of the walls is detected electronically without the need for any mechanical parts.
The electronic detector is a Hall device.
The Design Concept
The electronic barometer is based on an aneroid capsule. A coil, carrying a current, is attached to the rear of an
aneroid capsule. The magnetic field, which is created along the axis of the coil, is at right angles to the walls of
the capsule. A Hall device is attached to the front wall of the capsule. When the atmospheric pressure changes,
the front wall moves relative to the rear wall. This movement causes the wall of the capsule to be detected by a
Hall device in terms of its position along the magnetic field. The output from the Hall device is a measure of
the expansion and contraction of the capsule and is therefore a measure of the changes in the atmospheric
pressure.
Aneroid
capsule
Coil moved along
coil-holder until
x=R/2
Coil holder fixed
to rear of capsule
Coil
R
Hall sensor fixed to
front of capsule
R
x
Coil holder
Fig. 1: Mechanical structure of the barometer
Fig. 1c shows the basic structure of the electronic barometer. It is made up of the aneroid capsule (cell), a coil
(of radius R), and the Hall device. The aneroid capsule contracts and expands with a changing atmospheric
pressure. A coil is attached to the aneroid capsule in such a way that when the capsule contracts at the point of
attachment and moves inwards, the coil moves with it also. The same reaction happens when the capsule
expands. The coil is connected to a direct current source which generates a magnetic field B in the direction
shown by the arrows coming out of the coil. A Hall device is placed a distance x from the coil. When the
capsule expands, the Hall device moves away from the coil and detects a decrease in the magnetic field. This
generates a change in the output voltage of the Hall device. When the capsule contracts, the device moves
towards the coil and the magnetic field strength reaching the Hall device increases. Hence there is a relationship
between the magnetic field strength and the position (x) of the Hall device. The change in the output voltage is
therefore a measure of the change in the atmospheric pressur.
R
coil
x
P
Fig. 2: The magnetic field B at a distance x from the coil
The Magnetic Field
It can be shown that the magnetic field, B, at P, a distance x along the axis of the coil, is given by
1.1
=
(
)
Where R is the radius of the coil, I is the current through the coil, x is the distance from the coil and B is the
magnetic field at point P at the point where the Hall device is placed..
The rate of change of magnetic field strength with respect to distance is,
=
1.2
(
)
It is to the advantage of the design to place the device at the distance which experiences the maximum rate of
change of the field with displacement. By taking the second derivative, it can be shown that optimum distance
is R/2.. This means that the maximum
occurs at
= .
The Sensitivity of the Sensor
The sensitivity of the sensor depends on the change of output voltage, ∆Vout, of the Hall device caused by the
change in atmospheric pressure, ∆p. An aneroid capsule that has been removed from a commercial aneroid
barometer in our possession has been examined to discover the relation between ∆x, the expansion of the
capsule, and the change of pressure ∆p. By pressing on the wall of the capsule and noting the movement of the
pointer across the dial, it was shown that a movement of 0.5mm produced a change of approximately50
millibars as indicated by the pointer. This was adopted to provide an approximate indication of the sensitivity
of the capsule. Therefore
is equal to 100 millibars per millimeter of displacement. In other words, a
change of 1 millibar will cause a displacement of 0.01 mm.
The following measurements could be assumed for the aneroid cell:
Diameter of the cell = 20 mm
That would place the diameter of the coil at about 24 mm, for convenience. The prefered distance of the Hall
device from the coil would then be 6 mm. Using equation 1,2, the rate of change of B at this distance with
displacement, would be
dB
dx
3 x 6 x µo x I x R2
=
2 x (122 + 62)5/2
= 3.75 x 10-3 x I
T/m
-3
= 3.75 x 10 x I
where
mT/mm
I = current in the coil
The sensitivity of the Hall device, Allegro A1301, which was used is 25mV/mT. per applied volts. If the
applied voltage is 5V, the sensitivity will be 125 mV/mT. We may also assume that
I = 1 ampere
∆V = 3.75 x 125 x 10-3
Therefore,
mV/mm
= 0.46
mV/mm
As the displacement of the capsule wall is 0.01 mm/millibar, the change in output voltage relative to a change in
atmospheric pressure will be
∆V/∆p = 4.6 x 10-3
or
mV/millibar
µV/millibar
= 4.6
A more sensitive Hall-sensor that may also be used is an A1325 which has a sensitivity of 31.25 mV/mT. The
specifications of this sensor are shown in Table 1.
By-pass Capacitance
0.1uF
Sensitivity
31.25mV/mT
Supply Current
6.9mA
Quiescent Voltage Output
2.425V to 2.575V
Output Load Resistance
4.7kiloohms
Output Referred Noise with Ta=25°C
4.4mV (peak to peak)
Maximum Junction Temperature
165 °C
Operating temperature range
-40 °C to +165 °C
Forward Supply Voltage
8V
Reverse Supply Voltage
-0.1V
The Circuit Design
Numerical
display of the
atmospheric
pressure
R1
PIC16F872
AD620
A1325
R2
Fig. 3 Circuit design
Cost of Components
Table 2: Cost requirements for the sensor
Item Description
Quantity Price for 100
units
Item cost
1
Hall-sensor, A1325
1
$ 4.00
$ 4.00
2
Microcontroller, PIC16F872
1
$ 2.00
$ 2.00
3
7 Segment display
1
$ 0.54
$ 0.54
4
Operational amplifier, AD620
1
$ 2.00
$ 2.00
5
Resistors, Capacitor, PCB and hardware
Various
6
Magnetic coil
1
$ 5.00
$ 0.50
$ 0.50
Conclusion
The maximum range in the change of atmospheric pressure that could normally be expected is about 100
millibars. Therefore the maximum expected change in output is about 0.46 mV. If a microcontroller is used to
measure this voltage, the amplification should not be much more than 10,000. This would require an
operational amplifier with very low offset voltage, possibly about 40 µV, which is available with the AD620
operational amplifier. The offset voltage of this operational amplifier can be externally adjusted to be less than
4 µV. It is suggested that this operational amplifier be used as a preamplifier with a gain of 100, followed by a
second operational amplifier with a further gain of 100. With such an arrangement, the final sensitivity of the
electronic barometer would be 46 mV/millibar with an expected error of less than 1% at full range.. This is a
very low cost sensor and should cost less than $25 including the PCB and other hardware.