The Ion Detector
Ion Detection
A different type of device intended to respond to the ions present in the
atmosphere consists of a rectangular metal tube containing an insulated central
conductor which is connected to a meter. This responds to either positive or
negative ions depending on the potential applied to the tube.
In use either +5 volts or -5 is used and the output is connected to the input
of the meter interface. The high voltage power supply is not required as the
meter interface (and hence the meter) is always connected to ground. This also
has the advantage that a data logging system can be used.
The device consists of a box (open on two sides) inside which is an insulated
metal plate connected to the meter via a coaxial cable. A small fan fitted to
one open side draws air through the box and when connected to +5 or -5 allows
the current generated by positive ions or negative ions (respectively) to be
measured.The voltage on the meter can vary from +5 to -5 and the corresponding
currents are +/- 50 pico-amps full scale. The minimum resolution will depend on
the actual meter used.
The inside dimensions of the box are 200 mm square by 40 mm deep. A plate 140
mm square is suspended on insulators in the middle of this box connected via an
insulated coax connector to the measuring equipment. NOTE. The screen of the
connector is insulated from the box because the screen is used as the guard and
therefore will carry the same potential as the plate.
The fan used to draw air through the box is a brushless 12v DC fan 40mm sqare
with a maximum airflow of 210 litres per minute.
The sketches show how the device was constructed from sheet aluminium 1.3 mm
thick. The thickness is not terribly important but should be sufficient to
provide stiffness, it is likely 1mm is about the minimum.
It is possibly a good idea to build a mock-up from cardboard to appreciate how
it should all fit together.
The suggested course of construction is shown in the sketches. Start with
the baseplate A, 300 x 220 mm.
(1)
Make the top of the box B so that there are no sharp points inside. For
this reason it has flanges turned outwards so that it can be fastened to the
base by screws on the outside.
(2)
Supported centrally on insulators G within the box is the plate C 140 mm
square.
The insulators are best made from solid plastic rod, and glued to B and
C. Pieces cut from sticks of hot-melt glue are useful here, particularly on
account of its excellent insulating properties.
Fit a coaxial connector just at the mouth of the box and insulated from the box
to make connection with the plate C. Note. The screen of the connector must be
insulated from the box and the plate.
(3)
Fasten the box top to the base plate with two screws on each side near the
corners.
(4)
Next is the fan unit. Two wings E are attached to the fan F as in the
sketch. The wings are the same width as the measurement from top of box B to
base plate A. So that when the top plate D is fitted the wings fit the gap
between the underside of the top plate and the top of the base plate.
(5)
When the fan unit is in place on the base plate the ends of the wings should
just fit round the end of the box.
(6)
The top plate should be cut so that it is about 10 mm larger than the fan unit
all round.
(7)
The top plate clamps the fan and wings to the base plate by means of long
screws from top plate to base plate. The screws are on the outside of the wings
E.
IN USE
Assuming the device will be used for external measurements it is best supported
verticaly on an outside wall (or whatever else is convenient) with the fan at
the top. Then use TV co-ax to provide the connection between the plate and the
input of the high resistance meter unit. A length of two-wire cable is required
for the fan connection to a 12 volt DC supply. And finally a single wire to
connect the body of the unit to plus or minus 5 volts.
Some form of waterproof cover is required which must provide exit for the air
from the fan at the top, and the bottom must be open to allow free entry of
air to the box. This cover can be constructed preferably from plastic or metal
sheet. If metal it should be connected to real earth.
Constructed in this way it is most convenient for the cables to emerge from the
bottom forming a loop and then running up to their destination. This loop helps
to prevent rain from entering any of the electrical connections.
When running the output from the meter unit will vary between +5 and -5
volts. That is from 0 to +5 when the box is connected to +5 volts and from 0 to
-5 when the box is connected to -5 volts.
SOME RESULTS
The graph shows 3 days results from 21st Aug 00 to the 23th. The box
was connected to -5 volts. Three traces are shown, the lower one
being the output from a photo-cell, the middle one the output from a
temperature sensor and the top one the output from the voltmeter interface
connected to the box. As the box is negative the internal plate will be
positive and will therefore attract negative ions.
Remember that the waveform shown by the box output is a negative one and the
corresponding current is inverted. The highest points on the waveforms being 0
volts ie the current is zero, whilst the lowest points are at minus 5 ie the
current is maximum at 50 pico-amps. (5 volts across 100,000 Megohms).
I find the interpretation of these results beyond me and I should be very
interested in comments.
At first sight the box output would appear to be temperature sensitive. The box
waveform is similar to the temperature waveform but is about 2 hours later.
However on the 1st the dip in allumination due to heavy cloud cover is followed
in about 40 minutes with a dip in the box current. This seems to be a
significant difference.
On the 3rd there is a dip in box output with no corresponding dip in
illumination or temperature.
The next graph shows a similar 3 day event with the box connected to +5 volts
from 29th to 31st Aug 00. As the box is positive the central plate will appear
negative to the environment and therefore attract positive ions.
Notice again the relationship between the box waveform, now inverted, and the
temperature waveform. The peaks seem to follow temperature without the 2 hour
delay.
The voltages and currents are as before but inverted. The highest point is plus
5 which is also the highest current 50 pA. The lowest points the voltage is
close to zero and the current zero also.
Finaly. It really does look as though the system is temperature sensitive but
remember the input is a voltage follower accepting from +10 to -10 volts and
the changes in voltage are up to 5 volts.
Finaly I present the following, recorded a few days after the one above and
under the same conditions, from the 4th to the 6th of September 2000.
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