The Environmental Comparator
C. L. Fogliani and I. T. Townsend
Many ecological investigations depend upon measuring accurately the
light intensity and the temperature in conditions where it is inconvenient to
use a conventional calorimeter or mercury thermometer. An
environmental comparator has been developed to fulfil this need. In
addition, the comparator can be coupled to a simple calorimeter which can
be readily constructed in schools1 and used for teaching a number of
principles of chemistry.2
Construction of the Environmental Comparator
The comparator and its probes can be made in Australia for an
approximate cost of $20. The environmental comparator may be
constructed from the following components:
5A or 500mA ammeter; 10kΩ linear or logarithmic carbon track
potentiometer; 5kΩ linear or logarithmic carbon track potentiometer;
double pole double throw switch; 2 banana plug terminals; 2x9V
transistor radio batteries; radio knobs to fit the potentiometers
preferably with a built-in scale; protective diodes (see Fig. 1); solder and
insulated copper wire.
Our environmental comparator was made using a 'Peak' brand
microammeter which has sufficient room inside to fit the necessary
components making a very compact easily transportable instrument.
Assemble the parts as follows:
1. Convert the ammeter or milliammeter to a microammeter reading
0—5 or 0—500 units. This is done by removing the shunt wire and
series resistor and replacing the latter with a simple copper link as
shown in Fig. 1. Most modern meters are fitted with protective
diodes to prevent damage due to accidental misuse. These should not
be removed. If the diodes are absent in the meter, they should be
fitted (Fig. 1).
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2. Using a cork borer, bore holes in a plastic case in positions as shown
in Fig 2. The size of the holes depend on the size of the potentio-
meters and switch to be used.
3. Wire up the circuit as shown in Fig. 3, using the wiring aid shown in
Fig.4.
The original terminals on the front of the meter are now used to connect
the various probes. AlOkΩ linear or logarithmic potentiometer is used as a
balance control. Its function is to allow the meter needle to be brought on
the scale regardless of the value of the resistance of the probe. Once the
needle is on the scale, the value of the balance potentiometer is left
constant. A 5kΩ linear or logarithmic potentiometer is used as a standard
against which the unknown resistance is balanced. Its value must remain
constant for a set of readings. A 5kΩ linear or logarithmic potentiometer
is used to adjust the sensitivity of the instrument. For the temperature
probe, it is used at full sensitivity. For the light probe, it is set to give full
scale deflection for maximum light level and then left constant.
The double pole double throw switch is used to select either the power pack
or the 9V batteries as a power source for the instrument. The current drawn
from the batteries is approximately 6X10-3 amps. The batteries may be
conserved by supplying power to the unit only when a reading is being
taken.
Banana Plugs
f for Power Pack mounted on rear
Meter TerTnmats
Sensitivity
Batance
Heavy
Copper
Shunt
(remove)
Standard
Battery —Power
Pack Switch
Protective
Diodes
Series Resistor
(essential)
(replace this by
insulated copper/
wire)
Terminals for Probes
Figure 1 Diagram showing shunt and protective diodes
Figure 2 Positioning potentiometers and switch on the
of meter viewed from below.
meter.
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Figure 3 Circuit diagram for the environmental
comparator. Figure 4 Wiring aid for environmental comparator.
Construction of probes
In the construction of the probes, any variable resistance may be used
provided that its value varies within the range of 200Ω to 40kΩ. The
greater the variation in resistance, the greater the sensitivity.
1. Light Sensitive Probe
Place a cadmium sulphide photo-resistor inside a glass tube as shown in
Fig. 5.
The light sensitive probe is very sensitive and the sensitivity control needs
to be used to keep the needle of the meter on the scale when going from full
sunlight to complete darkness. Made carefully, the probe is water proof
and may be immersed indefinitely in water but not in strong acid bases or
in organic solvents.
2. Temperature Sensitive Probe
The temperature sensitive probe may be constructed in two ways
depending on the type of the thermistor available. The best type of
thermistor is the bead type. This has a very fast response with changes in
temperature. A temperature sensitive probe using a bead thermistor was
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constructed as shown in Fig. 6. The clear plastic sleeve must be water tight.
The top end of the glass tube is sealed with araldite to make it water tight.
To comparator
\\
To comparator
Plastic sleeve Araldite
Insulated leads
soldered
Figure 6 Bead thermistor temperature sensitive probe.
to photores.tstor leads
Figure 5 Light sensitive probe.
A cheap readily available thermistor is the brimistor (catalogue No. CZ10
available from Dick Smith Electronics in Australia). A temperature
sensitive probe using the brimistor was constructed as follows:
Remove one lead from the brimistor, solder a piece of insulated copper
wire to the other lead and then'embed one end of the brimistor and the
bared end of the second lead in Wood's metal as shown in Fig. 7.
The brimistor temperature sensitive probe is not as quick to respond to
temperature changes as the bead thermistor one. However, if the brimistor
is small, the response time should be less than one minute. Temperature
readings taken by the temperature sensitive probes should be accurate t o ±
1/3°C.
Calibration of the brimistor temperature sensitive probe
The temperature sensitive probe was connected to a meter and placed in a
vacuum flask. Water at various temperatures was placed in the vacuum
flask and the temperature of the water was recorded with a thermometer
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calibrated in tenths of a degree. The meter reading was also recorded and a
calibration curve was obtained by plotting the meter reading against
temperature (Fig. 8). The brimistor should not be exposed to temperatures
greater than 60°C.
Tube sealed with
araldite as in Fig. 6
(Brimtttor Type)
50
40 .
I
Cft
• •.Plunge into molten
c
• Wood's metal
-
/
J 30
J 20 -
/
10
Wood's metal
i i i i i i
10 20 30 40 5O 60 70
-Plastic sleeve
Temperature *C
Figure 8 Calibration curve for the temperature sensitive
Turned from solid
copper in lathe
probe.
1cm
Figure 7 Brimistor temperature sensitive probe.
Notes
1. Fogliani, C. L.and Townsend, I.T. (1974) Australian Science Teachers
Journal, 20:3, 109.
2. (1974) Teaching Principles of Chemistry by a Simple Calorimeter,
Mitchell College for Advanced Education Publication, pp. 38-53. This is
available at a cost of $5.
Detailed information on the construction of the colorimeters are contained in the
following two publications available from Mr C Fogliani, Mitchell College of
Advance Education, Bathurst, 2795, Australia.
1. Fogliani, C L (ed) Proceedings of the Workshop on Low Cost Locally Produced
Equipment. MCAE 1984 (Cost $10)
2. Fogliani, C L (ed) Proceedings of the Workshop on Fabrication of Low Cost
Instrumentation MCAE, 1985 (Cost $6)
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