The voltmeter could be connected either across the potentiometer or the fixed resistor. I tried both setups and discovered that more simple and readable readings where achieved by connecting the voltmeter across the fixed resistor.
While performing the preliminary tests I discovered the potentiometer would not cause a change in resistance until the cork was lifted above roughly 35º from its starting position [Fig.1]. To solve this problem I positioned the potentiometer so that its starting position was now about 40º [Fig.2], this way even when the potentiometer was at rest it was causing less resistance in the circuit than if it was at 35º this means that a voltage is measured even when the float is at rest. However this will not affect my results because I can subtract that initial voltage from the voltage after the water was added to give the change in voltage. I concluded that it would be possible to add water in 200ml increments as the setup seemed sensitive enough to allow me to do this.
[Fig.1] [Fig.2]
Method:
I will set up my experiment as shown in [Fig.4] and as shown in the circuit diagram [Fig.3]. As I will be using a potential divider in my circuit, therefore I will require a fixed resistor as well as the rotary potentiometer. I will repeat my experiment 4 times overall, using 10kΩ, 22kΩ, 47kΩ and 100kΩ once each. This will allow me to test how different resistance affect the sensitivity of a sensor and which setting gives better readings from the voltmeter.
I will use the retort stand and clamps to hold the potentiometer in a fixed position, the metal rod with the cork on the end which is attached to the potentiometer will drop down into the container, this will have water added to it in 200ml increments. The potentiometer will be connected to a dry cell battery pack and a fixed resistor, which will be changed to achieve the 4 different resistances I will be using, connected across the fixed resistor will be a voltmeter that will be used to take the results.
Circuit Diagram [Fig.3]:
[Fig.4]
Safety:
I will be using an electrical circuit in my investigation, which will be in close contact with water, so I will need to make sure that they do not come into contact with each other, even though the current flowing through the circuit will be very small.
I will also be using a 500ml glass measuring jug and therefore will have to be careful not to smash it, and make sure if it does smash that I clear up all the glass fragments straight away.
Results Table:
No.1:
No.2:
No.3:
Average:
Graphs:
The graph is plotted using the average of the three sets of results. The results tables showed a few anomalous results, however these seemed to have been ironed out when the average set of results were taken. The graph shows the general trends that were expected. As the resistance was decreased for each test the voltage rose and therefore the height of the graph increased but retained the same pattern of peaks, this shows that the change in voltage is proportional to the resistance of the fixed resistor. From these sets of results I would be able to calibrate the sensor so that it could be used to measure the amount of liquid in a container. E.g. similar to the set up used in a vehicles petrol tank, however the sensor would have to be re-calibrated when used with different volume containers.
Analysis:
The experiment went well. I set up the experiment as in [Fig.3], using a 10kohm fixed resistor. From my preliminary tests I knew that my cork wouldn't float with less than 600ml of water in the container, therefore I started the experiment with 600ml of water in the container and added water in 200ml increments, recording the voltage across the fixed resistor each time. Every time I added 200ml of water and took down the reading for the 10kohm resistor, I changed the resistor to a 22kΩ resistor and took down the reading, then I changed it to a 47kΩ resistor and then a 100kΩ resistor. In my preliminary tests I decided to set up the rotary potentiometer so that its resistance fell as the water level rose, this meant that as I added more water the reading on the voltmeter went up. I continued adding water in 200ml increments until I reached 2000ml. I repeated all this again two more times so I would have a reliable set of results. Whenever I found an anomalous result I repeated that reading again, if they appeared again I left them.
Many things could have caused the anomalous results. There may have been loose connections in the circuit, which caused the readings to differ. The dry cells may also have been going flat, which would have caused the voltage to drop, this may have been likely on the occasions when the voltage reached a peak and then fell for the last 3 readings. See [Fig.6]. Another factor that could have affected the readings is the rotary potentiometer, it was very fiddly when it had to be connected to the circuit, with only thin strips of metal to attach the crocodile clips to, these also bent very easily and if they touched the voltmeter would obviously read zero. Perhaps the potentiometer had slightly too a high friction to be very reliable when it was being moved by the cork floating on water.
Evaluation:
I think my results were reliable, however there were a few anomalous results before the average readings were taken. This leads me to believe that the majority of results are reliable.
I could improve the experiment by, using a new or larger dry cell batteries so that it removes the possibility that the anomalous results were caused by the batteries running flat. I could also check the results and redo the experiments more than once to achieve a more reliable set of results.
Furthermore any of the components may not have been functioning properly. I could also take into account things like rounding up and the accuracy of the voltmeter. If the reading on the voltmeter is changing a lot then it is hard to get a reliable reading and therefore the overall trend of the graphs and the results table could be affected, this could explain the reason why the voltage peaked before the last of the water was added and then fell again at the end.
Overall I would say that the results are reliable although perhaps a more sensitive rotary potentiometer would be better.