Smaller volume Greater volume
Fewer molecules and metal Same concentration but more
ions met on journey molecules and ions met, more
backscatter & absorption – less light reaches LDR
As I am dependant on the concentration affecting the passage of light I aim to keep the depth of solution constant. This will be done by using the same container and volume of solution.
I want to test this theory of volume and ensure it is true so I will perform a preliminary experiment. This will also familiarise me with the apparatus and expose any other factors which might affect my sensor.
Preliminary experiments
In this experiment I varied the depth of the solution but kept the concentration fixed at:
Concentration: 6 drops of squash/200ml = 0.03 drops/ml
Independent Variables: These influence a dependant variable and are therefore controlled. They may either be varied quantitively or kept constant. Such variables involved in this experiment are:
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Distance of LDR from light source – This must be kept constant; as light travels through the slit of the light box it diffracts. The greater the distance from the slit, the more diffraction that has occurred so less light enters the tube and is directed to the LDR. If the tube and LDR are positioned nearer to the light source for one particular solution, less diffraction will have occurred before the light reaches the tube. Therefore more light will enter the tube and be directed to the LDR, despite the solution being greater in concentration than others.
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Intensity of light source – For the test to be fair the same amount of light must enter each solution. This will be achieved by using the same lamp, power 6v, distance from each solution and position of lamp under the light guard (tube). The lamp will positioned directly underneath the LDR so more light is likely to enter the tube and reach the sensor. The more light that enters each solution, the more obvious the variation in the amount of light reaching the LDR:
Lots of light entering lots of backscatter
and absorption
Little light entering same proportion but
smaller amount of . light absorbed
Lots of wasted light is also wasted energy and on a large scale - in a factory, this would not be cost effective.
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Volume – This will be varied quantitively every 50ml over a range of 200ml but no reading will be taken at 0ml so four readings will be obtained.
Dependent variables: These variables depend on an independent variable; if the other variable changes, the dependent variable will consequently change. We do not control these variables directly but through the independent variables. This is:
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Amount of light reaching sensor - observed as voltage. Dependant on factors discussed above. In this experiment it will be dependant on concentration.
Other factors to consider
The majority of refraction occurs at the surface of the solution as light travels into air and speeds up.
Air
Solution
By placing the light guard (tube) just below the surface, light enters before it can be significantly refracted and scattered so more light is captured and directed to the LDR.
If the tube were placed above the surface it would produce a systematic error.
Therefore if the tube is above the surface then much light is scattered before it can reach the tube hence little light is captured and directed to the LDR.
Method: The circuit will be setup as shown in the diagram below.
Six drops of squash will be added to 200ml of water and the solution stirred well to ensure there are no regions of greater concentration. 150 ml of this solution will be transferred to another beaker.
The beaker containing 50ml will be placed on top of the light box and the tube positioned just above the 200ml mark. Preferably, the tube would be just below the surface however this would mean moving the LDR nearer to the light source with each solution, which, as explained earlier, is unfavourable.
The battery for both the lamp and LDR will be switched on and a reading taken from the voltmeter and recorded.
50 ml of solution will then be added from the second beaker to raise the level to 100ml and another reading recorded. This will be repeated to 200ml .
I must also consider safety as electricity and water are a dangerous combination. I will minimise the risk of electrocution by:
- Removing the beaker from on top of the lamp and away from other apparatus before adding more solution.
- Turning off the power pack between additions of solution.
- I will dry my hands before touching the wires and other electrical apparatus.
- The voltage will be kept low which will not minimise the risk of electrocution but will minimise danger.
Hypothesis: The more light falling on the LDR, the less its resistance.
So as volume increases, voltage increases as less light reaches the LDR and resistance increases.
As the components are in a series circuit (same current flows through variable resistor and LDR - no alternative route) the voltage across the LDR increases. This can be explained by:
V=I x R If the current is 2A and resistance 0.5 ohms then V = 1
If the current is 2 A and resistance 1.5 ohms then V = 3
Predicted Results – I had to first take a reading at 50ml and then 200ml to determine a suitable range.
Circuit Diagram
Actual results
Analysis
These results support my hypothesis and are very similar to my predicted results.
The only anomalous result was at 150ml when there was no increase in voltage, suggesting the amount of light reaching the LDR remained constant and did not decrease as expected. This might have been due to a change in external conditions such as an increase in sunlight. I will have to modify my experiment so that there are no external influences such as ambient light.
Evaluation
Problems encountered:
- I had to keep adjusting the apparatus between volumes to ensure the bulb was in same position under the light guard.
- The beaker had to be removed each time more solution was added as it was dangerous to do so with the lamp underneath.
- Although light was scattered by the same amount on the surface of each solution, the effect was greater on the smaller volumes as the tube was a greater distance from the surface. As a result the light travelled further in the refracted direction thus less concentrated in the direction of the LDR so less light entered the tube.
I had to compromise as the only way to prevent this would be to lower the tube, thereby lowering the LDR which would have a greater affect on the results than this surface scattering.
An alternative could have been to use an extendable tube but I don’t have one!
I then remembered that light was being refracted at each surface, not just from water to air but also air to glass and glass to water. Ideally the light source and LDR would be within the solution and the LDR would span the whole cross section of the solution. Therefore I could have improved my experiment by using a measuring cylinder as a container instead of a beaker.
- Light from other sources (ambient light) also reached the LDR. This was from the light fitting on the ceiling and window. I had no control over the intensity of this light which varied during the experiment; the light from the ceiling remained constant but from the window, decreased with increasing cloud cover.
To minimise this factor influencing my results, I will turn off the lights and close the blinds when performing future experiments.
In my main experiment, issues such as distance of tube from solution will not be a problem as the volume will be kept constant. The tube can therefore be positioned just below the surface of the solution, minimizing light scattering.
I must now decide on a suitable resolution for my sensor. This requires a second preliminary experiment.
Mixing your drink and squash tasting
What magnitude of variation does the sensor need to detect?
Changes of a few drops or a few millilitres?
I will discover this by mixing various solutions of the same juice, varying the concentration slightly by measurable amounts.
I will test each one and record when the solution just becomes too dilute or concentrated. I will also ask my brother to test them to ensure my results are not biased. I can then decide on a concentrate range within which the squash produced is suitable.
I know already that variations of a few drops have little effect so I will start by varying by millilitres.
What concentration to start with?
My squash concentrate says dilute 1 part to 4. If the average drink is 300 ml then I need to add 60ml of concentrate. This is a squash concentration of 20%. I will then vary my solution in steps of 2ml above and below this value. There is no point in varying by any less as my own taste buds are not sensitive enough.
Results – me Results - brother
This shows that a variation of 2ml either side of 60ml is acceptable
(if 62 is ok then 58 will be too as the margin is equal either side of 60).
60 = 20% of 300 so 2ml is a percentage error of 62 ÷ 300 × 100 = 20.7% = 0.7%
2ml = 1% of total solution 300ml therefore a concentration increase of 1%
My sensor will therefore have to be sensitive enough to detect concentration changes of 1%.
As I didn’t perform my experiment with 63ml and 64ml, an acceptable error may be larger than this in which case the sensor would not need to be as sensitive.
However, on a scale of thousands of litres 2% could be hundreds of litres which could be expensive. The aim of the manufacturers is to save money so they would wish to minimize the amount of juice unnecessarily used.
Experiment to determine sensitivity of my sensor
I will now vary the concentration of my solution while maintaining the volume as far as possible to identify the magnitude of change my sensor can detect. The concentration will be varied by 2ml of concentrate as this is the minimum change it will need to detect. Ideally it would be able to detect smaller changes than this to minimise squash wasted.
Method
The same method will be used as in the preliminary experiment in which volume was varied. However this time the bottom of the light guard will be immersed just below the surface as volume will be kept constant and concentration varied.
Starting volume dilution 1: 4 total volume 120ml voltage on lamp 8v
Analysis and Evaluation
These results show that, as expected, increasing the concentration of the solution increases the voltage as less light is able to pass through the solution so the resistance of the LDR increases. My sensor is therefore sensitive enough to detect concentration changes of 1%. However the voltage change is not uniform, it varies between 0.06 to 0.09, a range of 0.03. As the variation in V change shows no trend, I can conclude that the relationship between volume of squash (concentration) and voltage is linear. This variation in voltage change is most likely to have been caused by small alterations in apparatus. It is unlikely to be caused by the resolution of my sensor; if the sensor was only just able to resolve voltage changes to one hundredth of a volt then the variation would only be of 100th of a volt, not 3 × 100th of a volt as can be seen in my results.
By calculating an average voltage change, to eliminate the effect of changes in apparatus, I can determine the sensitivity of my sensor:
(0.06 + 0.07 + 0.08 + 0.09) ÷ 4 = 0.075 resolution only to 100th so 0.08V/1% variation in concentration
However, taking into account the apparatus will affect the voltage readings, a change of 0.06 V would need to initiate the addition of more solute as this would be the minimum change for 1% concentration increase.
In conclusion, I believe I have achieved my aim of creating a suitable sensor for detecting variations in squash concentration. It is sensitive enough to detect relevant concentration variations and also means that a resolution of 0.00V is sufficient. I recognise that my sensor had structural faults; the apparatus had to be constantly adjusted between readings, but with the equipment available this fault couldn’t be rectified.
